Date: Wed, 3 Apr 2013 18:05:38 -0400
From: "Carl" <km1h@jeremy.mv.com>
To: "Bill Turner" <dezrat1242@yahoo.com>, "Amps" <amps@contesting.com>
Subject: Re: [Amps] original thread: Peter Dahl transformers, remark
about capacitors for switching supplies
Start with the 160M lowest reactance disc cap you can scare up at the base
of the plate choke. A .0047 is adequate for a sane power level and a good RF
choke.
Then add a healthy glitch resistor and bypass the other end also.The
resistor has inductance and is self resonant somewhere in the 50-150 MHz
range for the 20-25W variety. For the 3-500 and similar I also use a 10 Ohm
20W in series and .001 bypassed as that is good for the parasitic frequency.
Some amps have an unstable layout and need all the help they can get; in a
SB-220 RFC-2 is replaced by that resistor.
Any further potential damage to diodes and filter caps comes from
switching spikes and each complete string should be bypased with an adequate
.005/.0047 disc and another to ground at the HV output point.
Some add a 1N5408 reverse connected across each cap as a crowbar and a very
fast fault trip.
Carl
KM1H
## I used 2-3 x .0047’s at the base of the plate choke. Then also 2-3 x
500 pf ceramic doorknobs, like HEC variety..also at base of plate choke.
The doorknobs make for a better bypass at the higher HF freqs, while the
.0047s work better at the lower freqs.
## I use 50 ohm, 50 watt wire wound for the glitch resistor, then another
bypass
cap to grnd right at the B+ output. sometimes I will use a globar for the
glitch R. Or paralleled 100 ohm glitch R’s..... or even 4 x 200 ohm
wirewounds or globars in parallel. Depends on the application.
## I have some .0047s in both 10 kv and also 15 kv. I’ll try putting
one across each leg of the FWB, so 4 x caps in total. That’s actually
a good way to combat switching spikes.
## Installing a .01 disc ceramic cap across each lytic in a HV supply really
is
pointless . With 8 x lytics in series, the 8 x .01 uf disc ceramic caps are
also in
series. Your end to end total C is just .01 uf / 8 = .00125 uf
## Putting a reverse connected 1N5408 or 6A10 across each lytic is a good
safety measure. IF one leg of the FWB ever shorted out, you would be pumping
raw AC into the lytics on half cycles. The safety diodes across each lytic in
that
case would simply conduct, and blow the AC HV fuse. I install a HV fuse
between
sec of xfmr and FWB. The HV fuse can go in either leg of the xfmr sec. Sized
correctly,
that works a lot faster than a 240 vac breaker.
## I noticed on the Dahl FWB boards, he uses a MOV across each of the 6A10
diodes.
I like carl’s method of a .0047 uf across the entire leg.
later... Jim VE7RF
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