A single 100K 3W MOX is what I use on refurbs and my own PA 3 holer.
There is also a delay tantalum on the board as well as a 3.3K 2W ( iffn I
remember) that always goes way high on the amps I see.
Carl
KM1H
----- Original Message -----
From: "Jim Barber" <audioguy@q.com>
To: <Amps@contesting.com>
Sent: Wednesday, October 09, 2013 8:57 PM
Subject: [Amps] Alpha PA-76A bleeder/equalizer resistors
I'm working on a PA-76A, doing the obvious stuff - replacing the filter
caps, the tantulum timing cap, the low-voltage filter etc. All well and
good.
What raised my question is the bleeder/equalizer resistors across the HV
filter caps. Apparently stock, each cap has two 120K 2-watt carbon comp
resistors across it in parallel. With 2400V B+, that looks like 60K ohms
across 400V, or ~ 2.7 watts per 4-watt pair. All the resistors are either
open or close to it, interestingly enough.
First, I looked at a few posted images on the internet. Yep, it looks like
that's what Alpha used, although the schematic I have says each "unit"
should have 220K @ 2 watts across it...
So: Inquiring minds would like to know what to use to replace them?
I'm thinking maybe 2 220K Ohmite OY (in parallel of course) across
each? That would lower the bleeder current, but I'm not seeing a huge
problem with that at first glance. ?
For whatever its worth, the original problem with the amp was that there
was substantial 120hz hum modulation on the output signal.
Since this is an original two-holer, I'm hoping the problem is strictly in
the B+ supply since a cathode-filament short in one of the 3CX400's would
mean either a retrofit or something equally undesirable...
Tnx es 73,
Jim N7CXI
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