>
>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.
>
>...
There is undoubtedly no way to build a VHF suppressor that does not have
considerable loss at 29MHz. The hat trick is finding a resistor with c.
10nH that can dissipate what's needed.
cheers, Peter
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|