microphonic tubes...?

[withmittens]

I have some microphonic 12ax7's somewhere. I know this because I hear a high pitched rattle or sustained clanging when I play. also the input jack can be heard through the signal when tapped with my finger and the amp is on. I don;t know if the two things are related.

my question is, which tubes are most likely to be heard if microphonic, and what tubes to buy to avoid the issue?

I am guessing V1 in amp can be a major cause, what about the loop tubes?

can both tubes in mods be an issue? that's 7 tubes minimum to track down and replace, test, etc....

does anyone have a procedure that helps solve this? maybe it's not the tubes, maybe something else?

Thx

 

[rblyn]

You've got to get a new tube (that you know is not microphonic)- and replace one at a time with it.....just a process of elimination. V1 and the PI tubes seem to first. The PI works pretty hard- I change that one often...I also had the 12Ax7 that buffers the effects loop go whacko on me once too. Never know- Just start replacing one at a time- You'll find it....

 

[withmittens]

I have a box of 12axx tubes, ugh, dreading testing all of them.

any idea about the guitar input area coming through the amp when tapped? even when I tap the faceplate with my finger nest to the jack......

 

[withmittens]

i found a bunch of old Telefunken 12ax7's put them everywhere. wow, what a sound. even in the loops, I changed one at at time, i noticed a richness and sustain that wasn't there before. probably wasting them however

 

[kc2eeb]

Of course, this may be taken as heresy, but, tubes sound good because they are microphonic. Those little gray plates and cathodes physically vibrate when you play. The trick is is to weed out the tubes that are a little too microphonic.

 

[Oregon]

old Telefunkens? Yeah don't ever let any tube junkies know what you've done with them lol... they'll come after ya with tar and feathers! :lol:

I remember doing my first power amp tube replacement search for my old T100 head, and found a serious tube guy in SLC (where I lived for a time) and he sold me 4 killer NOS Sylvania 6L6 beauties - then he asked what I wanted them for... and almost took em back! LOL I held on tight though

 

[Jaded Faith]

Do you have a tube shield on the V1 tube in the amp? If not, put one on it. I had this exact issue setting up one of the amps for our booth here at the LA Amp Show yesterday morning. Move a shield onto V1 from the loop tube and its been flawless ever since.

For testing tubes to locate the offender you can always tap test them with a chopstick or pencil. Keep in mind there will always be some noise, you are just looking for one that's way out there. Remember V1 should naturally exhibit more noise in the test because the signal cascades through all the others behind it.

Rob

 

[withmittens]

I'll put the shield back on, no I find less microphonics put still a large amount of hiss. when i bypass the preamp with my Pod it stops so it must be in the one of the 3 preamp tubes. I guess the parallel loop tube since the Pod is in the series. hiss is present in both installed mods, also a bit if popping. it remains consistent when switching channels

 

[withmittens]

no change with the shield though, thanks

if it isn't tubes it's beyond me what it is....

 

[kc2eeb]

Try a different guitar and cable.

 

[withmittens]

so, a few tubes were microphonic, or too much I guess. The Telefunkens sound astonishingly good as do the old RCA tubes. Good enough that I'll go to Ebay for more. I mean, they sound incredibly good.

I was looking at old EI tubes and they are more expensive than a batch of tested and odd telefukens or RCA's. you know, one of those odd batches of six, maybe $10 each or less. and since they don't need to match......just test for leaks and strength. The ones the tube snobs won't touch.

I think I had 1 bad cable. The biggest source of noise was my Pod connected to my computer via USB. I had no idea a USB connection could add so much noise.

The other revelations just make the amp sound much better.

thanks for the input