Anyone with a weird oscillation problem in a tube amp might benefit
from reading the stuff by Richard L Measures, AG6K. A couple of his
articles (in QST for Sept and Oct 1990) attracted a great deal of flak.
They were seriously trashed - also in QST - by a number of folk who
might be argued to have an interest in defending their own designs ..
The criticism was published in QST, but Rich Measures was apparently
refused space to rebut the allegations. Full details, including
correspondence with Eimac and ARRL can be found on his website, which
According to Rich Measures, vhf parasitic oscillation is a major and
very much overlooked factor in the self-destruction of expensive
power tubes. (I see it as a form of shock excitation, or "ringing",
because its not necessarily a continuous oscillation).
Very briefly, his solution is to greatly reduce the Q of the parasitic
choke in the plate circuit, by using a bit of relatively lossy wire,
like nichrome. (I used stainless steel piano wire). Its an inexpensive
fix - but please visit the web site.
The web site contains a wealth of info on all aspects of amplifier
design, including the ARRL Handbook chapter on amp design which Rich
Measures was commissioned to write .... (Someone must have thought he
knew his stuff !!)
If your amp is out of warranty and it trashes tubes, then you may be
able to cure it. Unfortunately you may need to buy new tubes before you
can test the fix .. Maybe there is a lot to be said for the good old
811-A after all ??? I had 'em glowing, but I never blew one.
Oh yes; if its a tube amplifier, then please pull the plug and ground
the plate supply before sticking your hands inside.
73 - John G3JAG
On 03-Oct-99 Web Williams wrote:
> I'm not sure if this would apply to this problem or not- but
> here's my story...
> I had an Icom IC-735 that acted the same way. After operating
> for a few moments, the SWR would go off-scale. External meter
> AND internal meter. Obviously, this condition would cause the
> power level to drop. A trip back to Icom, and when it came back,
> the power level was turned down to 50 watts from 100, but the
> problem still manifested itself. The dealer told me I was bonkers,
> that the problem had to be in the antenna. I mounted the rig into
> a mobile, took it to his shop, and began operating in the parking
> lot on 10m (at his suggestion, since he could not cause it to
> fail in his shop). After about 5 minutes, it begain showing the same
> high SWR condition. I invited the dealer out to examine the
> rig. He brought out a service monitor, which showed the rig
> transmitting all the way from 160 through 6 meters! Obviously,
> an antenna can only match ONE frequency! So- what he discovered
> was that the rig was going into oscillation somewhere in the final
> amp. It was replaced with a new rig after almost TWO years!
> (but NOT by the dealer, ICOM replaced it.). Sometimes it
> just doesn't pay to deal with a dealer. Sometimes you just HAVE
> to go to the source, which in my case, paid off. Sending the Titan
> amp to Ten Tec was a wise decision. They'll get it fixed properly,
> I bet. Oh, the reason the dealer could never cause the fault in
> his shop is that he only tested the rig on a dummy load, which
> will show a low SWR no matter WHAT frequency you transmit
> on. Had he bothered to put the unit on an antenna, it would have
> saved me TWO YEARS of not being able to use my radio
> properly! Lesson #1: If your dealer doesn't have an antenna, and
> won't actively use your rig to test it out- go ELSEWHERE!
> 73 all, -Web, KR4WM
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