Preamp tubes always become microphonic

[Resonator Guy]

I built a Mojo 5E3 a couple of years ago, and got some great help here debugging it.
I don't use it a lot (I have a pile of amps I roll through), and I don't play loud. I am having a problem with the first pre-amp tube, which is supposed to be a 12AY7. They get unpleasantly microphonic after what I feel is a relatively short time. I have tried 12AX7s, 12AT7s and I have a 12AV7 in there now, which at the moment sounds magnificent. Usually after a few months they start squacking like Long John Silver's parrot.

Does too high a voltage cause tubes to become microphonic? Is there anything I can look for to fix this hassle, or will I just have to keep switching tubes every so often?

I appreciate any advice.

[jrfrond]

Combos can rattle tubes and cause them to become prematurely microphonic quicker than separate head amps, because they sit near a vibrational source. There's not really much you can do, seeing as a good 50% of the new stock 12A_7 tubes I've seen in the last decade or so are microphonic to some degree out of the box.

[Prairie Dawg]

John, what do you think about what will happen to the tubes in an SVT if it's sitting on a speaker cabinet and being played?

[Chuck H]

As noted, most 12**7 tubes start out microphonic to some degree. I build an amp that is fairly high gain and I really need to pick through tubes to find one that stays stable. On my own amp I use a NOS RCA gray plate 12ax7a in V1 because I still have a few and they're very stable. For the amps I've sold I tried the Sovtek 12ax7LPS, which to me has tone comparable to good NOS tubes. But they're ALWAYS microphonic out of the box. I sell these amps with a Sovtek 12ax7wa because the short plate design is very stable. The 12ax7ywa does lack the top end of NOS though. Most "used but tests good" tubes are also too microphonic. Especially the 12ay7 and 12av7 tubes.

The way I see it you have two options. You can buy tubes for V1 that have beed selected by the tube seller for low microphony. They charge extra and you'll still have to accept that the tube may go microphonic in an indetermined abount of time. Or you could use the Sovtek 12ax7wa tube and accept the slightly lower fidelity. I don't know if anyone is making a short plate version of the 12ay7 but if so it may be worth a try.

[Prairie Dawg]

I managed to acquire thirty used Phillips 6681 mobile radio tubes and they are dead quiet and noiseless. They're my go to subs for taming a noisy preamp.

[jrfrond]

John, what do you think about what will happen to the tubes in an SVT if it's sitting on a speaker cabinet and being played?

Well, you will have SOME vibration transmitted to the tubes, but it's not like the direct acoustical shock waves that happen in combos. However, a badly microphonic tube WILL rattle atop a cabinet.

You know, it COULD be worse. I build a pentode design, and EF86's are pretty much born microphonic. I have to pick through a half-dozen tubes or so before I find something acceptable, and then if it's the combo version..... OY VEY, what a squeal I get sometimes!!! In amps with a few 12AX7's (like Fender), I'll just swap around until I find the one that works in V1. Again, not much you can do, and you must deal with it on a practical level. It DOES piss me off that big factories like Reflektor and JJ cannot build a solid preamp tube though.

[Prairie Dawg]

The reason I asked is I have been thinking about the general unreliability of power tubes in SVTs since my last soiree with the fault detection on one, which played out here in living color.

It seems to have passed muster as it went out on a gig Sunday after a sixteen hour heat soak and since then my phone's been dead quiet. Either I fixed it or I didn't pay the cell phone bill, or this is the last you'll hear of me after the owner arrives with his goon squad. I was getting tired of lugging it around and I resent the idea of being on a first name basis with a heating pad and a bottle of motrin.

What a frackin' education.

I hadn't thought about the shock wave in a combo amp-standing wave, perhaps?-but there you have it. Isn't it good learning three or four new things every day?

Even the best of EF86s seem to have their problems. Have you considered using another pentode like the 6SJ7 or 12SJ7 in your design? They're easy enough to find, lots of military surplus, glass and metal, is available plus those red envelope 5693 missile tubes. They look great.

[jrfrond]

Even the best of EF86s seem to have their problems. Have you considered using another pentode like the 6SJ7 or 12SJ7 in your design? They're easy enough to find, lots of military surplus, glass and metal, is available plus those red envelope 5693 missile tubes. They look great.

I've considered it, but I haven't been able to find enough information on their sonic abilities/compatibilities. Know of any resources?

[Prairie Dawg]

Not really. I only mentioned it because I've just finished up a 5C1 clone and am working on a BR6F both of which use the 6SJ7 in preamp service. The 5C1 has its own sort of blues thing going on but I don't know whether that's what your project amps are aiming for. The thing to do would be to build one and I'll test it for you for several years.

[Enzo]

All 12AX7s are microphonic to some extent, and the first tube in any amp will usually be somewhat microphonic. Pick and chose to lessen the effect.

Tube shields can help, but that silicone foam stuff Fender sticks around preamp tubes is great for reducing microphonics. I first recall it in the CYberTwin, inside, but I find blocks of it on Hot Rod DeVilles and the like.

[dumbassbob]

microphonic tubes

I have a bunch of old NOS Mullards RCA Philips Telefunkens ect They are the only tubes that are still non microphonic.It took along time to find good tubes and alot of $$$ and luckPlaces like Tubedepot will sell Nonmicro tubes for a price,at least they let you return them.I would just save up and get a good NOS NIB tube for V-1

[km6xz]

Hi Mu tubes like a 12AX7 is microphonic no matter how well it is supported because of the gain of 100 or more. The applications were microphonics are important to minimize either cascade lower gain stages or install the tubes in a vibration dampening housing. Test instruments that needed low microphonics such as master clock oscillators used crystal ovens to control the thermal environment of the crystal and also often installed the oscillator tube in the same highly insulated housing. A typical oven would be the size of a 1 pound coffee can with a layer of insulation between it and another can inside that with a different insulation material so the barrier would have more thermal resistance, and possibly a 3rd insulated housing inside that. Mechanical vibration and thermal drift were both greatly diminished. At least one of the layers of insulation would be spun fiberglass material that offered both protections. phase jitter and microphonics from such oscillators still represent the peak of the art 60 years later. My service monitor has such an oven that is 5 inches in diameter and 6 inches tall, but its solid state. Microvolt meters used isolation to reduce vibration modulating that could swamp an measurement signal.
So other industries have resolved the problem many decades ago but M.I. design has never taken microphonics seriously. Lots of consumer products of the 50s had sensitive tubes mounted on suspended carriages or rubber mounts with damping material around the tube. Home builders of guitar amps seem not to care about the long life of tubes or their sound enough to deal with the problem at the source, and keep swapping tubes around until one is found to be a little more resistant...for a while. Why not make the environment it works in to be friendly to any tube? That is like swapping tires trying to find a tire that rides well on a pot-hole covered street. A better solution is to fill the pot holes.

[Prairie Dawg]

12AT7s and 12AV7s are almost all microphonic but the mu is less than 100. That's because of the way they're designed for the intended use. You will also rarely find a 6211 that is not microphonic either-which is a pity. It didn't bother the Carrier air conditioning folks which is where all mine came from.

[Resonator Guy]

Thanks for the responses, folks. The amp sits on a carpeted floor in my music room and doesn't move around much at all. Buying more expensive tubes tested for microphony wouldn't help, I don't think, because the ones I put in aren't the least bit microphonic at the start. I have tube covers on it and I will investigate other shielding/dampening devices to see what helps.
I have just ordered 5 NOS Tungsol 12AV7s and will continue seeking that perfect tone. I don't like the sound of the only brand of new 12AY7s that I could find, and 12AX7s have too much gain. This amp is my best sounding one, so I would like the tubes to last a little longer.

[jpfamps]

I've seen problems in amps with microphonic valves where the problem was caused by the wire (usually solid core) connected to the valve vibrating.

Using silcone coated wire helps a lot.