Weird interaction of pedal board and amp

Jim Robbins · From: Ontario, Canada

I've been rehearsing at a studio that has a Fender twin non master volume reissue and it does not like my pedalboard. The twin seems to be working normally although it has a kind of low level oscillating noise even when nothing is plugged in. The problem is this: the twin will intermittently reduce the volume a lot and start sounding distorted if I'm playing through the pedalboard even with the pedals off. It sounds as if the tubes aren't getting enough power. Going straight into the amp is fine. It's not a cable issue. I've tried every channel in the twin and tried unplugging the footswitch without making a difference.

I would have thought that it was a faulty pedal or connection on the pedalboard, but when I plug the board into my Milkman mini at home, it works fine.

Any ideas? Thanks.

Steve Lipsey · From: Portland, Oregon, USA

I had that issue with a fishman Aura...it needed a separate power supply to isolate it form the other pedal's power or I got that noise.
Anything digital in the chain will have a clock frequency that can leak into other devices...mine was going from the Aura, back to the instrument, and then out over the stereo cable to the other devices...crazy...a separate 9V fixed it.
Somehow there's a feedback loop in your chain?
remove the pedals one by one till you find it.,..
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Jim Robbins · From: Ontario, Canada

Thanks, Steve. Everything is on isolated power if Strymon is to be believed -- most of the pedals are powered by a Zuma. There are a few with their own wall-warts because of their power requirements and I've got a Sarno Black Box that gets straight AC.

I've thought about going through the whole chain but since it doesn't happen at home, I'd have to rent the studio to diagnose it. Any thoughts on why it would happen with one tube amp and not another?

Glenn Demichele · From: (20mi N of) Chicago Illinois, USA

The first thing I would do is plug your pedal board into the amp, then unscrew the guitar cord cover at the amp input end to expose the terminals. Use a voltmeter to measure the DC voltage at these amp input terminals. If you see DC (like more than 100mV), figure out if its coming out of the amp input or out of your pedalboard by either unplugging the cord from to amp input (checks pedalboard), or unplugging the other end of the cord from the pedal board (checks amp).
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Jim Robbins · From: Ontario, Canada

Thanks, I will try that next time it happens.

Bob Sykes · From: North Carolina

The noise could be conducted noise, radiated noise or a combination of both. Likely outside the audio range. My old Fender tube amps are kinda susceptible to high frequency radiated noise.

As described previously, conducted noise (DC) can be checked for with a meter. A scope would be needed to check above 22 kHz. Otherwise it's divide and conquer with the individual items in the signal chain.

If there's a radiated component, increasing the separation distance from the pedalboard (noise source) to the amp (noise victim) can help diagnose it. Keep in mind cable (shields) can play radiating antenna too.
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Jim Robbins · From: Ontario, Canada

Well, everything worked fine at the rehearsal studio last night so I did not need to use the fresh battery I got for my multimeter. One of life's mysteries. Or maybe they fixed the amp, which did not seem quite as noisy as before. Thank you all for your suggestions.

Dave Grafe · From: Hudson River Valley NY

The Fender reissue amps use push-on contacts for power transformer leads, rather than the soldered connections of the originals. These can become loose enough to cause inconsistent voltage on tube plates, causing intermittent changes or complete loss in volume. If it's not yours there's not much you can do about it but this is one known issue peculiar to these RI amps.

Jim Robbins · From: Ontario, Canada

Thanks Dave, good to know.

Bobby D. Jones · From: West Virginia, USA

Check each pedal, If the polarity of a plug on any pedal is wrong, Compared to amp or other pedals could be the problem.

I have a delay pedal that if I get noise when I plug it in, Reverse the plug in the outlet and things are quite. The plug in does not have a wide blade for polarity, So it can be plugged in either way.

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