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Old April 26th, 2012, 08:19 PM   #1 (permalink)
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Microphonic tubes? or something else...

Hey guys,

I know this may seem like a stupid topic to start, but this problem has seriously been haunting me for years. My tube amps always turn tubes microphonic- it used to always be with my crate v30 (a great little american made class A el84 combo I bought for $300) but it was a hotbox and the tubes were placed right over the speaker... it turned tubes microphonic left and right. Now I've had the problem again as of recent.

There is this rattling sound in my amp on certain noises- i'm almost positive its a microphonic tube, but I cannot figure out which ones are microphonic and I've tried many combinations of my collection of 12 preamp tubes consisting of tung sol, mullard, jj, sovtek reissues- and recently I bought some nice NOS tubes from KCA tubes. RCA triple mica black plate 5751, 80's jan 5751, and a sylvania 7025- i also have a nos jan philips 12ax7wa, nos jan philips 12at7 (not a very good tube at all- all the big tube stores are selling em for $10), i also have two unidentified "USA" 7025's with nice looking internal stucture... along with my reissue tubes I cannot for the life of me figure out which ones are microphonic.

Could this be something else? I've had noise from a loosely connected filter cap before that didnt sound anything like this. I've changed power tubes and rectifer tube, I've resoldered all sorts of connections. The weird thing is my high e string up high on the fretboard sounds really weird... like ring modulated... but its very slight, however if you're listening closely and slamming on a bend up there it's very harsh and piercing. I havent tried every combination of tubes, it's difficult with the pro taking 6 preamp tubes.

Does anyone have any suggestions...
Could a tube tester help me locate which tubes I should throw out? I've figured this out myself... as some of them just stopped working (the collection used to be 15)
The tap test is kinda difficult for me to judge, I mean its obvious when one is REALLY microphonic- but this isnt like terribly obvious, just mild and annoying

I've seen shock mount tube sockets before. Does anyone know of anything like this?
Should I try a 12ay7 or 12au7 in my reverb driver slot? for some reason I think this may be the problem and I dont have any very good 12at7's right now.
I've also been running a 12ax7 or 5751 in the phase inverter.

Sorry for posting such a simple tech problem, it's just that this is driving me insane. It has been a major problem continuously for years for me... sometimes hardly noticeable, and unhearable in a band, sometimes infuriatingly impossible to cure. I have recently(almost) completely overhauled my amp with everything from fitler caps, cathode caps and resistors, plate and slope resistors, phase inverter section, reverb components, tons of stuff! and it has never had a better cranked sound. It's so close to having that perfect blackface on 6 sound!

Any advice would be wonderful!

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Old April 26th, 2012, 08:46 PM   #2 (permalink)
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with the amp on, tap on the tubes with the erasure end of a pencil. that should give you a good indication of if one of the tubes is microphonic. If none are louder then the others, try disconnecting the speaker if you can and plugging into an ext cab if you can. If that does not cure the problem, almost certainly it is not a microphonics issue.
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Old April 26th, 2012, 10:18 PM   #3 (permalink)
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If you have the same problem with all those tubes there must be something else going on. Isolate the problem circuit.

I'm assuming it's a BF/SF amp? If so, first isolate the power section from the preamp by taking out the PI tube (V6) with the power off. You may have already done this. Jostle the amp a little to see if the power tubes make noise.
Continue along those lines to narrow down exactly what part of the amp is making all that noise. I'll assume you know which tube is doing what. Leave out V5 for simplicity while you troubleshoot.

Also, try a 12AU7 in the reverb send socket. They tolerate a lot more current than 12AY7s and that slot is very tough on tubes, super high voltage. Plus they are cheap as dirt.

I suspect you will have to open up the chassis & fix some little thing, or take it to a tech if you're not comfortable doing that.
A tube tester will not detect a microphonic tube.
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Old April 26th, 2012, 11:36 PM   #4 (permalink)
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with the amp on, tap on the tubes with the erasure end of a pencil. that should give you a good indication of if one of the tubes is microphonic. If none are louder then the others, try disconnecting the speaker if you can and plugging into an ext cab if you can. If that does not cure the problem, almost certainly it is not a microphonics issue.
the first gain stage tube is always going to be louder than any other tube you tap on... and i feel the tap test only works when v1 is microphonic.
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Old April 26th, 2012, 11:42 PM   #5 (permalink)
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If you have the same problem with all those tubes there must be something else going on. Isolate the problem circuit.

I'm assuming it's a BF/SF amp? If so, first isolate the power section from the preamp by taking out the PI tube (V6) with the power off. You may have already done this. Jostle the amp a little to see if the power tubes make noise.
Continue along those lines to narrow down exactly what part of the amp is making all that noise. I'll assume you know which tube is doing what. Leave out V5 for simplicity while you troubleshoot.

Also, try a 12AU7 in the reverb send socket. They tolerate a lot more current than 12AY7s and that slot is very tough on tubes, super high voltage. Plus they are cheap as dirt.

I suspect you will have to open up the chassis & fix some little thing, or take it to a tech if you're not comfortable doing that.
A tube tester will not detect a microphonic tube.
I'm thinking the problem is my reverb driver tube because I only have 1 12at7- it is brand new from the tubestore (a nos jan 12at7 to boot) and I paid $4 for extra testing for low noise and microphonics. So I was hoping they arent THAT unreliable as a store. I do feel I'd like a lower gain reverb driver too... I feel like my reverb pan gets hit too hard for some reason.,.. just the sound of my verb makes me feel that.

I do not understand the first test you are talking about... with the power off how will I hear anything... you're explanation of the process does not make any sense to me at all. I'd appreciate a rewording if ya dont mind. Thanks!

I do feel comfortable working on stuff... as I said I've completely overhauled my amp... I've heard a few loose connections in my day... but they dont sound like this. The high e thing is strange though. I wish there was an easier way to test capacitors with my multimeter... I cant use my test leads on em, I dont know if thats the same with all dmm's, is it?
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Old April 27th, 2012, 03:33 AM   #6 (permalink)
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If it is only your reverb driver, turn the reverb all the way down or D/C the tank, to isolate the problem. That tube draws a LOT of current and frequently the cathode resistor fails on it. Check your V3 cathode voltage for the nominal @10v on there. It's a good idea to use a 1w or greater rated cathode resistor anyway on v3.

I just meant power off when removing the tube. Turn the amp on for the test.

I hate troubleshooting these little noises, it is such a PITA. My guitar's tone cap was bad but that didn't stop me from rebuilding my practice amp 3 times to get rid of the sound. Are you sure it's not that, or something like that?

I had a nasty sound too, not just like yours but similar in that it came & went, & sometimes made the guitar unplayable. I figured, "It's just a bleed cap to ground, it couldn't be making that noise". I blamed my amps, cords, even replaced the pickups & switch when all I needed was to replace a $1 cap in less than 5 minutes.
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Old April 27th, 2012, 11:22 AM   #7 (permalink)
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Originally Posted by claytushaywood View Post
the first gain stage tube is always going to be louder than any other tube you tap on... and i feel the tap test only works when v1 is microphonic.
Clay, you are correct with the observation that V1 exhibits the most amplification in the chain. However, it is not accurate that the tap test is useful only in judging the qualities of the tube in V1. IT takes a bit of experience to find one microphonic tube in the chain...because the microphony is exhibited throughout the chain to some extent, but the tap test will/can lead you to the problem of a microphonic tube.
Be aware also that microphony can originate in components other than tubes. Tap tests with a non-conductive probe (Chopstick?) on the components on the circuit board can also reveal microphononic problems there.
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Old April 27th, 2012, 08:07 PM   #8 (permalink)
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Not sure if this will be helpful... or even remotely close to your solution, but I had a rattling noise in my 5e3 shortly after build. I found the unused pins on the 6v6 sockets were rattling in the ceramic sockets causing a "glassy" sound at certain frequencies. It took me swapping some tubes to begin looking in this direction. I just removed the unused pins and no more noise. Ya never know, it could be something simple. Good Luck
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Old April 27th, 2012, 09:29 PM   #9 (permalink)
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I found a source of some noise, a resistor I found by tapping components in my amp. Sounded like it was microphonic. I still have the noise though. I'm gonna check my guitar wiring for the hell of it tonight anyways. Was thinking about installing a shielded wire from pickup selector to output jack. would that be useful in a strat?

I've checked reverb cables, reverb tank, normal circuit... etc... etc... I found one culprit. This is making me wanna get an external cab and just throw my amp on top of it.

So...I definitely need to help less the vibration in my amp. I like cranking this amp up and it vibrates like crazy when I do. I hate the tube shield spring things on old fender amps. I feel like the metal on glass contact makes the amp sound weird.
Here's some ideas for reducing speaker to chassis and tube vibration in a combo amp

I was thinking. I have o-ring tube vibration dampeners on the way. I was thinking I could also put some hi temp rubber on top of the spring in the tube shields and the 6l6 spring retainers.

Shock mount some of the preamp tubes from the chassis using rubber grometts between the base and the chassis or the screw head... something like that... I've seen it done before, but cant remember where.

I could also maybe shock mount the chassis to the cabinet. Fender's use the hanging method where the chassis hangs from the top of the cabinet with long bolts that go through the amp. Putting some rubber grommets in between the underside of the top of the cabinet and the top of the chassis as well as between the bolt's nut and the bottom of the chassis. This seems like it could reduce vibration transferring from the cabinet to the chassis by a nice amount.

anyone have any experience with this?

Also my reverb seems like its getting too much signal or maybe the reverb recovery is too loud... actually the tap test is now showing the loudest tube to be my reverb recovery tube... even throughout tube switches... I just installed a medium decay 3 spring accutronics tank, and i gotta say the reverb decay is still VERY long... no difference that I can tell between my old 2 spring long decay. little better sounding though. I was thinking of trying a 12au7 in the reverb driver position, but I was also wondering if its cool to use a lower gain tube in the reverb recovery position as well- or even with a 12au7 in the driver?
Am I supposed to leave the foam piece that came with the tank in there?

I think some new tube sockets and doing some shock mounting with some lower gain tubes might get me on my way... what do you guys think?
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Old April 27th, 2012, 09:31 PM   #10 (permalink)
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If it is only your reverb driver, turn the reverb all the way down or D/C the tank, to isolate the problem. That tube draws a LOT of current and frequently the cathode resistor fails on it. .
I've actually had the cathode resistor fail on me previously, and the previous owner had changed it as well. I'll put a 1 watt in there. Is there anything I could do to the reverb circuit to cool it down some? It definitely sounds like it could use some cooling down... but I actually dont get that "wet" sound that you often hear with fender reverbs. is that usually just with the external reverb box going into the input?
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Old April 28th, 2012, 07:26 PM   #11 (permalink)
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Okay. I think I have seen this before.
1- the reverb send cathode resistor has blown before-twice.
2- kind of a ring modulator sound on certain notes.

I fixed the reverb on my drummer's TRRI & replaced the cathode resistor, only it was already a 1w resistor, which had blown... I felt like The Man. But there was a problem.
When the amp was turned up it sounded crappy, and kind of like a ring modulator.

I was stumped.

But you have to remember that 3 gain stages makes it technically a high gain amp.
A Parasitic oscillation was throwing off the V3 bias, causing huge current draw in V3. I don't know how but the grid voltage on V3 was just way off. I jiggled the V2 wires and separated the wires between V2a & V2b- magic fix. All voltages nominal & the amp sounds terrific.

Check your lead dress to V1 & V2. V3 & V4 for good measure. Make sure you minimize crosstalk between stages, not just tubes. The plate & cathode wires for a given stage can be right next to each other & even provide a measure of noise cancellation if they are.

And remove the foam from the tank. Keep the old tank because it's probably good.

Last edited by firemedic; April 29th, 2012 at 11:01 AM.
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Old May 3rd, 2012, 01:57 AM   #12 (permalink)
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Okay. I think I have seen this before.
1- the reverb send cathode resistor has blown before-twice.
2- kind of a ring modulator sound on certain notes.

I fixed the reverb on my drummer's TRRI & replaced the cathode resistor, only it was already a 1w resistor, which had blown... I felt like The Man. But there was a problem.
When the amp was turned up it sounded crappy, and kind of like a ring modulator.

I was stumped.

But you have to remember that 3 gain stages makes it technically a high gain amp.
A Parasitic oscillation was throwing off the V3 bias, causing huge current draw in V3. I don't know how but the grid voltage on V3 was just way off. I jiggled the V2 wires and separated the wires between V2a & V2b- magic fix. All voltages nominal & the amp sounds terrific.

Check your lead dress to V1 & V2. V3 & V4 for good measure. Make sure you minimize crosstalk between stages, not just tubes. The plate & cathode wires for a given stage can be right next to each other & even provide a measure of noise cancellation if they are.

And remove the foam from the tank. Keep the old tank because it's probably good.
Yeh i'm getting that ring mod sound, but its only on higher notes up the fretboard. What do you mean seperate the wires between v2a and v2b?
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Old May 3rd, 2012, 02:04 AM   #13 (permalink)
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I think I know what you're talking about like just make sure the wires that are coming off the different side of the v2 tube are not close? but could you explain in detail a little more?

I am also curious about the heater wires... I remember reading somewhere that they are supposed to alternate between which pin they go to between tubes for better noise cancellation. Is that right? mine are twisted but they go to the same pin---except for between the v3 and v4. Could that be a problem?
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Old May 3rd, 2012, 06:05 PM   #14 (permalink)
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Unless there is a big hum, the heaters are likely fine.

Parasitic oscillation (or whatever this is) can be a real b**** to track down. One way to prevent it is to make sure the wires to pins 1,2,3 are together & away from the wires to pins 6,7,8.

Do this with all the preamp tubes that are standard gain stages. In a BF/SF amp that's V1, V2, V4. This may be the problem; I mentioned it because it's a free & relatively easy thing to try.
Also, since it sounds like you know enough to poke around the amp's guts, it seems likely some wires may have been inadvertently moved to a noisy place- god knows I've done that enough.
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Old May 19th, 2012, 10:34 AM   #15 (permalink)
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Unless there is a big hum, the heaters are likely fine.

Parasitic oscillation (or whatever this is) can be a real b**** to track down. One way to prevent it is to make sure the wires to pins 1,2,3 are together & away from the wires to pins 6,7,8.

Do this with all the preamp tubes that are standard gain stages. In a BF/SF amp that's V1, V2, V4. This may be the problem; I mentioned it because it's a free & relatively easy thing to try.
Also, since it sounds like you know enough to poke around the amp's guts, it seems likely some wires may have been inadvertently moved to a noisy place- god knows I've done that enough.
Sorry for the late reply- been out of town- just got back and have been messing with moving wires around. It would be really helpful to see the gutshots of a blackface or silverface non master fender amp and compare to mine. Mine is a silverface non master that I have converted to blackface specs- it has this ugly as sin wiring throughout it, super thick insulation and thin wire underneath. I just got some nice insulated and cloth covered wire from weber in 18g I believe. Would it be worth the time to replace some of the wiring in the amp and try to do better lead dress like the pre cbs blackface amps?

Also, I just noticed that my reverb recovery tube has turned white on the tip (on the inside- where it's normally black)! The reverb recovery tube is also the most microphonic when doing the tap test... even a little bit more than v1 or v2! I have changed out a bunch of tubes in this position.

Is there anything else I can do to help stiffen this part of the amp? I did previously increase some coupling caps to allow more bass response and warm up the amp (nothing major) dont know if that could be contributing?

I have had the cathode bypass resistor burn out before- and I do have a 1 watt in there now... can I increase any values to help it out? 2 watt increase? Or is this all the wiring's fault?

All my A vs B sides of the tubes are well away from each other... I think. Anyone have some good pics of outstanding blackface lead dress?

Thanks a bunch for the help guys!
This is driving me nuts!
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Old May 19th, 2012, 10:43 AM   #16 (permalink)
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I just noticed that my reverb recovery tube has turned white on the tip (on the inside- where it's normally black)! The reverb recovery tube is also the most microphonic when doing the tap test... even a little bit more than v1 or v2! I have changed out a bunch of tubes in this position.
Cool! So that one is dead. White tube = dead. No exceptions! It's cool because we can elminate that tube as a problem and reduce the population of potential problem tubes.

The tap test: Stick the tube in V1, first preamp stage. Turn the amp volume all the way up. Most people tap the tube with the eraser end of a pencil but I ain't most people. I just give it a little finger kick. If it makes a "ping" it's microphonic. It's up to you to decide how microphonic is acceptable.


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Old May 20th, 2012, 01:55 PM   #17 (permalink)
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ok... the tube was actually in my tremolo spot- which i've never used the tremolo. and i thought they lasted forever there anyways.... Weird, considering I havent moved it or anything. just burnt up! it was an old nos tube that had been kinda microphonic previously so it got moved to the trem spot.

anyone got any ideas on wiring? A lot of the wiring in this amp doesnt go through the holes so it gets routed under the board like blackfaces do. paprtially due to having thickk shielded cable from the input to first gain stage and from first gain stage to tone stack on both channels. all the front panel wires are jammed up against the grounding plate- they are twisted for the most part.

Are any sections more important to twist and route like a blackface? or anything that needs to go under the board? Or should I just try something else... or rewire the whole damn thing!
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Old May 20th, 2012, 04:56 PM   #18 (permalink)
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the first gain stage tube is always going to be louder than any other tube you tap on... and i feel the tap test only works when v1 is microphonic.
If all the tubes are functioning normally then you are correct, being earliest in the signal chain the first tube will generally yield the loudest response to mechanical activation.

But what if it's not?

Suppose V2 is significantly louder than the tube that feeds into it?

Wouldn't that be a pretty good indication of a problem there?

Better yet, what if you get a loud response from a stage that is supposed to be at unity gain?

Edit: Also, tremolo (low frequency oscillator) is about the hardest wear a 12A_7 tube sees (but yours probably failed from the heating/cooling cycles alone.)
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Old June 6th, 2012, 08:55 PM   #19 (permalink)
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If all the tubes are functioning normally then you are correct, being earliest in the signal chain the first tube will generally yield the loudest response to mechanical activation.

But what if it's not?

Suppose V2 is significantly louder than the tube that feeds into it?

Wouldn't that be a pretty good indication of a problem there?

Better yet, what if you get a loud response from a stage that is supposed to be at unity gain?

Edit: Also, tremolo (low frequency oscillator) is about the hardest wear a 12A_7 tube sees (but yours probably failed from the heating/cooling cycles alone.)
its been a while since ive tried tapping... still freakin have this problem after ordering NOS tubes from a very reputable dealer... found two of em were just microphonic though... in other known good amps.

My reverb recovery tube had a really loud response to the tap test... no matter what tube was in there! Is there anyway I can calm down the reverb recovery tube- I tried checking some resistors and components around that tube, but everything looked good. Maybe the reverb choke? I dont know... I've got a 2watt resistor I'm gonna put in paralell with the reverb cathode bypass cap as I've heard that is a tough spot on that resistor (and I've previously had a carbon film burn up that must have burnt up before that one too!) But what about the recovery tube... I'm feeling this is the problem.

because the normal channel sounds almost perfect.

I STILL have nasty distortion on high pitched notes up the fretboard on the high e and b strings. really sounds bad when blasting this amp. I know what a good fender sounds like too. Maybe I should post some sound clips of my amp? Would yall be interested in hearing this thing? Maybe seeing pictures of my lead dress? That might help me out I'd think?

Btw- I took this to a tech and he couldnt fix it, my town is lacking good techs... I could take it to another guy that's reputable, but he charges an arm and a leg (seriously, more than other techs)

Pics? Videos? I'll do it if there's interest in helping, or if yall think it would help you help me!
Thanks!
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