At 12:40 1999-06-05 -0400, Tom Rauch wrote:
>snip
>While I can't say for certain an amp could never oscillate and
>cause arcing, it is probably one of the least likely causes of
>failures. I've never been able to get one to oscillate and arc,
>however, even when I intentionally introduced VHF oscillations.
>snip
I recently put a new diode rectifier board for the screen voltage supply in
my HF amp. I put it in the 'input' compartment of the amplifier and was a
little careless not to put the board in the screened box as I wanted to get
more room there and get rid of the box. The amp immediately started arcing
the bandswitch, BIG time. Most arcing was done between the 15 and 10 meter
taps. It has never done this before so I was a bit puzzled. I could not
tame the thing on any band, arcing was bad no matter what.
Measured voltages, looked all right. Put the box back, screended the diode
board and .. Voila, no arcing in the bandswitch. Voltages exactly the same
as before.
Worked the WPX CW contest without a problem.
The only thing I can say is that I'm certain that oscillation made the
thing arc the bandswitch. And at low power too, on 15 and 10 it was arcing
even when running 10% of normal power. The bandswitch is not a 'Mickey
Mouse' job, it is designed for high power I might add. The anode meter did
not flicker and wander off on it's own during these events, it behaved like
normal. I have seen amplifiers go into oscillation and put the
anode/screen/grid meters at full scale etc, but that was not the case this
time.
Can clearly see how an oscillation of 2 amps @ 5.5kV could weld a
bandswitch.. not so.
So, if this is proof of Rich's theories regarding oscillation/arcing or not
I can't say, but there is NO doubt the arcing was caused by oscillations in
the amplifier, so 'occasional' oscillations should be able to make any amp
arc the bandswitch. Now, it is a different story HOW and IF they can occur
but every amp has it's own problems.
For the purists, the tube in my amp is a QBL 5/3500, capable of plenty of
'goo' well into 200 MHz, use it also in my 144 MHz EME amplifier.
FWIW
Peter/SM2CEW
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