Amp Hum

Bob Hoffnar · Post by **Bob Hoffnar** » 22 May 2003 8:42 pm

I just picked up an old Fender Princeton amp to use with my Stringmaster ( A freakin unbelievable sounding combination but that is for another thread).
I think its from the 70's at some point. the serial is #A967644.

Anyhoo it is putting out a low hum right around 60 cycles just loud enough to be a problem.

The hum starts when the amp is turned on. It doesn't matter what any of the knobs are doing. The hum does not get louder or softer. Its not a crackle or a buzz. Just a hum.

I've tried the ground lift, polarity stuff already. Also the hum does not change if I move the amp around or even if I move it to another building.

Is there some easy fix I might try out before I pay somebody ?

thanks, Bob

Ken Fox · Post by **Ken Fox** » 22 May 2003 8:52 pm

Bob, it could be the power supply filters or an open 100 ohm resistor(s)on the heater supply (6.3 volt AC supply has both legs to ground via 100 ohm resistors). The power supply filters are a can style with multi- sections. I tried a new one from Mojotone and was very disapponted, it hummed worse that the original! The Champ uses a three @ 20ufd multi section cap, the Princetons use a 4 section cap. The same cap is used for both, one section left unused in the Champ. A reputable cap might be the JJ/Telsa caps. Check Eurotube.com for that cap. Changing one of these requires a very wide tip 80-100 watt iron or a 300 watt soldering gun. A lot of heat is needed to remove the old one from the chassis (the can is soldered to chassis).
Also tubes can cause hum as well. I had a fairly bad hum from a defective 6L6GC in a Twin a few years back. Cleaning tube sockets can help,too.
<FONT SIZE=1 COLOR="#8e236b"><p align=CENTER>[This message was edited by Ken Fox on 22 May 2003 at 09:54 PM.]</p></FONT>

Michael Brebes · Post by **Michael Brebes** » 23 May 2003 7:43 am

I agree. It sounds like power supply caps or heaters. Another source for power supply caps is Antique Electronics at www.tubesandmore.com

Pete Burak · Post by **Pete Burak** » 23 May 2003 9:14 am

Hey Bob, Ken got my '70's SF Champ working like, well... a Champ!
I'm sure that amp can run silent.
My amp was making a wierd hum, and one thing that Ken troubleshot was that the input jack was shorting itself out.
Ken can describe this symptom and fix better.
He also replaced a few caps.
The thing is near silent with all knobs on 10.
I recently replaced the original Fender speaker with a Weber, and the amp weighs like 3 times more, and it sounds really cool. No speaker distortion, just that tube sizzle.

Ken Fox · Post by **Ken Fox** » 23 May 2003 10:06 am

Actually the input jack shorting contacts were dirty, caused the shorting jack to be slightly open. The contacts can be cleaned by inserting a 1/4" jack, spray the contact with cleaner, insert a strip of clean white copier paper in between the contacts, remove the 1/4" jack and then slowly draw the paper through the contacts. That cleans and burnishes the contact. Sometime these contacts are not closing, realignment or replacement is necessary at that point.