Can a tube amp become microphonic?

Dave Zirbel · Post by **Dave Zirbel** » 17 Aug 2000 11:10 am

I've been using a Fender Twin with a JBL E-130. It sounds great but I'm getting some strange vibrating sounds when I hit certain notes. I think it's coming from the pre amp tubes. I replaced the tubes but that didn't fix the problem. I get a similar sound when I tap or rub the tube with the amp on. I'm guessing that the tube is vibrating in the socket and reacts to certain frequencies. Does this condition sound familiar to anyone or am I crazy?

Thanks, Dave Z

Bill Terry · Post by **Bill Terry** » 17 Aug 2000 11:32 am

I've got a Blues Deluxe that does that, but you can't hear it at stage volume. Hold the offending preamp tube while the note sounds and it goes away. Does your twin have the metal shields with the tensioning springs that go over the preamp tubes? If not, that might help.

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Dave Zirbel · Post by **Dave Zirbel** » 17 Aug 2000 12:20 pm

I have the tube shields with the springs and you're right, it's not a problem at stage volume but I'm concerned about recording with it. Thanks.

Bob Metzger · Post by **Bob Metzger** » 17 Aug 2000 1:41 pm

Is it a power tube or a preamp tube (or a phase inverter tube) ? If you replaced the tube, it must be coming from elsewhere, unless the new tube is microphonic also. Sometimes, shielded wiring on the grids of the 1st two preamp tubes helps this!

Bob[This message was edited by Bob Metzger on 17 August 2000 at 02:43 PM.]

Dave Zirbel · Post by **Dave Zirbel** » 17 Aug 2000 2:41 pm

Bob, it seemed to me to be the 3 preamp tubes on the left, farthest from the power amp tubes. I didn't replace them with brand new tubes, I took them from another amp that doesn't have a lot of hours. Maybe I'll try replacing the other tubes.

DZ

Jay Ganz · Post by **Jay Ganz** » 17 Aug 2000 2:48 pm

Does it do it when you tap the tubes with the
volume down? If it goes away when you
turn it down, then it's associated only with
those first 2 tubes (12AX7's). The first is
for the normal channel - the second is for
the reverb channel.

Donny Hinson · Post by **Donny Hinson** » 17 Aug 2000 3:22 pm

The First stage of that amp has a lot of gain, and any tube will be a little noisy. They vary quite a bit though, so you just have to try different ones until you find a quiet one. You might also try a 7025, if you can find one. This is a kind of a low-noise, Hi-Ri version of the 12AX7A, and were often used instead of the 12AX7A in Hi-Fi gear.

Dave Zirbel · Post by **Dave Zirbel** » 17 Aug 2000 3:28 pm

I have a 7025. Should I try using it for the second preamp tube (two over from the power tubes)?

Donny Hinson · Post by **Donny Hinson** » 18 Aug 2000 1:50 am

First Dave, I would try it to replace the second tube (which is usually the "noisyest" one). But whatever tube "bongs" the loudest when you thump it is probably the one causing the problem.

B. Greg Jones · Post by **B. Greg Jones** » 18 Aug 2000 10:23 pm

Dave, does it make the noise with the reverb on? If so, turn off the reverb and see if the problem still exists. If it does it probably the reverb driver tube (3rd from the left next to the reverb transformer. It is usually a 12AT7 in Fender amps. I have an old twin that had this problem and this took care of it. Also, I've had bad luck with Chinese tubes (noisy, microphonic, short life span.) I would for sure use NOS American made tubes or European made for replacement. These are available from Groove Tubes. They are a little more money but well worth the added expense!! Old twins are tough to beat so keep good tubes in them. The amp will love you for it! Hope this is of some help to you, Good Luck!!

B. Greg Jones · Post by **B. Greg Jones** » 19 Aug 2000 9:31 pm

Whoops! Sorry I made a big mistake! The reverb driver tube is the 3rd tube from the right not the left looking at the back of the amp. Sorry about that I must have been setting on my brain at that particular moment!
Greg

C Dixon · Post by **C Dixon** » 20 Aug 2000 5:42 am

"Microphonic" tubes have been the plague and nightmare of any old time electronic's technicians. I hated with everything in me to get an amp in when I used to service amps, with your complaint.

Sight you a bit of trivia. I have tried at times over a dozen different brand new 7025's (the quietist of them all) to find one that had no microphonics. It is the nature of the beast so to speak. I found that Sylvannia tubes were the quietest. I don't even know if that company still exists now.

All the suggestions are good as listed by the various posters. But I gotta tell you, it is not an easy thing to cure. I have fought them for hours trying to get a set of tubes to be quiet.

Good luck and may Our Precious Lord help you with your problem,

carl[This message was edited by C Dixon on 20 August 2000 at 06:44 AM.]

Jack Stoner · Post by **Jack Stoner** » 20 Aug 2000 6:29 am

Carl, I've been told that there is NO US production of tubes anymore. The mass market is no longer there and like most other products if there's no incentive to produce a product they will cease US production. The last US production plant was sold, lock stock and barrel, to the Russians.

I tended to like RCA tubes better when I was in the amp service business. The RCA 6L6GC's were the best on the market at the time. I did tend to go with the 7025's for the preamp's as they were a little less noisy.

Like Carl said, the microphonic tubes have been a bane for both the consumer and service, although since they must be replaced they were a boon to the service (sell more parts..).

However, I have found that poor tube seating or dirty contacts in either the tube socket or the tube pins will also act and sound similar to a microphonic tube. The first thing to do when you suspect one is microphonic is to unplug it and plug it back in (turn the power off while you unplug and plug the tube back in - it won't hurt the tube but sometimes you will get a pop that could blow a speaker) and see if that corrects it. Also make sure you keep the spring loaded metal covers (shields) on the tubes.