High pitched squeal in effects loop (Muff related?)
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Re: High pitched squeal in effects loop (Muff related?)
Glancing at the schematic, a few things occur to me.
Sometimes reverb tanks can be microphonic.
If you've been adding reverb post-FX loop, try turning the reverb down and seeing if the behavior changes. If not, try disconnecting the tank, just to see
Sometimes reverb tanks can be microphonic.
If you've been adding reverb post-FX loop, try turning the reverb down and seeing if the behavior changes. If not, try disconnecting the tank, just to see
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Re: High pitched squeal in effects loop (Muff related?)
Thanks Brad. I took the reverb tube out altogether and the issue remains. Can the tank still be a possible culprit?
I set everything right on the brink of oscillation and I think it might actually be v1 in the preamp. I was able to get the squeal to catch, and also able to dampen it with the eraser of the pencil I was using!
Though it still seems like it MUST be effects loop related somehow (this issue only exists when the loop is in play). Which makes me think it's position v4 in the diagram but I dunno...
I set everything right on the brink of oscillation and I think it might actually be v1 in the preamp. I was able to get the squeal to catch, and also able to dampen it with the eraser of the pencil I was using!
Though it still seems like it MUST be effects loop related somehow (this issue only exists when the loop is in play). Which makes me think it's position v4 in the diagram but I dunno...
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Re: High pitched squeal in effects loop (Muff related?)
Hmmm. And then I swapped out the reverb driver 12AT7 with the FX Loop 12AT7 and it got marginally better (and by better I mean I could turn the fuzz up slightly higher before oscillation, and that scratching the cables wasn't AS audible as before). Anyway, I've ordered a couple of NOS Tested USA 12AT7s so we'll see what happens.
Thanks for all the help and if anything else occurs to you, holler!
Thanks for all the help and if anything else occurs to you, holler!
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Re: High pitched squeal in effects loop (Muff related?)
Sounds like you've got some leads to explore!
you could also swap some 12AX7s around internally within the amp and see if you can find one that's less-borderline in V1 (which will typically be the most-critical tube in the amp, since it sees the most overall amplification) .
you could also swap some 12AX7s around internally within the amp and see if you can find one that's less-borderline in V1 (which will typically be the most-critical tube in the amp, since it sees the most overall amplification) .
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Re: High pitched squeal in effects loop (Muff related?)
NOS Tubes arrived, but sadly solved nothing.
So far I've swapped preamp, phase inverter, reverb and driver tubes to no avail. Will probably try the power tubes as well I've got a couple of 6V6 around...
New wrinkle, IT HAS NOTHING TO DO WITH THE MUFF (though that is what is making the high pitched squeal).
I jumpered the send/return with a patch cable, and while I can't achieve the squeal, a tap on the faceplate or the handrails can be heard pretty loudly through the speaker cabinet.
I'll give the (possibly microphonic) power tubes a go, but if anyone has any thoughts in the meantime...
So far I've swapped preamp, phase inverter, reverb and driver tubes to no avail. Will probably try the power tubes as well I've got a couple of 6V6 around...
New wrinkle, IT HAS NOTHING TO DO WITH THE MUFF (though that is what is making the high pitched squeal).
I jumpered the send/return with a patch cable, and while I can't achieve the squeal, a tap on the faceplate or the handrails can be heard pretty loudly through the speaker cabinet.
I'll give the (possibly microphonic) power tubes a go, but if anyone has any thoughts in the meantime...
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Re: High pitched squeal in effects loop (Muff related?)
Brad, you mentioned something about the reverb tank possibly being microphonic. Would I hear that with the driver tube removed and/or reverb set to 0?
Also, Power Tubes are OK.
Also, Power Tubes are OK.
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Re: High pitched squeal in effects loop (Muff related?)
It's not surprising that the power tubes are not the issue, but it doesn't hurt to have ruled them out. They're seldom microphonic, and aren't being re-amplified by more stages, so they're not a typical point for this kind of issue.
The best way to rule out the reverb tank is to simply unplug it, or to remove the recovery (make-up gain) tube, if you can work out which one that is (provided it doesn't share function with another key part of the circuit).
Other things to suspect now that we've entered "grasping at straws" territory:
The best way to rule out the reverb tank is to simply unplug it, or to remove the recovery (make-up gain) tube, if you can work out which one that is (provided it doesn't share function with another key part of the circuit).
Other things to suspect now that we've entered "grasping at straws" territory:
- It's totally plausible that your replacement tubes are also microphonic. Mark their original locations and swap them around to other locations and see if anything changes
- If you're brave enough to peek inside or can locate some internal photos on the internet, I'd be curious to see whether the amp has a sizable number of Class 2 ceramics. These ceramic caps (which have barium titanate as their dielectric, usually) can be highly piezoelectric under some circumstances. If you're thinking "hey, I've heard that word before, when talking about acoustic pickups" then you already understand how this could be a problem...
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Re: High pitched squeal in effects loop (Muff related?)
We can probably rule out the ceramic cap angle.
I see only six, and none seem especially likely to be a source of your issue.
Three brown ones seem likely related to power supply snubbing/slugging function (hard to tell without that portion of the schematic).
C18 and C19 are most likely RFI or parasitic oscillation suppression for the first preamp tube. Worth a smack with a chopstick? Yes, if you're in there already... but I'm not staking money on them being the issue.
The last one I see is labeled C42 on the board. I can't find C42 on the schematic you posted, but most likely it's not audio-critical since it's sitting right next to some silver mica caps used in the tone stack.
Most of the rest of the caps seem like regular box film caps (probably metallized polyester or polypropylene).
That doesn't seem like a profitable lead, for now.
I see only six, and none seem especially likely to be a source of your issue.
Three brown ones seem likely related to power supply snubbing/slugging function (hard to tell without that portion of the schematic).
C18 and C19 are most likely RFI or parasitic oscillation suppression for the first preamp tube. Worth a smack with a chopstick? Yes, if you're in there already... but I'm not staking money on them being the issue.
The last one I see is labeled C42 on the board. I can't find C42 on the schematic you posted, but most likely it's not audio-critical since it's sitting right next to some silver mica caps used in the tone stack.
Most of the rest of the caps seem like regular box film caps (probably metallized polyester or polypropylene).
That doesn't seem like a profitable lead, for now.
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Re: High pitched squeal in effects loop (Muff related?)
Yeah, pretty confounding. Will likely run it down to Fromel Electronics today. I really appreciate all the help though!
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Re: High pitched squeal in effects loop (Muff related?)
Will be interested to know what you find.
My money is still on a microphonic 12AX7, probably very early in the amp.
Some amps are *extremely* sensitive to even small amounts of microphonic behavior. This may be one of them
My money is still on a microphonic 12AX7, probably very early in the amp.
Some amps are *extremely* sensitive to even small amounts of microphonic behavior. This may be one of them
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Re: High pitched squeal in effects loop (Muff related?)
Definitely update the thread when you can, I too am very curious to hear what the problem was,
Resident Spartan.
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Re: High pitched squeal in effects loop (Muff related?)
Unfortunately the answer is about 3 weeks of backlog away. But I will most definitely report back.
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Re: High pitched squeal in effects loop (Muff related?)
Hmmmph. Mystery solved. It WAS a tube. <shakes head and grumbles> I don't know how it kept being harmonic when I swapped out multiple pre-amp tubes. Maybe all my spares were garbage?
From Simon @ Fromel:
One of the effects loop tubes tested badly and I replaced it and it
seems fine. Everything else on the amp is super solid and passed all the
diagnostics just fine. I don't have a Big Muff here in the shop to run
into the fx loop however.
The specific tube is run pretty hard in normal operation and if it's
being driven even harder by a boost type circuit it could cause it to
fail earlier than it should. A cranked Big Muff is probably mostly ok,
but a really loud boost might be too much. Just something to keep in
mind.
From Simon @ Fromel:
One of the effects loop tubes tested badly and I replaced it and it
seems fine. Everything else on the amp is super solid and passed all the
diagnostics just fine. I don't have a Big Muff here in the shop to run
into the fx loop however.
The specific tube is run pretty hard in normal operation and if it's
being driven even harder by a boost type circuit it could cause it to
fail earlier than it should. A cranked Big Muff is probably mostly ok,
but a really loud boost might be too much. Just something to keep in
mind.
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Re: High pitched squeal in effects loop (Muff related?)
That tracks
Glad you got it figured out, even if it was a bit annoying
Glad you got it figured out, even if it was a bit annoying
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