Peter:
Darn good technical answer.
73
Bob K4TAX
Peter Chadwick wrote:
>
> 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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