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How To Set and Tune A Tube Amplifier

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How To Set and Tune A Tube Amplifier

Post by Foxhunter » Wednesday, 24 December 2008, 17:51 PM

Recently I helped a couple of members figure out how to work their old D&A Maverick amplifiers. There are others also asking lately in threads about how to do so. I'm no expert but still I wanted to post an online article I found about the "do's & don'ts" of setting and tuning an older tube linear amplifier. I've seen several people asking about this lately and think this may answer some questions and be helpful. It is a fairly accurate and easily explained instructional. Hopefully this I think will be of some benefit to some of our Forum Members and Guests visting the CBRT Forum here.


10 Meter Ham and 11 Meter CB ---- "Peak and Dip" Tube Linears

Why?: Some people wanted to know how to tune a tube linear amplifier.

Well you must: " PEAK RF OUTPUT WITH WATTMETER AND DIP THE PLATE CURRENT. "

A. Peak means to adjust a control until you get the MAXIMUM indication on a meter.

B. Dip means to adjust a control for MINIMUM indication on a meter.

Typically the procedure is the following:

1. Hook everything up (radio to amp, amp to wattmeter, wattmeter to dummy load which is capable of handling the full output of the amp for as long as it take you to perform the tune-up).

2. Set your radio's output to as low of a setting as it will go to, usually about one watt or so is all.

3. Set the amp's " LOAD " control to mid-point and the " TUNE " control to about the same point.

4. Key your radio and " VERY QUICKLY " turn the " TUNE " control back and forth to get a feel for the proper direction to increase power, then " UNKEY " and wait a few seconds for things to cool down.

5. Key up the radio again and watch your " OUTPUT " wattmeter while rotating the " TUNE " control for maximum power out. The " PLATE " meter should indicate proper neutralization of the tubes " IF ", you see a pronounced dip which co-incides with maximum output.

If it doesn't, you need to neutralize/balance the load power between the tubes. There is a control " INSIDE " the amp for this purpose if your amp is worth a plug nickle. " BUT " *** BEWARE *** of the " EXTREMELY HIGH VOLTAGES " present inside the amp! " THEY CAN BE DEADLY !!! "

6. After tuning the " TUNE " control for maximum meter reading (highest wattage as seen on an external wattmeter) then unkey and let things cool a moment.

7. Next key the radio again and adjust the " LOAD " control for maximum meter reading.

8. Unkey and wait a moment, then go back and re-peak the " TUNE " control again.

9. Finish up by adjusting the " LOAD " again for maximum meter reading.

10. Now you have preliminary settings, and can begin increasing your radio's drive power. Increase to the wattage you expect to be driving the amp with and-----repeat the tune procedure again.

11. Once tuned into a " DUMMY LOAD ", then switch to your antenna, and re-peak. If your antenna is adjusted properly, you should not need to re-adjust the amp much, if at all. If your antenna is mis-tuned, then you'll have to re-adjust the amp.

12. For frequency excursions within the same band, you should be able to leave the load control alone (pretty much) and simply tweak the " TUNE " control a bit to re-gain power lost by changing frequencies. If you seem to lose a LOT of power, you may have to re-adjust the " LOAD " control accordingly.

13. Always make sure that your " PLATE CURRENT " dips at practically the same tuning point as " MAX POWER ". Slightly off a bit is " OK ". Some people advance the " LOAD " control slightly to lessen power a few watts because it supposedly makes the amp sound better. Use signal reports from others to see if this adjustment benefits you or not.

14. If you don't dip the plate, you're going to wind up burning up a set of tubes !!!

That's about all there is to know about the care and feeding of a tube amp.

EDIT: Hope this helps and-------Merry Christmas

Foxhunter 351 NJ
Last edited by Foxhunter on Friday, 25 September 2009, 18:57 PM, edited 2 times in total.

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Post by treetall » Wednesday, 24 December 2008, 17:57 PM

OMG. i think ill stick to pill boxes at least for now. :shock:
801 working this baby driver in the carolinas getting out the way.



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Post by Green Hornet » Thursday, 25 December 2008, 18:33 PM

What a great a helpful article !!! I am setting up a Browning Golden Eagle Mark III with a Browning Mark III 180 Amplifier and believe me this advice will help. THANKS & HAPPY HOLIDAYS

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Post by fatboy803 » Thursday, 25 December 2008, 18:35 PM

treetall wrote:OMG. i think ill stick to pill boxes at least for now. :shock:
I second that :D

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Post by linx » Friday, 26 December 2008, 4:56 AM

Fox, if it's ok, I may move this to the viewforum.php?f=70 section. Great job as always bud.
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Post by djrebel236 » Friday, 26 December 2008, 5:04 AM

Thanks for the memories Foxhunter, great topic too...Dj
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question about tuning a tube amp

Post by 464budmann » Friday, 26 December 2008, 13:11 PM

[/i]hey Foxfire gotta a question bout my setup, was wondering if you could give me any advice? Im running a galaxy/saturn base into a palomar 350 and running them into a old Galaxy 800 tube amp/base, Iv got the saturn base's RF turned all the way dwn & barely on, Iv got the Palomers variable knob at 3/4 open and then I adjust the Plate tune and Ant load on the 800 to the furthest point in the red that the needle will go bout 1+to2+ while hallering AUUUUUUDIO etc, my swr's are below 1 all the time My question is; Is that even close to {the wright way} or am I fixin to blow some tubes to pieces? I dont have a dummy load either so was wondering if I should stop talking w/800 tube amp till I can learn the correct way to set it up,. god bless & thanks ahead of time for not callin me a dummy if Iv been doiner wrong this whole time...

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RE question about tuning a tube amp

Post by 464budmann » Friday, 26 December 2008, 13:39 PM

464budmann wrote:[/i]hey Foxhunter gotta a question bout my setup, was wondering if you could give me any advice? Im running a galaxy/saturn base into a palomar 350 and running them into a old Galaxy 800 tube amp/base, Iv got the saturn base's RF turned all the way dwn & barely on, Iv got the Palomers variable knob at 3/4 open and then I adjust the Plate tune and Ant load on the 800 to the furthest point in the red that the needle will go bout 1+to2+ while hallering AUUUUUUDIO etc, my swr's are below 1 all the time My question is; Is that even close to {the wright way} or am I fixin to blow some tubes to pieces? I dont have a dummy load either so was wondering if I should stop talking w/800 tube amp till I can learn the correct way to set it up,. god bless & thanks ahead of time for not callin me a dummy if Iv been doiner wrong this whole time...

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Post by Foxhunter » Saturday, 27 December 2008, 18:04 PM

linx wrote:Fox, if it's ok, I may move this to the viewforum.php?f=70 section. Great job as always bud.
Thank you Linx it's appreciated. I hope the Holidays will be treating you well. Please feel free to post this thread wherever you feel it will be the most benefit to any viewers and be most visible & accessible to them. These instructions posted above and here below are the most clear I could find on the web and wanted to place them in one thread for new tube amplifier owners.

I hope that they may be of use when trying to familiarize yourselves with this type of "somewhat tricky" radio gear, I know it took me awhile to get the hang of. 464budman try and re-read the instructions listed with tuning this type of amplifier. I see you have a higher-power base radio feeding into a transistorized amplifier then into a tube amplifier. I would believe the tuning instructions would remain close to the same. I'm a little "leery" giving more specific advice because of the nature of tube units to begin with and I wouldn't want to tell you something more that could cause harm in your specific setup configuration as I am not an expert in the field. Maybe another member here would care to venture more specific advice to your particular situation?

WARNING
Please remember that the warnings of "Danger: High Voltage" that is present in these units and that IT IS NO JOKE. A couple months ago I read where family members had found a radio operator lying dead on the floor of his radio room. His electrocution had burned and completely severed his hands off at the wrists-----his blackened hands were found still there attached to the tube amplifier. Nearly every year without fail guys are killed from poking arouund inside a tube amplfier. Often it happens while making adjustments inside the cover or trying to change tubes or another repair/maintenance task. It doesn't matter if the amp is plugged in or not, they can still kill while completely powered down and unplugged. Words "to live" by.
*********************************************************************************

Three Articles Below To Read On The Use Or Setting Up Of A Tube Linear Amplifier
Because CB & Ham tube amplifiers share many common charateristics the articles below often overlap covering both types and the principles are nearly identical. Some CB tube amps will be found to be "toned down" versions of the Amateur tube amplifiers and maybe be a little simpler in design and with functions included. However the following directly applies to the use of your CB amplifier. The first and original article is the most basic and the others following expand upon it in a little more detail. This should answer some questions and serve as a reference. It is a good start, and like the following articles, further information may be found online or on the CBRT Forum here with a little effort & research.


*********************************************************************************
10 Meter Ham and 11 Meter CB ---- "Peak and Dip" Tube Linears

Why?: Some people wanted to know how to tune a tube linear amplifier.

Well you must: " PEAK RF OUTPUT WITH WATTMETER AND DIP THE PLATE CURRENT. "

A. Peak means to adjust a control until you get the MAXIMUM indication on a meter.

B. Dip means to adjust a control for MINIMUM indication on a meter.

Typically the procedure is the following:

1. Hook everything up (radio to amp, amp to wattmeter, wattmeter to dummy load which is capable of handling the full output of the amp for as long as it take you to perform the tune-up).

2. Set your radio's output to as low of a setting as it will go to, usually about one watt or so is all.

3. Set the amp's " LOAD " control to mid-point and the " TUNE " control to about the same point.

4. Key your radio and " VERY QUICKLY " turn the " TUNE " control back and forth to get a feel for the proper direction to increase power, then " UNKEY " and wait a few seconds for things to cool down.

5. Key up the radio again and watch your " OUTPUT " wattmeter while rotating the " TUNE " control for maximum power out. The " PLATE " meter should indicate proper neutralization of the tubes " IF ", you see a pronounced dip which co-incides with maximum output.

If it doesn't, you need to neutralize/balance the load power between the tubes. There is a control " INSIDE " the amp for this purpose if your amp is worth a plug nickle. " BUT " *** BEWARE *** of the " EXTREMELY HIGH VOLTAGES " present inside the amp! " THEY CAN BE DEADLY !!! "

6. After tuning the " TUNE " control for maximum meter reading (highest wattage as seen on an external wattmeter) then unkey and let things cool a moment.

7. Next key the radio again and adjust the " LOAD " control for maximum meter reading.

8. Unkey and wait a moment, then go back and re-peak the " TUNE " control again.

9. Finish up by adjusting the " LOAD " again for maximum meter reading.

10. Now you have preliminary settings, and can begin increasing your radio's drive power. Increase to the wattage you expect to be driving the amp with and-----repeat the tune procedure again.

11. Once tuned into a " DUMMY LOAD ", then switch to your antenna, and re-peak. If your antenna is adjusted properly, you should not need to re-adjust the amp much, if at all. If your antenna is mis-tuned, then you'll have to re-adjust the amp.

12. For frequency excursions within the same band, you should be able to leave the load control alone (pretty much) and simply tweak the " TUNE " control a bit to re-gain power lost by changing frequencies. If you seem to lose a LOT of power, you may have to re-adjust the " LOAD " control accordingly.

13. Always make sure that your " PLATE CURRENT " dips at practically the same tuning point as " MAX POWER ". Slightly off a bit is " OK ". Some people advance the " LOAD " control slightly to lessen power a few watts because it supposedly makes the amp sound better. Use signal reports from others to see if this adjustment benefits you or not.

14. If you don't dip the plate, you're going to wind up burning up a set of tubes !!!

That's about all there is to know about the care and feeding of a tube amp.

****************************************************************************

The Care & Feeding of Linear Amplifiers


What Is A Linear Amplifier?


In amateur radio terminology, a linear amplifier (also known as a linear or a brick) is an RF amplifier designed to amplify the output of a transmitter to a higher power level without introducing distortion.
Linear amplifiers are available for frequencies from 1.8 MHz to above 1 GHz.
Linear amplifiers can have an output power of 20W to 1500W+ PEP


Why use a linear amplifier?


Mainly, working DX on the lower HF bands may require high transmitter power to get through the high level on noise on these bands.


Types of Linear Amplifiers


Solid State
Active device is a power BJT or MOSFET
Available in powers of 20W – 1000 W
Commercially available for any amateur frequency above 1.8 MHz


Vacuum Tube
Active device is a triode or tetrode vacuum tube
Available in powers of 300W – 1500 W
Commercially available for amateur frequencies between 1.8 MHz and 144 MHz


Solid-State (Transistorized) Linear Amplifier Basic Operating Controls


Mode Switch
Receive Pre-Amp Switch
Power Amp Switch
Status Indicators



Vacuum Tube (Valve) Linear Amplifier Basic Operating Controls

ALC (Automatic Level Control) circuit
Level
Metering
Band Switch
Power Switch
Standby Switch
Metering Switch
Plate Tuning
Load Tuning



Solid State Linear Amplifier (Brick): Pros & Cons


Advantages
Requires no adjustments when changing frequencies within the amplifier’s design range
Generally RF switched
Generally include a receive pre-amp
Disadvantages
Requires high current (20 A – 140 A) Low-Voltage DC power supply
Designed to operate into a 50 ohm load
Relatively expensive ($1 - $4 per Watt)


Vacuum Tube Linear Amplifiers: Pros & Cons


Advantages
Requires no external power supply
Can operate into a wide range of loads (30 – 150 ohms)
Relatively inexpensive ($1 - $2 per Watt)
Disadvantages
Requires adjustments when operating frequency is changed, even within a band.
Requires some sort of external TR switching relay
Difficult to find for frequencies above 28 MHz



Setup/Operation of a “Brick” Amplifier


Connect the RF output of the brick to a dummy load.
Select the proper RF mode.
Turn of the power amp.

Power Amp ---Switch
Pre-Amp ---Switch
RF Mode ---Switch


Transmit into the dummy load, increasing the drive until the proper output power is reached.
Shut down the brick Connect it to the antenna.
Turn on the brick amplifier and operate, checking to see if it's working correctly as it should.


"But it’s not working….."


Is the DC power supply properly connected and of sufficient amperage & voltage??
Check to make sure that the SWR at the RF output of the brick is below the maximum allowable value (typically 1.5 to 2.0)
Check to make sure that the proper mode is selected. Example: trying to operate SSB in the FM mode can create problems.



Operation of a Vacuum Tube Linear


Band Switch
Plate Tuning
Load Tuning
Meter Function Switch


Connect antenna, turn on linear and set the “Meter Function Switch” to “Plate Current”
Apply ~ 30 W of drive and adjust the “Plate Tuning” for minimum plate current.
Set the “Meter Function Switch” to “RF Out” and adjust the load tuning for maximum power output.
Increase the drive until the output power reaches the desired level---and repeat the plate and load tuning steps


"I tried all that and it didn’t work…."


Check to make sure the antenna is connected.
If the plate current does not show a dip and the antenna is OK, one of the amplifier tubes may be bad.
Use the amplifier’s metering to check the output of the HV supply. No HV = no RF output!
Check the antenna impedance. A vacuum tube linear cannot generally match impedances less than 30 or greater than 150 ohms.
Make sure that the amp is in the right mode (AM/CW/SSB whichever you intend)



Which Type Amplifier Should I buy?


If money is no object, it is hard to beat a good solid state linear amplifier for HF (or VHF) work.
If money is a concern, vacuum tube linear amplifiers are widely available on the used market at prices as low as $0.75 per W.


Selecting A “Brick” Type Amplifier


Decide whether or not a receive pre-amp is important – this can add significantly to the cost of the brick type amplifier.
Make sure that the brick’s output will not exceed the rating of the antenna or pre-amp.
Remember that a DC power supply with adequate current (amperage) will also need to be purchased.

Selecting A Vacuum Tube Amplifier
There are several types among them

Triode Amplifiers
Generally grounded grid design
Do not require neutralization
Gain limited to 10 - 12 dB
Triode transmitting tubes are very robust
Tetrode Amplifiers
May require neutralization
Gain can reach 20 dB
Tetrodes may be destroyed by excessive grid current
Sweep Tube amplifiers
Gain limited to ~ 6 – 8 dB
Some "sweep tubes" can be destroyed by full duty cycle operation!!

****************************************************************************

HF Tube Amplifier FAQ's


This page should help the less experienced users solve common problems and questions about using HF tube type amplifiers.

Q: "Why must I "tune" the amplifier?"

A: For those who are not familiar with tube-type HF amplifiers, I will explain the reason. Why do you tune a guitar? So that the strings produce sound in the proper frequency. Very much the same in an amp. The process of "tuning" the amp causes the output circuit to resonate, and also matches the impedance of the tube to that of the output/antenna. The signal that drives the amp will not be amplified if the output circuit of the amp is not resonated and matched.

Q: "What amplifier should I buy?"

A: What are your needs? How much power output do you wish to use most of the time? These are questions that the user needs to answer before making a choice. For example, if the user wishes to only use about 500 watts output, there are a good number of units that will fit this requirement, the exact model will mainly be up to the users personal choice. There are a large group of amps in the 1kW output class. Each model has a given duty cycle and range of bands. The larger the list of user requirements, the smaller the list of units that will fit into those requirements. Narrow your list to only the units that fit best. For more insight into what makes a good amplifier read my article.

Q: "I'm afraid of those expensive tubes, which ones should I get?"

A: At this point in time, there are still several types of glass enveloped tubes available, but this is soon to change as tube manufacturers phase-out glass tubes. There are a few types of metal/ceramic tubes that are soon to be phased-out too. All is not lost, if you want a type that is soon to go the way of the dinosaurs, it would be a good idea to buy several sets of "spares", this way the amp/tubes will last the life of the user, or enhance a resale later. Research the type of tube in a given model amp that you are interested in, find out what gear (aside of cb or ham gear) it is used in, if there is a large commercial market for the tube, chances are that it will be around for some time.

Q: "Should I run my amp on 220vac instead of 120vac?"

A: For the 1kW output amp, 120vac is not a very good option, because most homes are not wired to handle the heavy current needs at 120v. If the amp pulls 15Amps on 220v, it will then pull 30Amps on 120v. Since most houses are wired with 12ga. wire, which can handle 20Amps, the voltage drop at 30Amps would seriously degrade the performance of the unit. On 220v, the amp will run cooler, with better high voltage regulation, and less "flickering lights" in the home. A dedicated circuit with 12ga. wire will handle 5kW continuous current draw. Be sure to use a circuit breaker that is near the upper limit rating of the amp being used, i.e. 20Amps. Good high voltage regulation will mean that the amplifier's RF output will be more linear with less waveform distortions.

Q: "How can I know if my amp is prone to "parasitic oscillations"?"

A: A quick and easy test to detect parasitic tendencies is as follows: Place the amp on the highest band it can operate on, be sure the unit is ON, and in STANDBY mode. Place the TUNE control at or near the setting it would normally be for that band. Slowly rotate the LOAD control from one end of it's rotation to the other, observe the GRID current meter during the control rotation. If there is any movement in the meter, it is likely that the amp might be prone to parasitics. Steps should be taken to suppress these parasitics.

Q: "Can I key my "older or vintage" HF amp, with my "newer style" radio?"

A: Don't always bet on it! Many of the late model radios now use small low voltage, low current relays or transistors to key outboard amplifiers. As a general rule it would be prudent to use an outboard heavy duty relay that is keyed by the radio, and then keys the amp. This isolates the radio from the amp keying voltage/current. These relays are cheap and easy to build, and can be bought ready made too. I have designed a relay that will work on most radios and amps.

Q: "How can I control the RF drive, when my radio does not have a "variable" RF power control?"

A: Use the ALC (if equipped!) on the amplifier. Regardless of whether the radio can control it's RF drive level or not. The ALC circuit in the amp will send a voltage back to the radio that will reduce the RF output level automatically. This will prevent chronic over-drive, and help prevent the "oops" factor. Here is how to set the ALC level: Tune the amp up as normal, apply enough RF drive to reach full rated output of the amp, now adjust the ALC level control (usually on the back of the amp), until a slight reduction in RF output is noted. That's it, all done. Now the RF drive will be limited to the same output on the amp regardless of the band used.

Q: "The input SWR is bad on my amp, what can I do?"

A: Many "older" HF amps had no tuned input network, or had tuned input networks that were badly designed. If your amp has no input network, it might be a complex project to build and install one. In this case, it would be best to use a rig's built-in ATU (Automatic Tuning Unit) if available, or use and outboard tuner of some type. If the amp has a "tuned" input network and the SWR is bad on only one or two bands, and good on the rest, the network might need to be retuned, or repaired. Sometimes components in these circuits change value and "de-tune" the network. The parts should be replaced and the network re-tuned for minimum SWR. If the input "Z" of the tube(s) are known, a resistor equal to that Z value can be placed on the output end of the network, and an SWR analyzer like the MFJ-259 can be placed at the input of the network to aid in tuning of the unit without "powering-up" the amp. A slight "touch-up" may be needed when the amp is placed into service. Some amps use a passive resistor input network, and these networks provide a constant resistive 50 ohm load to the exciter, while absorbing some of the RF drive signal and converting it to heat. The rest of the RF drive is applied to the tube. This type of network is very broad-banded, however harmonic suppression is not very good. Harmonic distortions are not attenuated. In many cases the ATU in the radio can be used to "tame" the input SWR of most amps.

Q: "I'm not sure if I'm tuning my tube-type amp properly, how should I tune-up?"

A: The "Tube Amp Tune-Up Article" following below this first Q&A section should explain this in a very in-depth manner. Read it.

Q: "My amp acts strangely on certain bands, arcing and causing TVI, what is the problem?"

A: It is very possible that the unit has what is known as a "series resonance" on or near that band. Many commercially built amps have this problem when operated near 25 MHz. How to fix it can be complex, but the first area to try is to change the plate choke for a choke that has a higher inductance. Seek the advice of an experienced amp tech to help you with it.

Q: "I have heard that capacitors can go "bad", when and how can I tell if mine are "going" bad?"

A: "Computer-Grade" electrolytic capacitors have an average service life of from 15-25 years dependent on the level of ambient heat they operate in. These caps fail in only two modes, "open" meaning loss of capacitance----or "short" meaning a dead short, usually resulting in an explosive destruction of the cap. A shorted cap would be pretty obvious, but if your caps are "open" how can you tell this? If the caps are open, there will be a marked drop in the plate voltage under a "no-load" condition. Also, when under load, the plate voltage will drop severely, and it is possible that an AC hum or severe distortion can be heard on the transmitted signal from this unit. If your amp is over 20 years old, it might be a good idea to buy a replacement set of capacitors for your amp. Always replace the full set of caps.

Q: "Can "any" amplifier operate QSK/Full break-in mode?" (Ham Radio only)

A: If the unit has an open-frame mechanical relay for T/R, it is not a good idea to operate QSK CW with it. Open-frame relays are much to slow and prone to arcing to operate high speed QSK. If the unit has vacuum relays such as the Jennings RJ-1A which switch at about 6ms, should be capable of up to 40wpm QSK operation. The fastest QSK system uses PIN diodes for T/R switching, but care must be taken not to "hot" switch them, or they might fail. An Enable/Disable keying system must be used to prevent "hot" switching. If your amp makes a loud "thump" or click, it will not be a good idea to use it on QSK CW. PIN diode T/R systems are also very sensitive to SWR, so any time the antenna SWR goes over 1.5:1, an antenna tuner should be put in-line between the amp and antenna.

Q: "My amp does not have 17 meters on it, can I still use it on that band?" (principle is the same for 10-Meters & 11-Meters for example)

A: If your amp has (for example---can be substituted for 10 & 11 meters) "15" meters on it, you can use it on the nearby "17" meter band. Simply use the 15 meter band switch position and tune it up normally, on some amps the 20 meter band position must be used. However, there is not a "tuned-input" network for 17 meters so the input SWR might be fairly high (1.7:1 or more), just use your rigs ATU or an outboard tuner to match the input. On 12 meters, the 10 meter band position is used, but this can be a problem band if there is a series resonance at or near 25Mhz like that mentioned in the preceding paragraphs.

Q: "I have tried to use my amp on "17" meters, by using the "15" meter position, but it doesn't work, why?"

A: Check the input SWR of the amp first, it might be that the SWR is high enough to cause the exciter/driver to reduce RF output to the point that the amp is not driven enough for output to be useful. If it is found that the input SWR is high enough to cause SWR "cut-back" in the exciter, engage the ATU (if available), or use an outboard unit. If input SWR is NOT a problem, try the unit on the 20 meter position, if it still does not work, it is likely that the tank coil is to far out of resonance to generate any useful amount of RF output. To fix this would require the tank to be "re-tapped", but this might cause the loss of 15 meters. The user would need to decide if that is worth getting operation on 17 meters. It might be better to get an amp that operates on 17 meters without any troubles.

Q: "Which is better, a "Pi" or "Pi-L" output networks?"

A: "Pi-L" is better by far. Another article can explain in greater detail.

Q: "Can I use a "CB" amp on the ham bands?" (and vice-versa)

A: That Depends. Keep in mind some older "CB" tube amplifiers are very poorly designed and constructed and you will need to make this decision depending on the type and design of the amplifier in question, judging on a case-by-case basis. Many CB amps are designed to operate on AM only (some have SSB option) and often use a class-C circuit which is NOT linear and can be distorted and full of harmonics, resulting in bleeding and higher SWR. It will vary greatly, like any product, on the design & manufacture of that particular amplifier. Some CB transistor amps use a method of biasing that makes it impossible to transmit a clean SSB signal, no matter the drive level. Simply look at the circuit designs to tell. CB amps will operate from 10-40 meters typically, but may or may not cover that many bands due to design and construction variations. Most Ham amps need modifications to transmit along the 11 meter portion of the spectrum and won't work in their stock configuration from the factory. Many "Ham amps" specifically WILL NOT be able to work using the CB "AM" mode and are not meant for this type of continuous-duty operation and the destruction of expensive tubes can result.

Q: "My amp runs very hot, even when idling for extended periods, what is the deal?"

A: If your unit is an "older" vintage amp, it's likely that most of the heat (aside from the heat of the tubes), is coming from the HV bleeder resistors, which are of a lower resistance than they should be. For example the Heathkit SB-220 uses 30K ohm 7 watts resistors as HV bleeders, which dissipate a great deal of heat needlessly. Change these resistors to 220K ohm 2 watt "flame-proof" 2% type, which will greatly reduce the heat output, extend the life of the capacitors they are in close proximity to, and stiffen the plate voltage. The only draw back is that the bleed-down time will be much longer, but that is a better thing than the heat. If this modification does not reduce the heat level enough, the air-flow may need to be increased.

Q: "There is a "ticking" sound from inside my amp, other than that it works fine, what is going on in there?"

A: A small "ticking" or random "crack" sound from inside your amp might be a high voltage arc, where it has "ionized" a path to ground somewhere in there. These are hard to find, so here is how it's done: First you need to take the cover off the unit (be careful with HV exposed!), next defeat the interlocks and shorting mechanisms (be very careful!!), now power-up the amp, and darken the room. Now your looking for the light from the tiny arc that is the source of the sound. If you see where it is, note it's position, and repair it, or clean the dust or carbon track it is following, it might require you to actually move a part to break the path of the HV. Restore the amp to operational condition. A sharp point in the HV circuit will readily arc, so file or trim down any of these you find. Wires that are left with a sharp point, or edge will be a likely suspect. Trim them to a blunt edge or rounded point. "Rounded and smooth" is the best answer here.

Q: "My amp starts "humming" louder after it has been on for a while, why is it doing this?"

A: A growing "hum" in an amplifier, is usually from the power transformer, which as the transformer heats up, the laminations become loose, and begin vibrating in the magnetic flux that flows in the iron core of the transformer. No real easy way to fix this aside of replacing the transformer. There has been some success with re-tightening the bolts that hold the transformer together, this does not always work. Another cause may be a steel cabinet cover that is vibrating in the presence of the magnetic field that extends out from the transformer. A piece of brass or copper attached to the cover in the area of the transformer may reduce the vibrations. Other methods of vibration damping may work but it will be necessary to experiment with them to find the best fix for your situation. Lastly, the hum might be caused by a fan/blower that is "out-of-balance", many times this is caused by a build-up of dust in the fan blades. Fan/Blowers that use "sleeve-type" bearings can cause vibrations also, especially when old and worn down.

Q: "My amp starts drawing plate current as soon as it is turned on, what is wrong?"

A: Check the T/R relay, insure that the contacts are not fused in the TX position. Next, check the operation zener diode (if used), insure that it is in operational condition. If all these are OK, it may mean that one or more of the tubes have a grid-to-filament short. This does happen from time to time with power tubes. Check the unit to tell if any AC hum is heard on transmit. If AC hum is found, a grid-to-filament short is confirmed, replace the tube or tubes, and the unit should operate as normal again. If no AC hum is noted, and the transmit signal sounds normal, it is likely that the "cut-off" bias is inadequate or not functioning properly. It will have to be repaired.

Q: "My amp "blows" primary fuses from time to time, aside from that, it operates normally, what is the cause of the "blown" fuses?"

A: This problem often happens to units that have no "step-start" circuit, or a circuit that is not functioning properly. Another possibility for this might be a "flash-over" of HV inside the tube. Insure that your amp "step-start" circuit is operating normally. Also, insure that the plate voltage used, is not in excess of the tube rating. If you have installed new tubes recently, they may be "gassy", it will be necessary to "burn-in" the tube by allowing the unit to idle for at least 24 hours prior to operations. Check the filter capacitor bank for shorted caps. If the fuses used are to close to the maximum operating current level, they can burn out over a period of time. It is best to check the operation manual for the proper value of fuse that should be installed in the unit. Always use 250v fuses, never, ever use 32v fuses.

Q: "How long does the typical power tube last?"

A: Life spans vary from one type to another. An example is: a 3-500Z should last about 12,000 hours of transmit time. Most metal/ceramic tubes are rated at 24,000 hours. This means that (barring abuse or mechanical failure) the typical tube should last, (based on the following formula: 2 hours per day X 6 days a week) = 19 years for a 12K/hour tube, or 38 years for a 24K/hour tube. Excessive tune-up time, and filament voltages will drastically reduce these numbers. EIMAC tube data sheets state, that for each tenth of a volt over the filament voltage rating, the tube life will be reduced by 10%. For example: a tube with a filament rated at 5V, and an amp that uses 5.5V, the tube may have a 50% reduction in life span.

Inexperienced, excessive "tune-up" times & overly high RF "drive" levels are the primary cause of premature failures in most of the tubes used in CB & Ham radio tube units.

Q: "My amplifier has been in storage for several years, how should I power it up?"

A: The old practice of using a variable transformer or a "Variac" to slowly increase the primary voltage is not always necessary, but is the most proper and safest way. However, a few precautions should taken in powering up the amp for the first time in many years. The main problem in the first power up is not the current "in-rush", but rather air contamination of the tube vacuum. The first power up of an amp with tubes that have become heavily contaminated by atmosphere can result in an HV flash-over and resultant plasma damage inside the tube(s). To prevent the possibility of a flash-over, disconnect the plate/anode connectors to the tubes. Next place the amp in the low voltage of CW plate voltage mode, then turn on the main power. If no arcing, smoke, or explosions take place after power up allow the unit to idle with no RF applied for no less than 10 hours. This "burn-in" time burns off, or absorbs a large amount of the contamination in the tube, which will reduce the possibility of an HV flash-over and resultant damage to the tubes, and or the HV power supply.

As for the possibility of the HV filter capacitors exploding, or "smoking", even with a variac, there is no sure way to prevent this from happening. The caps will either work, or they will not. If the caps fail, replace the entire bank, it was likely long past time to do that anyway. If the unit was stored in a damp or high humidity area, the power transformer may have high levels of moisture in the windings, which can be removed by placing the amp in a kitchen oven, and adjust the oven temp for about 200 degrees F. Leave it in the oven for about 3 hours, then remove it, and allow the unit to cool to room temp before powering it up. It would be best to do this procedure with the tubes removed from the unit while in the oven. Another thing to look for, insects or rodents that might have taken up residence in the unit during storage. Clean out any dust that may have built up too. Check for any component damage and make repairs prior to first power-up.

After the unit is "powered-up", look and listen, your looking for smoke, arcing, or other signs of abnormal operation. Your listening for the sound of arcing, unusually loud humming, unusual vibrations, or the sound of the fan/blower either working abnormally, or not working at all. Look at the meters. The HV meter should read normal, and the plate and grid current meters should not read anything during "burn-in" period. If an odor is noticed, and seems to get stronger as the unit continues to run, it would be a good idea to shut it down and investigate the source. Another warning sign, is unusually high levels of heat that seem to be increasing even though the unit is idling, again shut the unit down and investigate the source of the over-heating.

If everything checks out good, and no problems happen during the "burn-in" period, shut the unit down (allow the unit to sit and cool for a few minutes), re-install the plate/anode connections, and again power-up the unit. Again, use the "look & listen" procedure. If no problems happen during this final step, your ready to operate with RF drive applied. If all appears normal, you are, "good-to-go". Be sure to use the "tune-up" procedure.

Remember, the use of a variac to power-up an amplifier is no guarantee that a problem will not happen, but it can minimize the chances from this all occurring. Otherwise, just turn on the main power and "whatever happens is going to happen", regardless of whether you did it with a variac or not.

Q: "I just installed new tube(s) in my amp, and now it acts strangely and trips breaker or burns the fuse."

A: When installing new tubes, there is a possibility that the tube(s) will have a "flash-over" problems, this is due to residual gases inside the tube(s). "Burn-in" the tube(s) for several hours before applying RF drive to them. Read this article for more details. Another possible problem could be shorts within the tube, such as a grid-to-filament short, this will only happen in directly heated cathode tubes. With indirectly heated-oxide coated tubes the short could be from the heater to cathode. Once residual gas is removed as a cause of the flash-over, if the short still exists it could be one of these other causes.

*********************************************************************************

HF Tube Amplifier Tuning Instructions

Introduction

This guide is primarily written for those who are new to HF tube-type amps, or anyone who wishes to get the most out a Linear amplifier.

I have found that this procedure works well with all HF amps that I have used, regardless of the type tube used.

First, we must understand why a Linear amplifier must be tuned in this manner.

Plate impedance

The voltage used, relates to the "Plate impedance", each amp is designed to operate at a given plate impedance. The plate voltage can be reduced, or increased, but the plate current must be increased or decreased, to meet the plate impedance level. A person can think of a tube type RF amplifier much like a antenna tuner, matching the tubes plate impedance, to the output impedance which is usually 50 ohms on the output end. The relationship of plate voltage to plate current creates the plate impedance level, which can range anywhere from 500 to 5000 ohms or more, dependent on the voltages used and the plate current needed to reach full power output. Changes in plate voltage, can create plate impedance mismatches, which cause output waveform distortions, and loss of efficiency. Large swings in plate voltage, due to a poorly designed power supply, or a power supply that is operating beyond it's capacity, can also create this same type of distortion. In my judgment, a swing that exceeds 500 volts is enough to cause distortions in the output signal.

To tune an amp at a lower voltage, then increase the voltage to operate on, creates a mis-match in the plate impedance.

Problem: Many amps can't stand a steady carrier tune up in high voltage mode, due to limitations of the power supply design and components.

Solution: Use a reduced "duty cycle" method to tune the amp in high voltage mode.

Hence, the "pulsed" tune method, which has a duty cycle of about 50%. You can use a store bought "pulsar", "cricket", or even a CW (Morse code) keyer.

{For using a CW keyer (it is more commonly available), set the keyer to about 40-50wpm, then tune the amp with it.}

Step 1. Set the amp in the voltage mode, you intend to use on the air, low or high, it does not matter. For a person wanting to run lower power most of the time, use the lower voltage, it will be more efficient.

Step 2. Set the exciter to whatever level that increases the plate current by double above the idle "zero-signal" current level.

Step 3. Preset the amp, to the manufacturers suggested settings, if available. Set the exciter to CW mode, ready to start transmitting a "string of dits".

Step 4. Begin tuning the amp, by "dipping" the plate current (while transmitting), this resonates the plate circuit.

Step 5. While still transmitting into the amp, tune the load control, for "peak" RF out put. Un-key when this is done.

Step 6. Repeat the tune cycle by again doubling the plate current, or until the maximum plate current is reached.

Step 7. Increase the exciters output until maximum plate current is reached. (Note; care must be used, not to exceed the amps plate & grid current ratings, if you reach MAX, plate current before reaching MAX exciter RF drive level, that is it, you can't drive the amp any farther.) (ie. If your amp has a MAX. plate current rating of 900ma. & 225ma.grid current, do not exceed these ratings, if you do, reduce exciter drive level.)

Step 8. At this point leave all settings where they are, now tune the "load" control to "over-couple" the amp loading, refer to ("Grid current Rule # 1"). Heavy loading is known as "over-coupling". This is the final tuning step.

Step 9. Switch the exciter to the mode that you wish to operate on. SSB is usually used on the high voltage setting, low voltage, is used for all other modes, but SSB can be used there also.

Step 10. You can reduce the RF drive power, if you wish to run at lower than full output. Do not change the voltage, unless you retune at that voltage level.

Now that you have tuned up, the amplifier is operating at maximum efficiency----and linearity.

Tetrode tuning: A small variation for these amps is that most run a rather high level of idle plate current relative to a triode type amp (typically 20% of the full load current). So, rather than doubling the plate current, one should add drive signal until about 1mA. of grid current is observed, or max plate current is reached, then tune as one would a triode amp with the steps above.

ALC ! (If equipped----this is mainly for higher-end units and Amateur/Ham Radio gear. A sign of design quality in a unit)

Any ham operator that does not use an ALC feedback circuit to control the exciter RF output to the amp, is an idiot. It is there for a reason. Most amplifier manufacturers have installed an ALC output on their units. It does not matter if your exciter can put out enough RF to overdrive the amp or not. Drive level is only one item the ALC system controls. Should the amp be loaded into a high SWR, the ALC will reduce the drive level to a safer level. Every ham has had a "lapse" in memory, and left the RF drive level too high when using an amp, and this can cause unseen damage to the grids of the tube. This is especially true of delicate metal/ceramic tubes. The ALC system will act as a "safety net" should an error be made or a failure in the antenna system happen. It is inconceivable that someone would knowingly do without it.

Most ALC systems can be set in the following manner:

1. Load the amp in the normal manner to full output, with full RF drive needed to reach max RF output level.

2. Adjust the ALC threshold (usually a pot on the back of the amp), until the RF output is reduced slightly, then back up the adjustment slightly.

3. You're done. No further adjustment is needed. If the ALC system was designed correctly, the system will operate without re-adjustment on all bands.

Now you can rest easy, knowing that your amp is protected from overdrive, or other problems.

Grid current; Rule # 1, Less is better!

Once you have tuned the amp for max output, you can reduce the grid current, by increasing the loading. (Loading is increased by reducing the load capacitor capacitance, this couples the RF to the antenna where it should be.) This will reduce the grid current drastically, with a small drop in RF output power. (OK, you ask how much of a drop in RF output? The general rule is 10%, (i.e. 1000watts out, reduced by 100watts or more.) The drop in RF output, is well worth the lower grid current, which will greatly increase the life of the tube (especially metal/ceramic tubes). The drop in grid current should be somewhere in the area of 30-50%, (ie. full drive grid current of 200ma, should be reduced to 150-100ma.) This is for triode type class AB2 amps.

For a tetrode type amps running class AB1, NO grid current is the rule. In these amps any level of grid current indicates over drive or a mis-tune condition.

If you turn the load control the "wrong way" the RF output will "peak", and the grid current will remain high, but if you turn the load control the correct direction, the RF output will drop slowly, but the grid current will drop quickly. This can vary from one make of amp to another, to verify the correct tune direction, look inside the unit to see which direction "un-meshes" the load capacitor, that is the direction of "heavier loading", the "correct" direction.

Care must be taken not to "over-load" the amp, as this will cause as much problems as "under-loading" can. Do not exceed the levels stated above.

A good rule of thumb is; Tune for peak RF output, and lowest plate current, then load the amp slightly to lower the grid current slightly.

Peak RF output and minimum plate current should happen at the same tune point (if the amp is designed properly and has no instability problems).

Now that you have tuned up using this procedure, your amp will now operate cooler, and with a lower IMD level, for a "cleaner" output signal & your tubes will last longer.

The use of the CW keyer puts a lower stress on the tube, power supply, and all RF components. The speed of the pulses, gives a good meter indication, and makes it easy to tune.

Nearly all HF linear amplifiers can benefit from these methods. Even if your amp has only one voltage setting, you can use these procedures.

A suggestion: Make a "cheat-sheet", or chart, with the tuned up, settings at a given frequency. In this way, you can set the amp by number, and fore-go the tune-up procedure. Less tune-up time means longer tube life.

With some HF amps, the tune dials are marked, 0-100. I found that "setting by number", is fast, and works very well, with no noticeable drop in output power. A cheat sheet for high voltage, and low voltage are needed, as the settings WILL be different.

Caring for your RF power amp tubes.

Why does the plate current dip, at the same point the RF power output peaks?

This is called "energy transfer" due to plate/load "over-coupling". Imagine a round topped hill with a "dimple" in the very top of it. This depression at the top of the hill is where the plate current dip happens. If there were no antenna/load circuit, the plate current would peak, like a hill with no "dimple" at the top, but no output would take place since all the energy in the DC current returns as DC back to the B+ side of the PSU. Since all amps have an antenna/loading circuit, the RF energy flows to the load, and DC energy returns to the PSU. DC energy is converted inside the tube to RF energy, then couples to the load/antenna.

Neutralization?

This applies only to "grid-driven" type power amplifiers. Find out what type your amplifiers is to see if this applies. This circuit reduces the stage gain slightly, and cancels out any instabilities in the PA. Generally, this circuit is used in grid-driven tetrode amps, where the stage gain in grid-driven service can be extremely high and therefore can be unstable. The circuit damps any tendency for the PA to oscillate. The PA must be re-neutralized on each band it is used on, it IS a tuned circuit. Some older PA(s) were neutralized only on the band that was most likely to cause oscillations when used.

None of the currently available tetrode PA(s) are neutralized, since they all use cathode-driven, passive resistor input networks. This lowers the stage gain, and stabilizes the PA. It also means the exciter drives into a 50 ohm resistive load on all bands. This is not really the best way to do it, as it provides no harmonic suppression in the input network.

*********************************************************************************

Hope this answered some questions, helped avoid potential problems, and increased the tube amp's performance for you. For the very non-complex "basic" CB tube amplifier, the very first section at the top will provide maximum benifit and be easily understandable. I'd hoped that the 2nd & 3rd sections might help with troubleshooting, answering questions, and in providing even more detail if needed.


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Post by Green Hornet » Saturday, 27 December 2008, 18:20 PM

Some of it is way over my head, but nevertheless, it's food for thought. Thanks & Happy New Year !

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Post by 464budmann » Saturday, 27 December 2008, 22:33 PM

WHATEVER THERE PAYING YOU FOXHUNTER, I GARUUUNTEE YOU, IT'S NOT ENOUGH... IT'S NOT ENOUGH. THAT'S A LESSON I'LL NEVER FORGET, THANKS

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RE A NOBEL PEACE PRISE...

Post by Foxhunter » Monday, 29 December 2008, 23:43 PM

464budmann wrote:WHATEVER THERE PAYING YOU FOXHUNTER, I GARUUUNTEE YOU, IT'S NOT ENOUGH... IT'S NOT ENOUGH. THAT'S A LESSON I'LL NEVER FORGET, THANKS
Thanks 464budmann I appreciate it and you're welcome. I do it because in return I get free radios from our sponsors !

LOL joking. Just trying to provide a few pointers by furnishing this type of information (usually found scattered throughout the web) so it can be more easily accessible & readily available to members & visitors on our CBRT Forum.



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Re: How To Set and Tune A Tube Amplifier

Post by Double D » Tuesday, 30 June 2009, 1:09 AM

Good 101 Fox... :cheers:
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but to acquire wisdom, one must observe."



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Re: How To Set and Tune A Tube Amplifier

Post by 721HACKSAW » Tuesday, 30 June 2009, 6:31 AM

Great info Foxhunter, thank you. People on this Forum such as Foxhunter, Linx, and Circuit Breaker, are always providing very good info, thank you!

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Re: How To Set and Tune A Tube Amplifier

Post by 'Doc » Tuesday, 30 June 2009, 8:06 AM

Tuning a tube type amplifier sounds like quite a job, but it sounds a lot worse than it actually is once you get the routine down. The first time you do anything, it isn't very easy/simple. After a while, it's not something you worry about at all. Knitting is a hell of a lot harder!
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Re: How To Set and Tune A Tube Amplifier

Post by pitbull3825 » Wednesday, 16 December 2009, 19:33 PM

Wow this really helped me I'm picking up my 6 tube tomorrow. SWEET!!!!
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Re: How To Set and Tune A Tube Amplifier

Post by Tinman590 » Thursday, 17 December 2009, 5:51 AM

pitbull3825 wrote:Wow this really helped me I'm picking up my 6 tube tomorrow. SWEET!!!!

What kind did u get? pics?
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Re: How To Set and Tune A Tube Amplifier

Post by pitbull3825 » Thursday, 17 December 2009, 6:07 AM

Palomar. No pictures yet it will be here this morning.
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Re: How To Set and Tune A Tube Amplifier

Post by Sheriff Bart » Thursday, 17 December 2009, 6:48 AM

Wait a minute Foxy, I'm not letting you leave so soon. With many of us glass operators the only dip we can control is surrounded by a plate of chips. How about the Phantoms and (Nitro 300 owners) as examples of tubers that only have one meter and it measures output only we only have "tune" and "load". How about a nice read for them.
Ah well, back to the ranch dip...and chips :r&r:


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Re: How To Set and Tune A Tube Amplifier

Post by 'Doc » Thursday, 17 December 2009, 8:12 AM

... Ranch. Ranch? Eeuu, yuck!
Wish I had a dime for every gallon of that stuff I've made (work). One part buttermilk, 2 parts mayo, season to taste. At least a gallon per hour at the salad bar on a moderate day when the buffet is running. Try it on pizza, doesn't go well with cinnamon sticks, or anything else as far as I'm concerned! Although, I'm told it doew well as a topping for liver jello! I don't eat that either! Nasty!
- 'Doc

PS - Those 'parts' are measured in gallons, by the way. They give us a choice on how to mix the stuff. One of those 2 foot metal whisks, or by hand. If by hand, you have to shave your arms. I use the whisk. Tried to talk'em into using a paint mixing bit on a drill, but they said no. Some idiot would drop the drill in the bucket probably.

PPS - And just for grins... I have an AL-811 amplifier that there is no 'dip' with. Actually, it's so broad that it's about useless. Had to tune for max on a watt meter. Which 'slope' of that 'dip' do you wanna be on anyway?

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Re: How To Set and Tune A Tube Amplifier

Post by Sheriff Bart » Thursday, 17 December 2009, 8:19 AM

Big Time ROFLMAO at that ^^^^^


and thanks for the homemade recipe, I think I'll sub a little horseradish and use a little less mayo. and you're right, it does taste good on pizza.

well when I did the procedure with my Nitro 300 I started @ 12 o'clock but I just adjust both "tune" and "load" for max watts. I just want to know if after everything is maxxed do I back off either the "tune" or "load" by any percentage.


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Re: How To Set and Tune A Tube Amplifier

Post by pitbull3825 » Thursday, 17 December 2009, 8:44 AM

It has 6LF6 tubes in it. Does that sound right? How many watts can I put in to it safely?
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Re: How To Set and Tune A Tube Amplifier

Post by 'Doc » Thursday, 17 December 2009, 14:31 PM

Sheriff Bart,
Very basically, it's a tune for max then just a 'tad' more 'load', but NOT much, I mean really a little bit. Then, depending on the metering on the amplifier, watch the grid current and don't go over the max there, that's a sure way of buying tubes. So is 'bumping' the plate voltage over the recommended value. A couple or few volts isn't too bad, but if it's any great percentage more than 'normal', it can get to be too much real quick.
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Re: How To Set and Tune A Tube Amplifier

Post by warlock35po » Tuesday, 12 April 2011, 12:20 PM

Hello;Before you touch the inside of the BEAST unplug it take offcover take a peice of about 8 gauge insulated wire and while standing on dry surface preferably on a rubber mat, clip one end of wire to amp chasis and with one hand in your pocket short all the filter caps to ground,wait a minute and repeat.Don't worry the loud snap is only a reminder that its a heart stopper.This isDEADLY serious _ _ _ _.Do not trust bleed off resistors. Regards warlock35po

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Re: How To Set and Tune A Tube Amplifier

Post by 'Doc » Thursday, 12 May 2011, 7:31 AM

The best way of replacing tubes is to use what the thing was designed to use. If it's been modded, then you're on your own, or the judgement of the one who did that modifying.
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Re: How To Set and Tune A Tube Amplifier

Post by Ohio_359 » Thursday, 12 May 2011, 7:45 AM

The only thing I didn't see is that you need to final tune on a modulated signal. Tuning up on a dead carrier will only get you close.

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Re: Re: How To Set and Tune A Tube Amplifier

Post by 420Snowman » Saturday, 14 May 2011, 4:02 AM

pitbull3825 wrote:It has 6LF6 tubes in it. Does that sound right? How many watts can I put in to it safely?
[img]http://s3.postimage.org/lJmUJ.jpg[/img]
You really shouldn't have to drive it with more than 10 watts max in my experience. If it had driver tubes then you'd be talking like 2 watts. I drive my 4x4 tuber with 2 watt dead key and let her swing.
Maybe someone has more experience with the palomar, I had one and couldn't get it to work, everyone here helped me try though, and that's a fact!


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Re: How To Set and Tune A Tube Amplifier

Post by 420Snowman » Saturday, 14 May 2011, 4:03 AM

That palomar is like a straight 6 or 8 tube isn't it?



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