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Power Loss when hot

Lazybones1222

W9WDX Amateur Radio Club Member
Apr 6, 2005
948
29
28
Tampa Bay Florida
I run an old Varmint 250XL. It is a three tuber, one 6kv6a driving two 6lr6 tubes.

I have noticed the pep power output decreases about 70 to 90 watts if I bucket mouth for a long time DXing. It gets pretty hot. If I let it cool down the watts come back.

Is this normal? Or is it an indication of something no so good?

ps: I already have an extra fan added.


Varmint_close_up_small.jpg
 

As your tubes heat up, the internal "geometry" changes so, that would cause the tuning to change.

If, when the power changes, you re-tune, you will see your normal output however, when the tubes cooled, they would have to be tuned again.
 
QRN said:
I had a homebrew two tuber that would do that when I drove the pi$$ out of it. :shock: I think it was the tubes surrendering to the abuse. :LOL:

Hmmmm.....the operating instructions from Brewer Labs say it is designed for a 4 watt deadkey, but no more than 6 watts. I hit it with a newer 2950 DX with a 4 watt deadkey......and around 25 watts pep.

Think that could be overdoing it?
 
I had a amp do that once and it ended up being the filtering caps needed to be replaced.... Replaced them and it went back to working properly... Not to say this is your problem.. you need to get it checked out by a qualified tech...
 
Lazybones1222 said:
Hmmmm.....the operating instructions from Brewer Labs say it is designed for a 4 watt deadkey, but no more than 6 watts. I hit it with a newer 2950 DX with a 4 watt deadkey......and around 25 watts pep.

Think that could be overdoing it?

K.I.S.S.

Keep It Simple Stupid

Follow me here........

Tune tubes cold get "X" output, tubes heat up, output changes, tubes cool down, output returns to "X". The internal dimentions of the tube changes with heat and therefore the amout of capacitance and inductance needed to resonate the circuit will change.

Yes, the filter caps could be old and the B+ is dropping which would reflect in lower watts but, that is an difficult and expensive diagnosis to make; try the simple test of touching ou the "Tune" control after the "bucket-mouthing", the watts will be there.

BTW, when that amp was built there were no Hi-Powered radios, that's what that first tube is for, to provide drive to the final stage; if the instructions say don't put over "X" watts in, then expect shorter tube life however, tubes don't fail like yo-yo's, When the watts are gone they are gone; they don't sneak back in when the amp cools off.

.
 
The couple picofarads that would result from tube element movement is a drop compared to what it takes to resonate the tank circuit.You may be right HOWEVER I did see what I saw on my old amp and yes I did retune while the thing was cooking and got no more power out of it.The tubes were screaming.Ever see a pair of 6KD6's with the plates the same color as the filaments? :shock: ;)
 
QRN said:
That is true Justin.It was shortly before the output went to almost nothing.Could never figure out why. :LOL:

Now, I'm not telling you guys anything you don't already know, but,.....

You never want to see the plates of a sweep tube turn red or even orange.

I set them up with negative volts on the control grid and a few (+15 vdc ) on the screen and I make sure that the supressor (third grid) is grounded.

The only color you saw out of my sweep tube was a blue glow (almost the same color as the Varmit covers) and the lasted forever.

The only glass tube you want to see turn red are 3-500Z's with Graphite plates.

.

.
 
Yeah but that was the only way I could be absolutly sure that I was coming over the neighbors TV set when I wanted to make my feelings known about him. :twisted: A Colt 1000 with the mod limiter cut and a Turner +3B feeding a pair of 6KD6's into the Wilson Shooting Star.When I asked "Can you hear me now?" I meant over the TV. :twisted:
 
Hey guys - I did did some more checking/talking on the Varmint and discovered the following:

My first observation of power loss being heat related is wrong.

Here is the more correct information.

1. Retuning did not correct the problem

2. If I say a long ahhhhhhhhh into the microphone, the output peaks at around 450 watts and then, slowly at first and then more rapidly drops off to 300 to 350 watts. This happens regardless of how long the amp has been on.

As was suggested before, could it be the capacitors? There are three very large capacitors in the rear of the Varmint. Could they need replaced.

ed
 
The one way to confirm if it's the filters is to read the high voltage while keyed, and see if it falls off in step with the output power falling off. This is trickier than it sounds. The RF from the tubes will cause many DC voltmeters to go psycho until you unkey. A 'scope is good, but most probes aren't rated for 900 Volts DC. You have to use the probe's "Times 10" setting to see anything this way.

Old filters will usually look bad if they're getting ready to fail. If yours are original, you should replace them just to prevent a "KABOOM" later on. If they show cracks, swelling, or a "zit" on the rubber end insulator, it's past time to replace. In a Varmint, the bleeder resistors should get changed at the same time. If one of them goes bad, it will clobber a filter. They mainly serve to split the High Voltage evenly three ways across the filters. If one of the bleeders goes bad, one filter gets more voltage than what it's rated placed across it. That filter will short out inside. Tends to clobber one or both of the other two when that happens. Cheap insurance, those bleeder resistors.

My money is on one or more tubes being the culprit. If you have access to a tube tester, you will probably observe the tester's meter reading fall off while the knob for the "emission" or "quality" test is held to that position.

There are other possible causes, but most of them will cause the "peak" position of either "Plate" knob to shift. If 'rocking' either plate knob does NOT return the lost power, you're probably looking at tubes. If this brings back ALL the power you lost, tubes aren't the culprit.

It's only necessary to overdrive tubes like that once or twice to make them behave this way. Once damaged, there's no way to "fix" this failure.

73
 
Do you remember the old drug store tube testers??
Nomadradio??

I have an old U-TEST-EM tube tester...id like to sell....
its about 6' tall...with every pinout socket for every sweep
tube socket there is...

How much to you think its worth in 9 of 10 condition??

AM POWER
 

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