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View Full Version : Does this sound like a blown filter cap?


willhutch
01-09-2007, 09:59 PM
My Princeton Reverb is on the fritz.

A couple days ago it started humming badly. It was a low freq hum that remained even with the volume off.

Just a little while ago, I was showing off a new guitar to a buddy (I'll talk about the guitar in another thread). I was noodling around when all of the sudden the amp started humming very loudly. I abruptly turned the amp off.

My friend, a mad-scientist type familiar with electronics said he thought it was a filter cap going. He picked up the guitar and played a low Bb and announced "that was definitely 120 Hz we were hearing".

We pulled the chassis and could see no signs of anything having exploded. Of course we couldn't see into the cap can. My friend advised I should undo the leads to the cap can and check the filter caps with a capacitance tester (which he has). Or I could undo the leads, power up the amp and read the voltage from the caps.

First question: Are the symptoms consistant with blown filter caps? Second: any tips for dignosing this problem?

brad347
01-09-2007, 11:40 PM
first question: Yes this sounds like it could very well be a blown filter cap.

second question: No need to chase your tail trying to diagnose. Just get a repro cap can and replace it. It's "cheap amp insurance" anyhow and would need to be done eventually anyway even if it's not the problem. But i'd lay 5:1 odds that you have a blown filter cap (provided you've already tried swapping tubes and checked all grounds, etc).

Blue Strat
01-10-2007, 05:41 AM
1) 90% of failed caps show no visible signs
2) It could be a failed filter cap, bias supply cap, power tube, preamp tube, etc, etc.

Check the bias on the power tubes (or swap in a known good pair) to be sure they're both operating.

Pull the preamp tube closest to the power tubes and turn the amp on. Still humming? This would point to the power tubes, power amp or power supply (including filter caps). If it stopped humming, the problem is either one of the preamp tubes, bad tube connection, or something else in the preamp.

gldtp99
01-10-2007, 06:18 AM
yes, do as Blue Strat suggests and pull PI tube to isolate problem (preamp or power section)--- if the problem is in the power section, then try swapping the rectifier tube (GZ34 or 5u4gb-- whichever yours has)--- my friend's VibroChamp had similar noise problems and we found it was a bad rec tube (5y3gt in the Vibro Champ)--- installing new filter caps in an older amp is a good idea but be sure that the problem isn't just a bad tube before getting the soldering iron out...............gldtp99

mageerc
01-10-2007, 06:53 AM
If that PR hasn't been serviced for more than 10 years, take this opportunity to replace ALL the electrolytics in the amp. The filter caps, the cathode bypass caps and the bias cap... your symptoms sure sound like a failing filter cap.

boobtoob
01-10-2007, 07:44 AM
Could also be a bad coupling cap.

willhutch
01-11-2007, 11:47 AM
So guys, I went in to do further diagnostics on my Princeton Reverb per the advice given.

The situation has changed since last powering up:

Now, instead of a loud, harsh hum, I'm getting a tiny, thin, fuzzy guitar signal. It gets a little louder whn I turn up the amp, but is still very quiet.

I pulled the PI, per suggestion, and found that there was no sound at all - no hum. I swapped the rectifier tube and the problem persisted. Since I don't have spare 6V6s, I haven't swapped power tubes yet. Nor have I swapped preamp tubes.

Does this still sound like filter cap problems??

brad347
01-11-2007, 12:03 PM
hmm... maybe not.

willhutch
01-11-2007, 11:26 PM
bump to the top for John Phillips or OTM.

brad347
01-11-2007, 11:37 PM
i'm not those guys but I would bet that you have a bad preamp tube.

Try swapping their places and see if the problem moves/changes.

Kingofdogs1950
01-12-2007, 12:50 PM
I'm having the same (or similar...) basic problem with a Champ clone I recently built. There is a very loud hum when I turn on the amp. Filter/bias caps check out ok. Replaced all tubes. No change. With the loud hum I'm afraid to leave it on to test voltage. So what's next???

Mark

donnyjaguar
01-12-2007, 03:00 PM
I'm having the same (or similar...) basic problem with a Champ clone I recently built. There is a very loud hum when I turn on the amp. Filter/bias caps check out ok. Replaced all tubes. No change. With the loud hum I'm afraid to leave it on to test voltage. So what's next???
Check your wiring. :)

Without looking at it I can only guess. And my guess is a bum tube or tube socket on V1.

willhutch
02-09-2007, 04:02 PM
Gentlemen,

After a bunch of troubleshooting, I concluded that the filter caps are the culprit. I swapped them out just now and guess what......the amp is fixed!!!!!

I'm really having fun with the tech side of music these days. It's a rewarding adjunct to my love of playing the guitar!

Thanks to those who chimed in to help out!

Johnny Z
02-09-2007, 05:58 PM
If this amp is very old say 10+ years yep it could very well be a cap, even newer amps can blow a cap. Try grounding the center lug on your guitar cable to the amp chassis for a second if the hum gets worst it's most likley a ground loop problem.

Jay101
07-05-2009, 02:28 PM
try moving it to a diffrent power supply maby its noise from elsewhere///?????? just a thought

RussB
07-05-2009, 04:07 PM
try moving it to a diffrent power supply maby its noise from elsewhere///?????? just a thought


why the hell would you dig up a year and a half old thread and offer less-than useful advice? :crazy