RE: [drakelist] R4B hum

[Gerry]

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.   -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Mark Lakomski Sent: Saturday, October 25, 2003 1:46 PM To: John_K7FD; [EMAIL PROTECTED] Subject: Re: [drakelist] R4B hum   Hi John,   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 substitute.  There are a few other electrolytics but with a 100 and 200 uf section on the power supply I'd think it was most suspect.   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 for this one.  It's a physically small can with 200/100/100/20 uf @ 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.    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.   Next attempt will be to try using a section of 1-1/2" I.D. aluminum 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.   Anyone could comment on the comfort factor of using the 160v caps in place of the 200v (on a 160v B+ line)?    73, - Mark K9MRK (off to the hardware store, again...) ----- Original Message ----- From: John_K7FD To: [EMAIL PROTECTED] Sent: Saturday, October 25, 2003 1:53 AM Subject: [drakelist] R4B hum   Hmmm....no make that hummm...   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?   Thanks   John K7FD