Tom says:
>.....the claims tank components arc from VHF parasitics on frequencies
>far removed from the tank resonance is wrong.
With respect, Tom, I think you're falling into the same trap Rich does -
generalising from insufficient evidence.
Consider the case where there is a resonance of the the tube output capacity in
series with the pi tank input capacity, which series capacitor combination is in
shunt with the inductance of the lead from the plate via the blocking capacitor
(assumed to have a small amount of series inductance at VHF). The tuning C and
lead inductance are series resonant below this frequency; you can visualise it
like a pi network if you like. But you remember the old days of 6146s and 5763s
on 2 m, where you 'series tuned' the plate circuit? This put the tuning C in
series with the tube ouput C. Now if the tube output C is higher than the tuning
C, then we can get step up to the plate tuning C at the VHF frequency. The step
up will be in the ratio of the capacities.
Now IF (and it's admittedly a very big IF) the tube internals and the physical
layout and the grounding of the grid (or screen - same will apply to grounded
cathode tetrodes) is such that the feedback is in the right phase at the
resonant frequency, then you can get an oscillation which can potentially (no
pun intended) make the tuning capacitor arc over.
Having got an amplifier which has been working fine for a lengthy period, the
only reason I can see for this happening is that the parasitic suppressors have
got too hot because of under-rated resistors in a hot environment having gone
high if not open circuit. Under those circumstances, it's probable that nichrome
resistors would be a cure. Properly rated resistors may be cure that loses less
power - depending on layout etc.
Now, I figure there's probably a marginally (but not much) higher chance of that
happening than my XYL winning the first prize in the lottery - but I don't
totally discount the possibility.
73
Peter G3RZP
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