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Topic: Causes for Sound of (Maybe) Microphonic Tubes |
Chris Bauer
From:
Nashville, TN USA
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Posted 22 Mar 2014 11:28 am |
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Sooooo.. I'm still trying to trace the cause of unpleasant overtones through one of my amps. It's not strings because it happens after string changes and isn't a groove in the changer because it happens across guitars and none of this happens with other amps. The problem has, in fact, gotten worse with time in that it now happens on multiple strings.
The 6SC7 has sounded microphonic from the start. After swapping out five of them so far, the problem is still there and is worse with some tubes. I know that it is possible to simply have that many bad tubes but with each replacement that explanation feels less likely.
Is there anything us non-electronics types can do to test out if it's something having to do with that socket - or something connected to it - and not the tubes themselves?
Last edited by Chris Bauer on 22 Mar 2014 12:52 pm; edited 1 time in total |
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Chris Bauer
From:
Nashville, TN USA
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Posted 22 Mar 2014 11:57 am
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| By the way, in case this is at all diagnostic, the notes have more and more of a harsh clanking sound when plucked. Much more noticeable on the unwound strings but you can hear it everywhere on the guitar. |
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John Polstra
From:
Lopez Island, WA, USA
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Posted 22 Mar 2014 1:00 pm
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Chris, I would start by doing a test to find out for sure whether or not the objectionable sounds are caused by a microphonic tube. Cable up your amp to the speaker in one of your other amps. Use a long speaker cable so you can separate the speaker from the problem amp by a reasonable distance. This is so the vibrations from the speaker won't affect the tubes in the amp. Do you still get the same harsh sounds? If so, a microphonic tube probably isn't the culprit.
It's just a sad fact of life that a high proportion of tubes are at least somewhat microphonic. Sometimes a fairly stiff piece of 3/8" - 1/2" closed-cell foam can help. Cut a strip a couple inches wide, and long enough to cover your problem tube plus one or two adjacent tubes. Cut holes so that the foam will fit snugly over the tubes so as to dampen any vibrations. Don't do this with power tubes, though -- only preamp tubes. Power tubes get too hot.
It can also help to re-mount the offending tube socket on rubber grommets, so as to "shock-mount" the socket.
Finally, if all else fails, consider using your amp as a head, with your speaker in a separate cabinet.
John |
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Chris Bauer
From:
Nashville, TN USA
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Posted 22 Mar 2014 1:16 pm
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| Thanks, John. This amp is actually a head and the issue happens whether the amp is on a cabinet or several feet away. That tell you anything or narrow down what I ought to try next? |
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Stephen Cowell
From:
Round Rock, Texas, USA
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Posted 22 Mar 2014 1:24 pm
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It might help us to know the amp and your guitar/system. Starting to sound like parasitics to me.
_________________
Too much junk to list... always getting more. |
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Chris Bauer
From:
Nashville, TN USA
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Posted 22 Mar 2014 1:39 pm
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Email sent with details. Thanks, Stephen.
(I'm usually hesitant to mention brand names on the forum only because it inevitably seems to derail the thread into some kind of argument over who likes what better than what someone else happens to like.) |
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Tim Marcus
From:
San Francisco, CA
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Posted 22 Mar 2014 2:22 pm
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if its what I think it is, and it uses a 6SC7 in the preamp… best of luck 
If I were you, I would take it to a good tech and have him re-wire the socket for 6SL7. Or buy a socket adapter if you can find one. That is a much more reliable tube and had less issues with microphonics. They even make them in Russia to this day - they are not reliable compared to the mighty mini dual triode 12AX7 but if you desire that octal preamp sound its a better place to start.
The Sarno V-8 is based off of 6SL7 (or maybe its 6NS7) and the FYD octal amps are as well.
Fender stopped using 6SC7 in the 1950's for a good reason: its nearly impossible to find a good one. I think the metal bottle ones are supposed to be better.
_________________
Milkmansound.com |
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Chris Bauer
From:
Nashville, TN USA
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Posted 22 Mar 2014 2:38 pm
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| There's both a 6SC7 and 6SL7 onboard. Rectifier tube is a 5U4 and the power tubes are 6L6s. |
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rodger_mcbride
From:
Minnesota
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Posted 22 Mar 2014 3:02 pm
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Call or email Tyler at TCtubes.com and ask him to try to find you a quiet one. He tests them for microphonics and is no stranger to those of us that use them as a preamp tubes. Quiet 6SC7's are really difficult to find. Sometimes a microphonic PU will find a sympathetic resonance with the tube and complicate things. While the 6SL7 & 6SN7 are great, IMHO there is nothing like a good 6SC7 for that almost 3d tone.
rodger |
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John Polstra
From:
Lopez Island, WA, USA
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Posted 22 Mar 2014 3:10 pm
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| Chris Bauer wrote: |
| Thanks, John. This amp is actually a head and the issue happens whether the amp is on a cabinet or several feet away. That tell you anything or narrow down what I ought to try next? |
If the issue happens even when the amp is several feet away, I think it's probably not caused by a microphonic tube (unless you're playing really loud). If you want to double check, play so you're hearing the problem. Then have a friend / spouse / well-trained dog grab the offending tube, squeezing just hard enough to dampen any vibrations. If the tube is microphonic, that should make the bad sounds stop or at least change.
Any chance you could put a recording on the web somewhere so we could hear what you're hearing?
John |
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Tim Marcus
From:
San Francisco, CA
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Posted 22 Mar 2014 4:15 pm
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an amp feeding back in proximity to a speaker may also be caused by a miswired output transformer.
don't ask how I know this  … but if one were to wire the output transformer backwards, and there is global negative feedback in the circuit, negative feedback (good) becomes positive feedback (bad) and that will regenerate and sound similar to a microphonic tube when its near a speaker.
_________________
Milkmansound.com |
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Chris Bauer
From:
Nashville, TN USA
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Posted 22 Mar 2014 4:18 pm
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Okay, you win, Tim. I won't ask how you know.
But I gather that's something that would appear right away, yes, and not after the amp has been used for a while. (?) |
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Tim Marcus
From:
San Francisco, CA
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Posted 23 Mar 2014 10:07 am
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true - its likely the 6SC7 acting up because the structure has become loose in the bottle
_________________
Milkmansound.com |
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John Billings
From:
Ohio, USA
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Posted 23 Mar 2014 11:07 am
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| 6SC7s are notorious. Whatever you do, DON'T buy an adapter that allows use of a 12AX7! Tried that with my '52 Pro Amp, and it just killed the good sound! |
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Chris Bauer
From:
Nashville, TN USA
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Posted 23 Mar 2014 11:55 am
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| I'm trying to figure out how many 6SC7s I go through before deciding that it's actually something else... I'm up to number nine. I got a lot of them and I'm wanting to think that they can't all be bad. (?!) |
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Mark Fowler
From:
Minnesota, USA
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Posted 23 Mar 2014 12:37 pm
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I seriously doubt it is a tube problem I run 6SC7 tubes and occasional get a bad one or microphonic. I always use o-rings to help with vibration.
Sounds like some other issues which Tim has already noted or it could be a bad filter cap which will make the amp sound out of tune. Coupling caps are few on this amp but one could still go sour or bad solder joint.
Mark Fowler
FVA |
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Bob Lawrence
From:
Beaver Bank, Nova Scotia, Canada
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Posted 23 Mar 2014 12:53 pm
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I seem to be having technical difficulties posting 
Last edited by Bob Lawrence on 23 Mar 2014 1:01 pm; edited 3 times in total |
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Bob Lawrence
From:
Beaver Bank, Nova Scotia, Canada
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Posted 23 Mar 2014 12:54 pm
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Chris,
It may not be microphonics your dealing with. If you are unsure have a look at the video demo on the Tube Depot website "What is a Microphonic Tube?" It also demonstrates how you can test the tube for microphonics. I tried to post the URL for the video but the forum won't accept it
As always, when working on tube equipment remember SAFETY FIRST and if your unsure about what you are doing find a amp tech.
If after swapping out 2 or 3 tubes don't solve the issue I would look elsewhere, as suggested by others. |
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John Billings
From:
Ohio, USA
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Posted 23 Mar 2014 2:37 pm
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"If after swapping out 2 or 3 tubes don't solve the issue I would look elsewhere, as suggested by others.
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Took about ten tubes for me to find good ones for my amp. And the good ones SING! |
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