tube microphonics

[neilium]

Can differences in operating conditions (plate voltage, bias, grid resistance, and how it is coupled to the next stage) make a tube more or less microphonic? I realize that if you raise the gain, you'll be amplifying more of the microphonics. But is there more to it than that?

The reason I ask is that every time I read about octal preamp tubes in guitar amps, the common gripe is that they are microphonic. I've used an Ampeg Portaflex that didn't have any such problems, and I 've built guitar amps from 3 to 30 watts that have octals in front and I can only think of a couple times where a tube was microphonic.

So, what do you think? I end up running things close to book value; I know a lot of people in the land of rock like to run things hot. Is that the cause for this difference?
-neil

 

[CJ]

Depends on what type of noise. The crackling noise can be minimized by lowering the plate/screen voltages.

I do this on the EF86, as it is famous for turning noisy after time. Run it real low and it will stay quiet for ever.

Most tubes will make a little noise if you tap on them. You can get tube coolers that will dampen microphonics.
I use vinyl grommets on 1st stage preamp sockets.
A trick I stole from the V76.

 

[adrianh]

Build the amps and be happy.
If you find a tube that is very noisey
or microphonic do not use it.
Some tubes brands are more microphonic.
It is a mechanical resonance of the tube
parts inside mostly the grids.
As you increse the mass the frequency gets lower.
So octal tubes are generally worse than miniatures.
Wow physics really works!

Make sure the tube has rubber isolation mounts
on the socket. Do not sweat it.
It is a fact we live with using
"The worlds most perfect amplifying device"

 

[neilium]

I'm not really having problems with microphonics; my question was more like: I'm not having a significant problem with microphonic octal tubes, yet all these guitar amp guys are. What am I doing differently, if anything? I don't buy special tubes; I get mine where everyone else does (Antique Electronics, Angela, and the flea market when I'm lucky.)

I wonder if it has to do with the fact that I prefer to use two medium gain stages in places where most guitar amps have one high gain stage.

I also wonder if "microphonics" has become a blanket term for all the various unintentional noises a tube can make. In which case, lead dress, grounding, RF, and a scadillion other things have to be addressed before laying the blame on a type of tube.

-neil

 

[CJ]

Those guitar amp tubes take a lot of vibration, although they are supopsed to be able to handle 6 zillion G's or whatever the spec is. But still, they take a lot more abuse than a tube in a micpre.

I would think that driving the tube harder would increase the risk of microphonics.

Yes, tube noise seems to get bundled up in the "microphonic" category.

There are other sources for that noise. Heck, I just read that a bad mic pre input transformer can make noise on startup.

 

[Test Point]

I have noticed in my many years of repairing tube amps that the tubes being made today are quite a bit more microphonic than the tubes of yore. I am almost convinced that a large part of that comes from the glass envelope being thinner and also maybe a less dense type of glass. Mesa Boogie actually slips their first preamp tube into a large piece of shrink tube and shrinks it around the envelope of the tube...it seems to help quite a bit.....I have tried it on tubes that I just couldn't get quiet enough for my taste and I have had pretty good results doing this as well. I wouldn't do it on power tubes for pretty obvious reasons....heat retention being one of them...I've also noticed that Fender is putting a fairly dense foam piece around their 1st and 2nd preamp tubes in some of their newer amps.....unfortunately, it looks like most of the tubes made today are "iffy" at best when it comes to microphonics. Of course if the elements are loose inside and that's causing your problems, no amount of exterior deadening will help.

Power tube microphonics are a different animal for the most part...usually these tubes hang right behind the speaker in an amp and get blasted and pounded all night long at a gig.....it's any wonder they last as long as they do....EL84's are the worst...especially when used in a Peavey Classic 30...I've measured their lifespan in days sometimes, especially if the guitarist is a balls out player and plays the amp maxxed all night......almost all power tube microphonics come from the elements getting bashed around inside due to mega sound waves......

TP