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Re: [Amps] Removing grid shorts

To: Doug Renwick <>
Subject: Re: [Amps] Removing grid shorts
From: Roman Ganovych <>
Date: Tue, 28 May 2019 13:45:31 +0300
List-post: <>
Hello Doug,

you should talk to OM Power people to get the most
comprehensive answer, based on their huge practical experience.
They gave up long ago, and swithched to chinese FU-728F type,
but still a large number of OM-  GU-78B amps are in
daily use all over the world.

It is a typical way of dying for this tube type,
at least half of those I've seen (many dozens) end up like that -
due to required high VHF gain, entire structure of GU-78B grid is very
close to cathode,
burning it with cap discharge rarely had stable positive effects,
it starts develope same fault after few hours or even emmidiately after.
You can do temporary fix with brute force by medium level hummer punches,
but you would need
good X-ray device to see in which spot to make those punches, otherwise
things may get even worse. Btwy high quality X-ray tubes are also currently
build end exported
on a big scale by Svetlana :) .
The easiest you can do is to reduce heater voltage, if you are at near 28V -
go down at least 10%, or more if desired effect is not reached yet.
Now imagine the innards of this tube if so relatively small heater power
does the trick (and all know that heater there is separate from cathode).
Yes, power output drops much further, but ~2kW from PA is still more then
100W from bare radio :).
Anyway, get ready to look for a good tube replacement next couple months.

If anyone has, or knows the sources with quantities of faulty soviet power
tetrodes like GU-78 type family (10+
of most popular types that existed), I can privately direct to sources
where they can replace them
for new, or used but good, same or other type, depending on choice, GU- GS-

Roman in Lviv, west Ukraine, at the ruins of Polyaron tube factory :(.

вт, 28 трав. 2019 о 04:09 Doug Renwick <> пише:

> I have a GU-78B that has developed a grid short. I have determined the grid
> short initiates about 2 minutes after power on and ends after about 5
> minutes.
> I have tried unsuccessfully to 'burn' the short by hooking up a 560µf
> capacitor fed with 300vdc.
> What other methods that can be tried to 'burn' this short?
> Doug
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