Larry, your descriptions sounds like a classic example of self oscillation.
The usual fix for that is adjustment of the grid/plate neutralization circuit.
There is plenty of info about this in any ARRL handbook printed in the last
50 or 60 years.
73,
Gerald K5GW
In a message dated 3/30/2008 9:15:31 P.M. Central Daylight Time,
n1miw@cox.net writes:
Hello guys, trying to get one of these amps back on the air, and I am
having a little bit of difficulty. I hope this is an easy question for some
of you, and would appreciate the knowledge feedback! Here's the history of
my amp... purchased on ebay with the hopes of learning about tube amps from
restoring it, but the electrical problems were more elaborate than I was
told (said it needed the HV transformer, and would be including the spare
with the amp). I found the diode stack blown, the capacitors were moldy, so
I knew they needed to be replaced. The HV xformer is fine, so I have a spare
now. Also needed to replace the tune resistor since it was broken (4k 100
watt ceramic). The screen supply xformer is reading 140V instead of ~35V, so
I installed a small xformer for that circuit. Now, she powers up fine with
no sparks or blown fuses, so I think I'm getting somewhere with it. My
problem is this... when I turn the HV on, I see an output on the wattmeter -
about 80 watts or so, with my plate current at ~70ma (70ma is spec for the
4x150a). I am NOT transmitting into the amp when this happens! If I provide
a 5-10 watt input at this point, nothing happens on the bird. The dummy load
IS heating up also. Now, if I try to tune the amp with an RF input present,
I get the bird to INCREASE in what I'll call "stray RF", and the meter will
also DECREASE more and more as I tune the amp. As an example, if the bird
reads 50 watts - with 10 watts input, no change. Tuning the amp more, if the
bird reads 150 watts and I apply an input of 10 watts, the needle drops to
130 watts. With 200 watts with no input, after I apply the 10 watt input,
the meter drops to ~75 watts, but at this point, my plate current is WAY
over 350ma, and the fuse blows. It almost seems like something is backwards,
but I don't know what. Where is this RF coming from if I'm not giving the
amp an input? Is there a voltage somewhere that's too high, which is causing
the "stray RF"? I've also heard a carrier on my receiver which "comes &
goes" while I'm tuning this thing. Again, you experienced guys might have
fixed something like this before, but I'm too new to understand what's
happening. Any help will be appreciated. Oh, I did try 3 different tubes -
all with the same result. They are known to be working from my FAA
amplifier. Thanks... Larry
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