From: "Gerry" 
Subject: RE: [drakelist] R4B hum


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Modern electrolytic capacitors do have a certain amount of overrating =
but
there is no reliable way to determine how much. You would have to have =
the
spec sheets from the manufacturer to give you a better idea. Still I
wouldn't do it. You can never tell what line voltage will do over time. =
Then
there is temperature rise, another unknown. I would suspect your B+ may =
be
over the line at initial turn-on. After the tubes warm up it probably =
falls
to a lower and safer value. Manufacturers only say their capacitors will
perform up to a specified maximum voltage, after that, you're playing =
with
fire (literally). As for hum, I would first make sure the AF output tube =
is
good. When my R-4B developed a loud hum, I tried a new tube and that =
fixed
it.

=20

-----Original Message-----
From: owner-drakelist{at}www.zerobeat.net
[mailto:owner-drakelist{at}www.zerobeat.net] On Behalf Of Mark Lakomski
Sent: Saturday, October 25, 2003 1:46 PM
To: John_K7FD; drakelist{at}zerobeat.net
Subject: Re: [drakelist] R4B hum

=20

Hi John,

=20

I just restored an R4A that isn't humming yet but that cap gets

very uncomfortably warm (and is bulging) and am working on a=20

substitute.  There are a few other electrolytics but with a 100=20

and 200 uf section on the power supply I'd think it was most suspect.

=20

I've seen nothing new or old stock to match it.  Even if NOS caps

were available it's likely they'd also fail shortly.  I've "rolled my =
own"

in a Heathkit and starting on an AC-3 by cutting the base off with a

hobby razor saw and installing discretes.  It's pretty easy except=20

for this one.  It's a physically small can with 200/100/100/20 uf {at}

200v.  With discrete electolytics coming in 160v and 250v it would

be much safer to go with the 250v.  But there is no way they will fit.

2 100/160v in parallel and a 22uf/250v fit a similar can perfectly

side by side with room for 2 more 100/160v on a second deck.

But with B+ at 160v using 160v electolytics is a little uncomfortable

even though I'd like to think they were underrated. =20

=20

Tried to fit 100uf/250v in just the electrolytic paper covers alone

and they work out to be about 1/8" too short though they fit

side by side fine, but no shielding.

=20

Next attempt will be to try using a section of 1-1/2" I.D. aluminum=20

tubing that should slip right on the old base.  To make it "pretty"

I got to find a way to cap the end.  Wish I had a TIG welder...

Not as OEM looking it may be a PC board disk fitting the I.D.

and using the foil inside as a common ground.

=20

Anyone could comment on the comfort factor of using the 160v

caps in place of the 200v (on a 160v B+ line)? =20

=20

73,

- Mark K9MRK

(off to the hardware store, again...)

----- Original Message -----=20

From: John_K7FD  =20

To: drakelist{at}zerobeat.net=20

Sent: Saturday, October 25, 2003 1:53 AM

Subject: [drakelist] R4B hum

=20

Hmmm....no make that hummm...

=20

I just purchased a B line...unfortunately the R4B has a bad case of hum. =
Is
it the big Can that needs replaced or some other electrolytics? Is there =
any
source for these or am I dead in the water?

=20

Thanks

=20

John K7FD


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