High pitched squeal in effects loop (Muff related?)

[echobaseone]

Just posted this on the Gear Page, but I should've asked here first...

I have an Orange Rockerverb 50 (4x 6v6 version), and it DETESTS drives/fuzzes/most overdrives through the front end, turning them into mush, even though I play through the dirty channel with the gain lower than 10 o'clock.
I rely pretty heavily on Muffs as a secret weapon in a few songs, and thought I had cracked the code by running the muff at the start of the effects loop. Until recently, this approach had worked pretty well, but after a show this weekend, I noticed that with the fuzz on, but the guitar volume OFF, there was a terribly shrieking sound coming from the amp.
It turns out that even with the amp volume at ZERO, this unearthly screech was still present.
So far, I've tried isolating just the muff, different patch cables, different cables to/from the amp, and different power supplies (currently using a Walrus AETOS). I'm about out of ideas.
Anyone got any thoughts on this? Much appreciated!

[zhivago]

Could it be a tube problem?

[echobaseone]

Hi Yannis!
Perhaps. but i did swap the two 12AT7s (1 is loop, 1 is reverb) with no diff.
it's pretty nuts that it produces this noise with the volume off though, do you think that could be tube based?

[JackFawkes]

Complete stabs in the dark, and no idea how to go about testing them but, could it be either grounding or impedance related?

Jack

[echobaseone]

It could be Jack...I ordered this: PYLE we'll see if that helps. It really is weird. It makes my RC-20 loops go into a higher treble register too. Hopefully this gets solved when the Pyle ground lift arrives.

[zhivago]

Hi Yannis!
Perhaps. but i did swap the two 12AT7s (1 is loop, 1 is reverb) with no diff.
it's pretty nuts that it produces this noise with the volume off though, do you think that could be tube based?

It's always hard to trouble shoot this stuff...my rule of thumb with tube amps is trying fresh tubes to eliminate that before diving in deeper.

It certainly sounds like an odd problem....could it be something with the speaker itself? Can you plug the amp into a different speaker?


I understand how frustrating trouble-shooting this must be...I had volume fluctuations with one of my amps a while ago and it turned out to be that the power socket in the back was missing one of the two screws causing it to move a bit. I put a second screw in that secured it and the problem went away...sometimes it is something crazy like that.

[øøøøøøø]

This kind of thing requires methodical troubleshooting

Questions:

1) was this behavior always present with this pedal? Did it just start occurring, or is there a chance you just didn’t notice until now?

2) can you replicate the behavior with any other pedal (particularly other fuzz pedals)?

3) you tried isolating cables and power supplies—good! Make sure this is done in a methodical way, changing one element at a time. A 9V battery is the best way to rule out power supply (eg intermodulation of the SMPS frequency)

4) the tubes that buffer the effects loop (if applicable) are another important variable, especially the one on the input side. It’s probable that it’s slightly microphonic, and the added gain from the muff sends it into runaway oscillation

The fact that the behavior persists with the amp volume at zero is perfectly logical; the effects loop is well after the volume knob

Worth acknowledging that a fuzz in an effects loop is an unconventional use case and would not have been designed-for, so there’s a chance you’re just finding a behavior that the designers didn’t envision.

But my money is on a microphonic tube

[echobaseone]

1) it's a new pedal, an Op-Amp based Muff, I only noticed it because in order to make it heard at live volume, I had to turn up the output beyond where it normally lives.

2) yep. tried it with a Ram's Head type Muff. still there, though slightly different.

3) tried a different Muff with a battery. Same result.

4) Doesn't appear to be the tube. I did a swap (though NOT with a brand new tube...a 12ax7 from another amp...it's meant to be a 12AT7, but I reckon that's not especially relevant here?)

You might be right Brad, just a contingency that was never foreseen, The noise reminds me of those pedals you could control the whistle pitch with your tone knob. Not sure if it was ZVex thing or???

[øøøøøøø]

I wish we had a schematic for the amp’s effects loop. That might give some clues.

One thing I still can’t help but suspect:

Perhaps using a fuzz in the FX loop is providing so much additional gain that it makes the choice of tube *really* critical.

Almost all tubes are at least a little microphonic. Class 2 (barium titanate) ceramic capacitors, too, while we’re at it…

It could be that introducing a fuzz to the equation pushes these factors beyond what the designers anticipated, but that it could be made to work with an extremely-carefully-selected tube.

One easy test: hook the fuzz up and get it *just* to the point right before it’s going to squeal.

Then lightly tap the glass envelope of each small tube with a pencil.

Can you excite one into runaway oscillation? If so, try tubes in that spot to see if you can find a quiet one

[echobaseone]

I couldn't get the tubes to oscillate via pencil tap, but holy hell, the whole thing is "alive".
You can touch the knobs and hear it through the speaker. Touching any of the cables with fingernails is transmitted through the speaker. Tapping on the faceplate, etc.
As far as tapping the tubes goes, the PI/Reverb/FX loop are the loudest when tapped, but the preamps ones are quite live too.

[echobaseone]

Complete stabs in the dark, and no idea how to go about testing them but, could it be either grounding or impedance related?

Jack

Tried the Pyle, no luck with the ground on/off.

[andy_tchp]

4) Doesn't appear to be the tube. I did a swap (though NOT with a brand new tube...a 12ax7 from another amp...it's meant to be a 12AT7, but I reckon that's not especially relevant here?)

Well, it might be. The 12AX7 has even more gain.

Brad alluded to runaway oscillation; I've seen this with Fender Reverb equipped amps (usually Silverface) when the 12AT7 Reverb driver is marginal or highly microphonic. Crazy frightening screeching.

The 'fix' (more a 'workaround' really) is to use a good quality NOS 12AT7 in that slot.

Or stop putting a Big Muff in the effects loop. Drop the level of distortion down and EQ differently if it 'disappears' in the mix. Though a lot of that is just a Big Muff doing what a Big Muff does (vanishing).

[øøøøøøø]

Oh! I missed that you'd replaced the 12AT7 with a 12AX7.

Get a good, non-microphonic 12AT7 in there and reëvaluate.

Regardless of that, if things are "live" and microphonic to the degree you're describing (where touching a knob is heard through the amp) then something microphonic is almost certainly your issue. Whether it's a tube, or something else that's gone microphonic (ceramic capacitor?) that's most likely gonna be your issue.

[echobaseone]

Sorry for any confusion...I replaced the 12AT7 with a 12AX7 temporarily to see if the tube itself was the issue.
But yeah, maybe that's not a good delta. I'll see about getting a tough12AT7 and see what happens.
Thanks for the help and the input...Something IS microphonic somewhere.

[øøøøøøø]

You’ll find it!

I like to proceed methodically.

If you turn the input volume knob on the amp down and the microphonic behavior goes away, then it’s most likely the first preamp tube.

If it doesn’t, then it’s later, but that at least helps. If you turn the master all the way down and it goes away, then it’s somewhere between the first tube and the last tube before the master volume, etc

You can triangulate a lot of information this way without unscrewing or unsoldering anything at all.

Sometimes I like to “go down the line” starting with V1 and remove tubes one at a time to see where the behavior changes or stops.