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

[Carl]

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 at telus.net>
To: <amps at 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 at q.com>
> To: "Amps at contesting.com" <Amps at 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
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
>
> -----
> No virus found in this message.
> Checked by AVG - www.avg.com
> Version: 10.0.1432 / Virus Database: 3222/6238 - Release Date: 10/10/13
>