Hum on a Fender Champ 12

[merkaba22]

I just got an old Champ 12 that supposedly came with MM iron (w/ Nos.: 1-02403 and 51A606-711 that I saw when I looked into the chassis) and a Celestion Greenback -- it had a hum and I took it into get some Sozos, etc. expecting that this would fix the hum.

What I really liked about the amp was that besides it sounding pretty good in view of the improvements, there was additional detail in each note that I am not normally used to hearing. I remarked about it to a amp tech/designer and he said it was because it was a single ended amp.

I finally got the amp back and found out with the MM iron and Celestion that the amp weighs in about 6lbs heavier and was told that it makes about 22 watts now -- iirc, there was something about 6 or 8 volts that when calced gave that output. There were three grey and black "caps" that were like the originals that had been replaced -- about .5 dia and about 1.5" long.

BUT -- and this is the reason for the post: it still hums when all controls are zero and nothing is plugged in -- and my PR II and recording equipment do not hum in the same room/outlet -- I searched the internet and found this:

"This amplifier uses a single 6L6GC output tube. The output stage is rather unusual in that the screen voltage is only +242VDC, while the plate voltage is +496VDC. this was done in order to achieve the exact sound desired for both clean and overdrive modes while maintaining call "A" operation within the proper family of curves for a 6L6GC.

A reasonable way to provide such a 2 to 1 B+ ratio is to use a voltage doubler in the power supply, and tap off one half of the doubler for the screen, and use the total output for the plate.

The trade-off is a slight increase in hum. It puts 60Hz. ripple on the screen and 120Hz. ripple on the plate. Audible hum is noticeable only in a very queit enviroment, so the Chanmp 12 is better used as a guitar or practice amp, rather than in a recording studio application(editor note: a room or distance microphone would work).

We don't want you chasing ghosts, so if someone complains about hum on a Champ 12, measure the ripple on the output tube with a scope. You should see about 1Vp-p ripple on the screen (pin 4), and 15Vp-p on the plate (pin 3). Ripple of 15Vp-p translates to 5.3Vrms riding on +490DVC, or about 1%.

If you scope across the speaker, you should see 50 or 60 millivolts peak-to-peak. That is what you hear and is normal for this amplifier."

Is there really no way to get this hum out?

[dobbhill]

It could probably be improved with more power supply filtering and a choke, but that would also change the sound and "feel" of the amp.
D

[merkaba22]

It could probably be improved with more power supply filtering and a choke, but that would also change the sound and "feel" of the amp.
D

Thanks, I decided to keep it for the weekend and give it a chance since it was arguably better than when I took it in.

However, today, I was playing it and the bigger hum came back in two stages one after the another over the duration, of say, 10 second.

Where the hum level is now, defines not only the improvement gained by the repair but also points to quality of the hum standard for the design of this amp.

Any ideas where the "repairable" hum might be coming from.

Also, for clarity's sake: the replaced electrolytics were by Illinois Capacitor and seemed to visually match what was taken out.

[Structo]

The schematic is very hard to read.

An old trick on some of those single ended amps is to reference the heater voltage from the cathode voltage on the 6L6.
This lifts the heater voltage above ground and usually the result is less hum.

YMMV

[selloutrr]

tubes or a bad chassis ground i'm not familiar with the champ 12 but the champs i have worked on all greatly improved with a grounded power cord. you could have a broken ground wire even if it looks good solid core wire can break in the sleeve when tweaked one to many times.
a tube could also be going bad, try swapping with know good tubes. can you check the amp with another speaker and the speaker with another amp? then work backward.... using the schematic trace for voltage.. unplug the power and drain the caps and test for continuity. all champs are a simple circuit you should be able to trace it fast. if this is one of the solid state 80's models???? sorry it's probably more hastle then it's worth use it for an end table and enjoy the conversation.

[Structo]

No not SS but it is a different animal from the old 5F1.

Here ya go.

http://www.schematicheaven.com/fenderam ... _schem.pdf

[JD0x0]

You could try some sheilding in the cab. Put a small peice of sheet metal that covers the entire top of the chassis on the top, inside of the cab this should reduce hum.

[selloutrr]

its pcb? make sure everything is snug to the chassis. i had a '65 twin reverb come in that had a similar problem turned out the wave solder was shotty at best and all points needed a touch up a few wires were not ever soldered and transformers were loose by a 1/2 turn... it worked by luck and nothing more.