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[Amps] Alpha PA-76A bleeder/equalizer resistors

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Subject: [Amps] Alpha PA-76A bleeder/equalizer resistors
From: "Jim Thomson" <jim.thom@telus.net>
Date: Wed, 9 Oct 2013 23:29:01 -0700
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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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