Interesting Peter,
> 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.
HF oscillations and instability will cause arcing, VHF oscillations
however is the topic here.
VHF oscillations will not arc bandswitches because they are
shunted to ground by the low reactance of the tuning cap. It is
impossible to develop enough VHF voltage across the tuning cap to
make it, or the bandswitch, arc.
So while I agree you might have had a HF or VLF oscillation that
made a component arc, the likelihood a VHF parasitic damaged
anything is very remote.
Of course it is possible if you used a tuning cap that had a very
high reactance at VHF.....but then your amp would also be a
harmonic nightmare.
To date, I have never seen a VHF parasitic cause any of the
problem Rich claims. Even when intentionally created.
73, Tom W8JI
w8ji@contesting.com
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