Welcome back, Tom. I missed you.
>> >Parasitics, even assuming they would occur, can't cause tube failures
unless
>> >you see some solid evidence of excessive current on meters.
>> >
>> A jump to 250mA of ZSAC is not a clue?
>
>Rich, maybe you can put aside all the innuendo and diversion and explain how
>a parasitic can mechanically displace a filament, while thermal cycling or
>material problems are not likely to do so.
Both apparently can. Eimac cautions against having more than c, 29A-rms
of filament inrush current - presumably to avoid distorting the filament
helices. I have autopsied a number of grid-fil shorted 3-500Zs that were
funtioning normally before their grid choke imploded, and/or the grid-I
meter and/or shunt exoloded, and/or the vhf parasitic suppressor resistor
more than doubled in resistance without showing external signs of
heating. (tubes removed from amplifiers that had c. 60% of 29A of
inrush).
- A friend took his SB-220 to work, coupled the anodes to a spectrum
analyzer, and found there was damped-wave ringing at c. 110MHz at the
anodes when sending 50wpm dits, even though grid current was normal -
thus, no vhf oscillation was present. In other words, if spark
transmitters produced RF by pulsing DC current through a resonant
circuit, it should be no surprise to find vhf ringing in the anode's
vhf-resonant circuit. This phenomenon would be no problem unless there
was a feedback path between the SB-220's anode output and cathode input.
Unfortunately, there is 0.3pF of feedback C. At 110MHz, 0.3pf = c.
4800-ohms of XC. This doesn't seem like much until one discovers that
the length of RG-58/U coax used at the cathode input of the SB-220 is
resonant c. 110MHz. Thus, the SB-220's cathode-input receives no help
from the pi-tuned input in attenuating the 110MHz damped wave signal
being fed back from the anodes. -- Eimac states that the frequency for
maximum ratings of the 3-500Z for "Amplifier and Oscillator Service" is
110MHz.
-- As you may recall, during our last telephone conversation, you told
me you had seen many SB-220s that had sustained what appeared to be
parasitic oscillation damage.
>
>It would be interesting to follow through step-by-step with your logic.
>
>Exactly where does the bending force come from Rich?
Electro-magnetism, Tom. Get thee to a welding shop and observe arc
welder cables when an arc is struck.
have phun
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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