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Microphonic chassis phenomenon

7/19/2004 2:01 PM
Matt Microphonic chassis phenomenon
I built a 'tweaked-for-guitar' version of the 6G6B circuit into the Bass Channel side of my late BF Bassman. The Normal channel is just regular BF AB763 non-tremolo/reverb type of circuit. V2 or the associated circuitry is microphonic. I've tried several different tubes but no change. I've tried moving the treble circuit to the last gain stage and even removing it, it got worse when I removed it. I tried a 500pF cap across the plate load resistor and swapped for a new resistor, no change. I've tried changing the B+ nodes a few different ways, no change. Originally, I used a 12AX7 in V2 but it was too much gain and more microphonic. The circuit with the 12AY7 sounds really, really good. But I can't get rid of the microphonic chassis phenomenon. I can hear it when I tap on the tube shield of V2. A link to the schematic is below, the power amp been changed to a regular BF "guitar amp" circuit a long time ago, so I'm pretty sure that part is good. Any ideas? TIA!
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7/19/2004 9:14 PM
Dai Hirokawa

it would seem to me that it's the *tube* that's picking up the microphony, but?
7/20/2004 12:30 AM
Yeah, the schematic won't help us much on microphonics. Lots of things can get that way. Tubes are most likely, but ceramic caps can turn into little piezo mics. Coax can be microphonic even. Tap on every part with something insulated.
7/20/2004 12:01 PM
I've tapped around a little bit and just about everything in that area of circuitry is somewhat microphonic. When I check the plate voltage on V2a, I get a very loud 'bump' at the speaker. Even poking on the GROUND side of the cathode resistor/cap combo gives me noise, and I've resoldered/moved/checked that ground several times. The wire to the grid of V2a is coax, I'll check it. No cermaic disks in that area, I don't think there are any in the amp at all IIRC. I'll poke around some more and see if I can determine anything.  
7/20/2004 2:25 PM
Rob Mercure

Hey Matt,  
First off you can remelt all of the solder joints on the eyelet card - often times one of the leads in an eyelet isn't perzactly securely soldered. And then there was a thread a few years ago on this forum concerning microphonic eyelet cards - some just seem to be such. I don't remember if there was a consensus for a "universal" solution but you might try removing the tubes and baking the chassis in an oven at about 120 degrees F for a couple of hours to absolutely remove any moisture. Then, if this doesn't cure the problem (still assuming that you remelted the solder joints) you might flush the circuit card repeatedly in a solvent such as anyhydrous isopropyl alcohol (not rubbing alcohol which is 30% water) or electronic contact cleaner - sans lubricant. Let the board dry thoroughly and see what happens. But I do seem to remember that there were just some boards that didn't yeild to any "cure" and had to be replaced.  
7/20/2004 2:55 PM
Thanks Rob. I had sort of suspected that this might be the sort of problem I may be up against. What a pain that will be! I have access to pure IPA, or mole seive pellets to dry rubbing alcohol if I have to.  
Now that I think about it, I always had a balance problem in the PI/power amp. Something was always pulling down one side of the PI and I've rebuilt that area at least once. The 82k side always has less plate voltage than the 100k side, by 20-25 volts (it's not the cap, I've replaced it more than once). It's that grid that is pulling it down but I can't figure out why. When cranked, you can see one power tube doing most of the work (grid going crazy inside). I even lifted the board out and sucked every bit of extra solder from underneath and resoldered every connection. Waste of time other than learning how much extra solder was in there (a LOT!). This amp has always been a mystery, even though it has always sounded really good. Might be time to break out the G10.
7/20/2004 10:37 PM
Rob Mercure

The old thread that I referred to was a "spin off" from a thread about conductive eyelet boards - apparently some Fender boards like a little current flow. Generally this is due to metallic salts forming from atmospheric corrosive elements/compounds - seaside air, heavy pollution, working on the metals within the chassis. Still, baking and solvent flushing usually take care of this (still recommend melting the solder in ALL of the eyelets first). But there were some cards that had specific conductive paths - still fairly high resistance - that could be found with a ohmmeter - and cut in two by scoring the board with an X-Acto knife.  
A lot of work but perhaps worth it. While I've no major complaints with the Fender style eyelet board - after all the number of microphonic/conductive boards is relatively small with these long-lived amps - I still like mil-style turret boards. I've never seen a phenolic turret board conduct or carbon track and they're too damned stiff to move enough to affect a signal even if they're microphonic. So, if you wind up replacing the board you might consider replacing the eyelet board with a turret board - no stray blobs of solder underneath to worry about.  

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