>
>
>-----Original Message-----
>From: Tom Rauch <w8ji@contesting.com>
>To: amps <amps@contesting.com>; Steve Thompson <rfamps@ic24.net>
To: <amps@contesting.com>
>Date: 07 September 2000 22:11
>Subject: Re: [AMPS] Blown TL922A... What to do?
>
>
>>
>>> characteristic remains the same. There is a single series resonance in
>the
>>> low/mid vhf region, and the impedance stays low either side of it. When I
>>> next get the bench clear (might be a while), I'll run some plots. Please
>>> say if there are any particular configurations you (or anyone else) would
>>> like me to try and simulate to measure.
>>>
>>> Steve
>>
>>Hi Steve,
>>
>>What I found useful and educational is the measure the
>>impedances along the path from anode to the switch contacts, and
>>calculate the amount of current the tube must sink in order to
>>develop the voltages necessary to cause the switch to arc at
>>say...100 to 200 MHz.
>
>>
>>The 3-500Z, in the fiberglass socket with groundplane on it, has
>>minimum S21 (or S12) loss at about 180-200MHz, indicating the
>>grid is "ungrounded" at that frequency. This would be near the
>>frequency where the tube oscillates in a parasitic mode.
>
>
>Has anyone been able to measure the frequency of oscillation in, for
>example, TL922?
The only case I have knowledge of involved loosely coupling a spectrum
analyzer to the anodes of a seemingly stable* SB-220. Whenever
anode-current changed, damped wave ringing at c. 110MHz was observed.
/*note: no bandswitch or Tune-C arcing took place during the test/
>What dip frequencies are found with a GDO in a 922?
Measuring at the dc blocking C, typically c. 120MHz. There is a slight
variation depending on the setting of C-tune.
>In other amps?
>
Henry 2k-4s and B and W PT-2500s reportedly dip at roughly 95MHz.
>>
>.....
>>
>>It should be very simple, with the right test equipment, to prove the
>>possibility of switch contact failures from VHF parasitics.
>>
>>I maintain contact failures are caused by problems at or near the
>>resonant frequency of the tank, where driving impedance matches
>>or nearly matches the tube's optimum operating loadline. It is easy
>>to get several times the operating voltage of the tube in the tank if
>>the tank is improperly loaded.
>>
>>Do you agree or disagree with the above statement?
>
>Without hands on experience of real amplifiers, I'm treading very carefully.
>I chipped in because I'm lucky enough to have the gear to measure impedance
>'properly', and those who haven't might not realise what happens in hf sized
>layouts as the frequency goes up. I was also hoping to gain an insight into
>how Rich visualises these things, as he and I use language that's
>sufficiently different to cause problems in discussion.
>
>For sure, if you have a low impedance at a given point and frequency, it is
>difficult to generate high voltage there, and if you mistune the output
>network you can get a high impedance and lots of volts at the tune C. That's
>simple fact.
The simple fact is that when one mistunes, the tank goes out of resonance
and peak V decreases. On my SB-220, with a full drive mistune test, it
is not possible to arc the Tune-C. (the SB-220 Tune-C breakdown
potential measures c. 3700v on a high-pot.)
>
later, Steve
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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