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Disclosure: I do not have a scope.
I just finished an amp build & let the buyer hear it the other night. He loves it. I decided to make a few changes to "clean up" the insides. After changes, was testing with my guitar. I know the pickups are microphonic, too much so for my taste. So I was testing my amp out. I usually give it an "extreme" test before letting go, just in case the buyer tries something "crazy." I'm also fairly close to the amp while testing.
First, it's a single-channel 2x6L6 Fender "Normal" channel. I'm playing it through my 15" Weber bass speaker (which has more than enough treble for guitar musing). My guitar is a Tele w. Duncan singles in it. So, I had the treble on 10 & cut control (added) on "minimum treble cut", and the volume pretty much maxed too. Bass was on 0. I was getting a high-pitched whistling noise, almost like microphonic squeal. I could dial it out with the 2 controls or the tone control on my guitar.
Testing of the amp ensued. Weird findings:
I ended up undoing all of my changes trying to get rid of it except for upping the coupling caps to the 6L6s, which are now at .022. The squeal is still there with the extremes of controls. I can also induce it at a slightly lower volume by keeping a finger close to the 6L6 closest to the front panel. I swapped the tube positions & it stayed with the socket. I haven't tried a different set of tubes yet.
If I unplug tbe guitar & cable from the amp I can run all the knobs on 10 (V, T, B, Cut) with no squealing or tube cacophony if I tap the tubes. No strange changes in the control knobs if turned that indicate amp-based oscillation to my ears. Just normal changes that come with adjusting knobs. No redplating without signal that may show oscillation. I'll do some voltage checks for leaky caps later today.
Guitar otherwise sounds good on all pickup combinations, controls work smoothly after some cleaning, nothing out if the ordinary at more "normal" settings... Didn't notice any issues theother night while testing woth buyer's guitars & speaker cab (1x12" closed-back Celestion) other than there was not much bass, probably because I trimmed it out in my own testing cuz I have a bass speaker & a modded Tele.
So it seems to me that SOMEthing related to my guitar is setting off something. A different guitar with a P90 in the neck does it too, though not as badly. The humbucker doesn't do it. I know I'll need to pot my Tele pickup(s) & I'll get around to it. And I know that "nobody plays with the Treble on Max & the Cut on Minimum!" But since it seems to only happen with a guitar plugged in, & with microphonic pickups, can I reasonably conclude my guitars are the problem? And why I can trigger it by getting a finger close to the 6L6 is beyond me; it's new to me!
This is more a "fun discussion" to me, though if anyone has any quick&dirty theories on what's going on I'll hear em.
Jusrin
I just finished an amp build & let the buyer hear it the other night. He loves it. I decided to make a few changes to "clean up" the insides. After changes, was testing with my guitar. I know the pickups are microphonic, too much so for my taste. So I was testing my amp out. I usually give it an "extreme" test before letting go, just in case the buyer tries something "crazy." I'm also fairly close to the amp while testing.
First, it's a single-channel 2x6L6 Fender "Normal" channel. I'm playing it through my 15" Weber bass speaker (which has more than enough treble for guitar musing). My guitar is a Tele w. Duncan singles in it. So, I had the treble on 10 & cut control (added) on "minimum treble cut", and the volume pretty much maxed too. Bass was on 0. I was getting a high-pitched whistling noise, almost like microphonic squeal. I could dial it out with the 2 controls or the tone control on my guitar.
Testing of the amp ensued. Weird findings:
I ended up undoing all of my changes trying to get rid of it except for upping the coupling caps to the 6L6s, which are now at .022. The squeal is still there with the extremes of controls. I can also induce it at a slightly lower volume by keeping a finger close to the 6L6 closest to the front panel. I swapped the tube positions & it stayed with the socket. I haven't tried a different set of tubes yet.
If I unplug tbe guitar & cable from the amp I can run all the knobs on 10 (V, T, B, Cut) with no squealing or tube cacophony if I tap the tubes. No strange changes in the control knobs if turned that indicate amp-based oscillation to my ears. Just normal changes that come with adjusting knobs. No redplating without signal that may show oscillation. I'll do some voltage checks for leaky caps later today.
Guitar otherwise sounds good on all pickup combinations, controls work smoothly after some cleaning, nothing out if the ordinary at more "normal" settings... Didn't notice any issues theother night while testing woth buyer's guitars & speaker cab (1x12" closed-back Celestion) other than there was not much bass, probably because I trimmed it out in my own testing cuz I have a bass speaker & a modded Tele.
So it seems to me that SOMEthing related to my guitar is setting off something. A different guitar with a P90 in the neck does it too, though not as badly. The humbucker doesn't do it. I know I'll need to pot my Tele pickup(s) & I'll get around to it. And I know that "nobody plays with the Treble on Max & the Cut on Minimum!" But since it seems to only happen with a guitar plugged in, & with microphonic pickups, can I reasonably conclude my guitars are the problem? And why I can trigger it by getting a finger close to the 6L6 is beyond me; it's new to me!
This is more a "fun discussion" to me, though if anyone has any quick&dirty theories on what's going on I'll hear em.
Jusrin
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