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[AMPS] Blown TL922A... What to do?

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Subject: [AMPS] Blown TL922A... What to do?
From: RMcGraw@InfoAve.Net (Bob & Linda McGraw K4TAX)
Date: Sun, 10 Sep 2000 20:33:16 -0700
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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