Jim, the tolerance is not that critical since there is plenty of voltage
overhead and the original 5% carbons worked for about 35 years.
Standard 5% 3M MOX from Mouser or elsewhere is fine for another 35 years.
All the ones Ive checked are within 2% anyway and modern replacement caps
are listed at 20% but come in around 10% or less at the values used in most
amps.
Carl
KM1H
----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Thursday, October 10, 2013 2:29 AM
Subject: [Amps] Alpha PA-76A bleeder/equalizer resistors
Date: Wed, 09 Oct 2013 17:57:23 -0700
From: Jim Barber <audioguy@q.com>
To: "Amps@contesting.com" <Amps@contesting.com>
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
## forget the carbon comp crap..and ditto with a pair of 220k Ohmite OY.
## use a single 100k, 3 watt MOF resistor across each lytic.
## VISHAY 1% TOL.
## PN- 3 100K 1% T – 2 B14
SAP PN: CPF3100K00FHB14
## Stock item from MOUSER. Mine came in boxes of 100. I measured all
300 of em from 3 x boxes...and there is only 20 ohms at most between any
of them.
That equates to .2 % Tolerance. Great, no more mucking about trying to
play matchup.
The late Tony W4ZT put me onto these beauties. The actual value is not
critical. Anything
from 90-110 k is fine. What is critical is that they are all the same,
or very close in value.
With these Vishay resistors, the V drop across each lytic is identical !
later......... Jim VE7RF
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