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

[Carl]

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 at q.com>
To: <Amps at 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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