Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TerryD on January 03, 2016, 01:42:53 pm

Title: Is this dangerous?
Post by: TerryD on January 03, 2016, 01:42:53 pm

I replaced my filter caps and along with those I changed that electrolytic cap in the power supply section right next to the diodes.

The Fender PA 100 had a 80mfd 75V cap in there.
Silverface Twin calls for 50mfd 70V
BF Twin calls for 50mfd 50V.

I just happen to get a Sprague 50mfd 50V cap for no particular reason so I stuck that in the PA 100 power section.   Am I still safe?  Will that do?  Is it good?  What exactly does that cap do anyway?

I have on hand a xicon 47mfd 350V and a xicon 100mfd 160V .  Should one of those go in there or should I get an exact 80mfd 70V?  Thanks, Terry

I'm not usually so cavalier.  It's just that I read after I got my parts how guys that do the power caps often change out that cap as well.

Title: Re: Is this dangerous?
Post by: sluckey on January 03, 2016, 02:50:06 pm

You're flirting with disaster! Use the 100µF @ 160V. In fact, there are two of those 80µF/75V caps. If you're gonna change one, you should change both.

Title: Re: Is this dangerous?
Post by: PRR on January 03, 2016, 03:02:43 pm

> What exactly does that cap do anyway?

You mention three amplifiers and do not say which one you are working on.

I will *guess* that a 50V-75V spot is your Negative Bias (and filter) for the Power tubes.

In typical implementations, if this cap(s) fail short, your power tubes zero-bias, red-plate, melt, a hundred bucks down the drain. This is not a place to be low-balling just because that is what is "on hand".

In "many" implementations the bias is derived from a 50V AC tap on the PT. 50V*1.414= 70.7V DC. A 70V cap is very marginal. 75V is good for years. A forever-rebuild (40 years) wants a 100V part or better.

Given that Sluckey says there are 2 caps, this may be a 2-stage filter. In that case the value isn't critical. They do not have to be the "same" (tho that's most economical). Under 30uFd may let a little buzz in. Over 1,000uFd may take too long to charge-up from a cold start. Your 47/350 and 100/160 set will work, but 350V is way overkill.

Title: Re: Is this dangerous?
Post by: TerryD on January 03, 2016, 03:35:17 pm

Thanks!

 I see "now" that Doug has replacement caps for just this situation.  I'll  just slip the original back in for the moment and order a set  asa       p.