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[AMPS] Capacitor Identification

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Subject: [AMPS] Capacitor Identification
From: (John Lyles)
Date: Tue, 31 Mar 1998 10:08:19 -0700

>Hi John,  Thanks for the info.  Do you think these will be ok for bypass
>capacitors at the base of my plate choke then?  They would be subject to
>6000 volts, and would be grounding any stray 1.8 through 30Mhz rf leaking
>past the choke.

We had a bunch of the 0.02 uF 20 KV made exactly the same by Murata. They
were mounted in
series/parallel, across the center tap of the push pull output transformer
for a 2-6 MHz amplifier, plate voltage was 15 KV DC. So there were four
capacitors in the network, two in series to get 0.01 uF and then another
string in parallel to that. The total capacitance was then 0.02 rated for
40 KV DC.
I measured the total network with the vector Z meter, and got series
resonance at 7 MHz, where the Z was 1.5 Ohms, going from capacitive to
inductive. I'm sorry to report that I did not sweep out to 30 MHz.

I would suspect that a single 4700 pF version of the same would have a
series resonance somewhere out in the HF band, but don't know where.
Perhaps you might take one and try a GDO on it. Or feed a small RF voltage
through a series 50 Ohm then the cap to ground, and put a wideband scope
across it, and sweep the frequency, while seeing how it attenuates. You
should see if it becomes lousey, on the inductive side of series resonance.
It could be bad at 10 meters. Or it could be fine. At a minimum I recommend
you test it first. Besides the insertion impedance being high way above
series resonance, the general capacitor construction should be OK for
bypassing on the DC side of choke, if the choke is adequate to prevent more
than a few hundred volts of RF at the cold end. If you know the impedance
of the cap and the choke, then it is all a simple RF voltage division
calculation done for all the operating bands you wish to use.

Otherwise you might do better to stick a handful of good ceramic doorknobs,
the bigger Centralab types, that are rated at 1000 or 2200 pF each for your
plate bypass. They are better from the standpoint if internal lead
inductance than the Murata DC types, also the cermic is lower loss (hence
you can only get them in smaller pF values, not 0.01 or higher).

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