My Fender 6G15 is sick

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I hope this is the right place *L* My colleagues at referred me to this forum for assistance.

I have a Fender Reverb, blackface era.

When I first got it, the caps were stock (40+ years old), so they got changed out. I changed out the tubes, and I replaced the 2-pronged power cord with a grounded cord.

This was fine for several months, until recently.

The unit sounds fine in bypass, but when I switch it on (which, is all the time), I get a nasty buzz.

If I adjust the Spring Dwell, no change. If I adjust the Tone control, it changes the tone of the buzz. When I adjust the Mix control, it increases in volume as I add more "wet" signal.

I'm hoping it's not the Power Tube, as it's a 6K6GT, and difficult to find. It might be the 12AT7, or the 12AX7/7025... or it might be something completely different. The 7025 was brand new. It's a Sovtek/Fender 12AX7A. The 12AT7 is a used Hewlwtt Packard. The 6K6GT is a used (Not NOS) RCA.

Anyone have any thoughts or experiences they can help me with?
I am not familiar with the particulars of your amp, and I'm not sure what you mean by "sounds fine in bypass", but I have built several amplifiers.

Whenever I hear about a buzz or excessive hum, I think bad capacitor. Check the filter capacitors (big electrolytics) in the power supply circuit. Make sure all the caps are discharged before doing anything with/to them else you could wind up dead.
If you aren't accustomed to dealing with several hundred volts of electricity, find someone who is to help you.

Good luck.
Mike L.
I think you caught everyone off guard. This is the Fender outboard reverb unit correct.
Checking the schematic, the power supply uses a half wave rectifier. This has high ripple and can be hard on filter caps. I suspect one or both of the new caps installed is probably going bad.
Like MikeL said, if your not comfortable with high voltage, get some professional help. 300V is lethal.

The other thing to check is the cables leading to the springs. If they're not shielded properly the unit will be very noisy.

oh sorry!

Yes this is the stand-alone reverb unit. The "Bypass" is when the Reverb is not engaged, thus clean signal is passing from input to ouput.

If the caps are bad, I won't be happy *L* I just dropped those in several months ago. Those phenolic circuit boards are a bit of a pain to deal with.

I'll look at the cables. I move the unit alot, so one of the cables may actually be loose or damaged.

Thanks for the help!
It is not the power supply or it would be hummy no matter what.

The 6G15 does not have a bypass, it has a reverb kill switch - the footswitch - and it has a balance control between the dry signal and the reverb. The footswitch is simply in parallel with the return sensor on the reverb pan. It just shorts the inoput to the recovery stage to silence it.

I doubt it is the tube. In any case it wouldn't be the 6K6 since that is the drive tube, not on the recovery side.

Isolate the problem. Something is introducing hum into the recovery circuit. My first guess and way out in front of the others is the reverb pan itself. After that I suspect the footswitch wiring.

Disconnect the footswitch from the jack. If that helps, there you are. If not, then move on to the pan. Disconnect the pan cables from the chassis jacks. If that cuts down the hum, then the pan is at fault or its cables.

Then test the cables. An open shield on the return cable will cause loud hum.

And remove the reverb pan and inspect inside it. At each end there are a pair of thin wires - black and green - from the jack on the pan wall to the transducer at the end of the spring assembly. These wires like to break off. You may be able to tack solder them back on. Otherwise replacements can be had from Antique Electronics at reasonable cost. And even if the wires are intact, measure continuity acros each jack on the pan. One will measure very low - les than an ohm, while the other will measure 150-200 ohms. DOn't worry about the value, the thing is either open or it is right.

Antique is at

Look on your reverb pan for a number in the format: 8AB2C1B. If you need to replace the pan, match the first three characters and you are OK.
Well, it seems to be working now.

I unplugged the foot-switch, and I don't have the nasty buzz I had before. Since I use the reverb all the time (just at different levels) this is fine.. and it's one less switch on my pedal board.

I looked at the cables for the spring-pan, and they appeared to be fine. I haven't had a chance to look at the pan itself. This weekend, I'll prolly do that.

Thank you for all your help!!!
If unplugging the footswitch makes the hum go away, then the footswitch is the problem. If the unit works well without it, then there is no point in tearing into the pan.

The hot lead of the footswitch is connected right to the signal point, so it has to be a shielded cable. Also, the shield must be connected to the ground of the chassis via the jack.

So you either have a wiring problem, or the footswitch has the wrong type cable.
Yeah, I resolved it ahwile ago.

Actually what happened is a friend borrowed a cable, and so I ended up using what I thought was an instrument cable to go from the Reverb to the amp.

It wasn't. It was an unshielded speaker cable. When I switched back to using a regular instrument cable, there were no issues, whatsoever.

That was what 2 years ago? It's been running like a sherman tank since then!

If any of y'all get your hands on one of these things, don't let it go!
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