So-called 'microphonic tube' noise

[Dave Sloven]

For a couple of weeks a noise has been bothering me now and then when practicing.  I'd hit a certain frequency and this loud 'bbbbbbrrrrrriiiiiiiiinnnnngggg!' would sound out.  I thought it might be in the amp, so I mentioned it casually to the local amp manufacturer/repairer when I was in his shop asking about some Celestion Classic Lead 80s.  He didn't sell those, and his prices on Eminence speakers were quite high, so I didn't buy anything, but I did mention this noise.  His diagnosis was immediate: 'It's a microphonic tube, man. Those cheap Chinese tubes they use in Peaveys don't last long. Bring it in, I'll sort it out for you.'

Having had some dealings with this guy before and him being well known around town I was skeptical. Some people think he's wonderful but his answer for everything to me seems to be for me to bring it in for an expensive check-over and probably some parts it didn't need (and some have suggested, aren't fitted!).

So one of my friends comes over on Saturday to try his Laney Lionheart through my Orange PPC412 and experiment with my guitars to see how they differ from his to help him decide on some replacement pickups.  The Lionheart has no master volume (I don't know if this is because it's 'vintage' or if its an issue with all Class A amps) so it is quite loud by comparison with my Peavey (which I keep turned down) when he starts playing, as he is not used to the 4x12 cab.  Anyway the first thing I notice that his amp has this so-called 'microphonic' problem too!  I then proceeded to tell him the story about this guy - who he knows well - and we eventually after getting annoyed with this sound (it was doing it a lot more often with his amp and playing) we started moving stuff away from the amp, being convinced at that point that it was something in the room vibrating.  The first suspect was a tall metal floor lamp, as it was directly in front of the amp and the sound was very metallic, such that we suspected it might be the large filament in the light.  Nope.  Then we looked at the painting on the wall with the metal frame, nope.  The music stand, nope. Then I looked where my acoustic guitar was sitting - right in the firing line of the bottom left speaker in my 4x12.  Moved that to the other side of the couch and it stopped!  It was the steel strings on the guitar the whole time!  It only started doing it as I increased the volume a couple of weeks back and my friend was playing louder and making it worse.

You can bet that I told this story about the amp guy's 'microphonic valve' noise at the party that night!

I think we can call this one SOLVED!

[jpfamps]

It's a fairly common problem tracking down the source of vibrations in room with other kit in it.

Microphonic valves are not uncommon, but are easy to diagnose. Often a gentle tap on the side of the offending valve will provoke the noise.

Substituting the valve for a new one should cure the problem; if it doesn't then the valve you swapped out isn't microphonic.

Regardless, there do seem to be tech out there who immediately assume that any problems with valve amps are valve related and start by changing all the valves, whether this is needed or not.

[Dave Sloven]

Yeah it's important to go to someone who is trustworthy and who has a steady flow of work.  The guy I was talking too is going through a bit of a quiet stretch and probably has bigger overheads than my local guy, who works out of his garage.  My local guy is very cool.  I took my guitar to him and he said that it had an issue that should be fixed under warranty before he would do a set-up on it, so he sent me to someone else.  I can't imagine this other guy ever telling me to go elsewhere. He always rubbishes the competition.  Still he is a local character and quite entertaining in his own way - I'm just not sure I'd trust him to work on my stuff.  He generally looks after the bands that practice in his practice rooms though, as he needs their return business.

[timmy_b]

I rehearse in a really tight room, we can usually get our vocalist's mic to feedback through my stack pretty good! Fairly annoying, but at least we know there's no serious problem, just healthy levels of volume :-)

[JJretroTONEGOD]

I had the same problem in my recording studio, because I have the acoustic guitars on wall stands quite near my studio monitors, it took ages to find the source but now I put a cloth under the strings, and it's no longer a problem. I put a lot of acoustic treatment inside my room which tends to make the room sound quieter so you can hear slight noises even more lol both a blessing and a curse.

Microphonic tubes sound awful in fenders by the way, very harsh and nasty, you'd definitely know when it's gone bad.

[fourt6and2]

Also, those "cheap Chinese tubes" your tech was talking about are actually some of the most durable, least microphonic tubes being made right now. The Ruby Shuguang 12AX7 AC5 HG+ is a sturdy tube and some of the lowest noise preamp tubes out there.