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Tube Driver Amplifer Tuning

Cajun Invader

Cajun Invader

Membership Moderator
Oct 13, 2008
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I have a Varmint 250XL that I am going to use as a driver for a 4 tube 572B.

The 572B amplifier only needs 100 watts to excite it to full power.

The Varmint has a Hi and Lo power switch. On hi it does 250 watts and on low it does 70 watts.

I can tune and load the amp on high to only do 100 watts but I'm not sure if that is recommended. Should a tube amp always be tuned and loaded to it's maximum power to operate properly? Will de-tuning it hurt the tubes or other components?

I need an online elmer.
 

The wattages you are listing.........are they dead key or swing? Don't remember if the 250 has SuperMod or not. If you have a radio with a variable dead key you can drop it like that. The Clipperton should be able to take a key of 45-80 and swing to upwards of 300, peak watts that is. 100 watt dead key will be too much IMO. You will get better results and longer tube life by dropping the key. Letting the Clipperton key around 300 or so and swinging it should run cool and last.

BC
 
I can tune and load the amp on high to only do 100 watts but I'm not sure if that is recommended. Should a tube amp always be tuned and loaded to it's maximum power to operate properly? Will de-tuning it hurt the tubes or other components"
Click to expand...


Don't do that. You WILL hurt it. If you want less power output from the amp, you need to drop the input drive lower. See QRNs tutorial on tube amplifier tuning:

http://www.worldwidedx.com/amplifiers/26225-how-tune-tube-amp.html
 
I was talking PEP.

On high I can adjust the carrier with the radio between 40-200 watts. It will swing to 250-275 from any carrier setting.

The amp does have the supermod setting. I can set the carrier from 0-70 and it will swing to 250-270.

For some reason I was thinking 100 pep for the input.
 
1. Try thinking in terms of Average power instead of Pep, which doesn't really do anything meaningful for tuning an amplifier.
2. Take Mole's advice and read QRN's post.
3. De-tuning a tube amplifier is bad news, so don't. Unless you like replacing tubes or fixing things.

I really suggest finding out more about these things before jumping in to it. One of the best things you can do is forget all the 'CB' lingo/buzz words and do it right. That's not a 'put down', just very good advice. It isn't easy to forget/change the way you think of things so count on some work when doing it. It really does pay off though.
- 'Doc
 
W5LZ said:
I really suggest finding out more about these things before jumping in to it. - 'Doc
Click to expand...

That's what I'm trying to do Doc.

I read the post about tuning an amplifier then I found some information on cbtricks about the Varmint 250XL. The tuning instructions say to detune for AM operation.

The instructions also say the Varmint is rated in RMS watts. Now I'm confused. The power rating is 250. I'm only seeing 125 average watts on a Palstar WM150. Should I be seeing 250 RMS? I really doubt it.

From what I understand so far about watts, RMS can not be used to measure variables only constants. So average watts would be accurate. You are correct, it's hard to forget the lingo and buzz words. I'm sure the only reason this amplifier says 250 on it is because it's marketed towards CBers.

Here are the instructions from cbtricks. http://cbtricks.com/Amp/brewer_labs/graphics/xl150-xl450_tune.jpg
 
Tune the Varmint (with the Dentron inline and not keyed) like you normally would. Get your peak swing and then drop the dead key to around 50 watts with your radio. Probably it will work out best with high power/SuperMod. Key the Dentron and Varmint and adjust the Dentron for max dead key. You have the amp pretuned then. Now key both and say audio and touch up the Dentron for max swing. You are there now. 80 watts max on dead key into the amp.....50 or so will be better. I've ran a Clipperton like that for years and never a problem. The higher the key..........the shorter the tube life. Watch the tubes. If you are running them too hard they will change colors. Dull red you can get by with for a short time, anything else, bright red, orange, yellow, or white (OUCH) is not good. A slight change in the tube plate color is acceptable, just enough to tell it though.

BC
 
Always remember that you should NEVER detune an amp or reduce loading in order to lower the power output from it. Normally you should tune and load the amp for maximum output and then reduce the drivel level to obtain the power output that you want to achieve. This will ensure maximum stability and greater IMD performance from it.Tune the Varmint for max on high and reduce the drive from the radio to obtain around 100 watts and then drive that into the Clipperton.If the Clipperton is being driven too hard, further reduce drive level from the radio until proper operation is achieved.
 
QRN said:
Always remember that you should NEVER detune an amp or reduce loading in order to lower the power output from it. Normally you should tune and load the amp for maximum output and then reduce the drivel level to obtain the power output that you want to achieve. This will ensure maximum stability and greater IMD performance from it.Tune the Varmint for max on high and reduce the drive from the radio to obtain around 100 watts and then drive that into the Clipperton.If the Clipperton is being driven too hard, further reduce drive level from the radio until proper operation is achieved.
Click to expand...
That's exactly what the manufactures instructions say.
 
mackmobile43 said:
That's exactly what the manufactures instructions say.
Click to expand...

Imagine that! (y) Seriously though, they should know how to tune their own amps. Detuning is very bad as it increases the plate dissapation as well as increases the plate current and causes the amp to be run on a freq it is not tuned for. (Duhh, I did say detuned didn't I?) Reducing the loading may cause the amp to go into oscillations and that is very bad and quite destructive for the tubes amd possibly the power supply components as well.
 
Ideally 572B's should not show any colour at all when operated within specs especially if they have a graphite plate. In heavy service like AM or FM or one of the digital modes it is acceptable to see a small amount of deep cherry red in the middle of the plates but do NOT run them until they are bright orange.That colour is for the filaments only. :D
 
I see in BC's post to say audio into the mic while tuning, it says nothing about that in the instructions for tuning the Varmit amplifier and I have not read the instructions for the Dentron so I could'nt say if they tell you to do that but as far as I read the Varmit instructions all it tells you to do is peak the plate voltage, peak the load and then peak the driver, rinse and repeat, nothing about injecting an audio signal into the input while doing any of this.

By doing this are'nt you basically detuning the amp? at a set drive input the tubes should only produce so much output power and by introducing an audio signal while tuning the amplifier is not called for?
 
mackmobile43 said:
I see in BC's post to say audio into the mic while tuning, it says nothing about that in the instructions for tuning the Varmit amplifier and I have not read the instructions for the Dentron so I could'nt say if they tell you to do that but as far as I read the Varmit instructions all it tells you to do is peak the plate voltage, peak the load and then peak the driver, rinse and repeat, nothing about injecting an audio signal into the input while doing any of this.

By doing this are'nt you basically detuning the amp? at a set drive input the tubes should only produce so much output power and by introducing an audio signal while tuning the amplifier is not called for?
Click to expand...



the PROPER method is to inject a dual tone while tuning to prevent IMD.

i really don't think the manufactures of the Varmit amp care about "splatter" or IMD . they just want you to tune for max smoke.

if you really want to get up to speed on tubes, go here and read these files. i think these are in the fifth reprinting now. the answer to your question starts on about page 2 of ref c&f4.
get a cup of coffee (or two) :blink:and take a good set of notes too. lots of good reference material here:

the links to Care and Feeding of
Power Grid Tubes, which was revised in 2003:

http://www.cpii.com/docs/related/22/C&F1Web.pdf

http://www.cpii.com/docs/related/22/C&F2Web.pdf

http://www.cpii.com/docs/related/22/C&F3Web.pdf

http://www.cpii.com/docs/related/22/C&F4Web.pdf

http://www.cpii.com/docs/related/22/C&F5Web.pdf

http://www.cpii.com/docs/related/22/C&F6Web.pdf
 
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