Thanks, Carl.
I might as well replace all the carbon-comp resistors on the
control board while I'm at it. The tubes don't show any element shorts
with an ohmmeter and hi-pot fine, so I'm going to go on the assumption
that the hum modulation is coming from the B+ supply.
73,
Jim N7CXI
On 10/9/2013 6:17 PM, Carl wrote:
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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