Simple Microphony sensor for valves (tubes)

[Jon Snell]

Here is a useful tool for the workshop full of valves that are possibly noisy.
This simple circuit will detect any noise and a gentle tap with a biro will show up microphony.
Select diferent bases as required. This is just the basics. Anode resistors 100k
Cathode 1k8
Grid 10k
Heater smoothing 6k8u
HT smoothing 100u
the rest is nominal.
I hope you find it useful.

[mhuss]

I like the idea, but how do you judge 'too microphonic?' Certainly some just are awful and can be rejected out of hand, but where's the cutoff?

I have at times thought of something similar with a "standard weight" to tap the tube and some sort of peak/hold metering. Of course, it's not just the amplitude but also the characteristics of the sound it makes. I had some old GEs that sounded like you were dumping a barrel of junk on the floor when you tapped them.

[Jon Snell]

I like the idea, but how do you judge 'too microphonic?' Certainly some just are awful and can be rejected out of hand, but where's the cutoff?

I have at times thought of something similar with a "standard weight" to tap the tube and some sort of peak/hold metering. Of course, it's not just the amplitude but also the characteristics of the sound it makes. I had some old GEs that sounded like you were dumping a barrel of junk on the floor when you tapped them.

You use your ears and judgement.
All down to experience.

[Leo_Gnardo]

I like the idea, but how do you judge 'too microphonic?' Certainly some just are awful and can be rejected out of hand, but where's the cutoff?

I have at times thought of something similar with a "standard weight" to tap the tube and some sort of peak/hold metering. Of course, it's not just the amplitude but also the characteristics of the sound it makes. I had some old GEs that sounded like you were dumping a barrel of junk on the floor when you tapped them.

I've had some Amperex and Mullards that sounded like the percussion section dropped all their metal instruments at once. OW! Others besides...

Our long missing correspondent from Russia, Stan, built a calibrated tapper gadget as you mentioned for Magic Parts (Ruby tubes) to test for microphony. I mentioned it to Sharon there some years ago, she said it was still in use. I imagine, something like a piano hammer, the test person can simply lift and drop on the UUT while it's energized and mounted in a socket (of course!). Then watch scope screen, also listen. More than "this much" deflection of the scope trace, no pass, that's a second rate tube. Now Sharon's moved on I can't ask her again, but I hope their tube tapper is still in use. Someone will have to ask Danny there. I only call for an order once ... well since covid started not since fall 2019.

Tap with a biro. We 'Mericans don't know what that is. (Bic pen over here folks.) I lost a customer who played only solid state amps all his life, say 4 decades or so. He finally sprung for a reissue Super Reverb, and a year later it was making too many noises for him. Mostly output tube rattle. I set him up with a fresh pair of output tubes then the calls started. "I'm tapping the tubes with a pencil, and I can hear noises coming out the speakers." My answer - "Do you ordinarily tap your tubes WHILE PLAYING YOUR GUITAR?" ... "No, of course not, but the idea that extra noises could be coming out bothers me." My advice buddy, ditch that Super Reverb ASAP and stick with transistor amps. I haven't the time nor patience. Tubes DO make noises, especially when you tap on them, the tradeoff is tone and feel that don't seem to be found in transistor amps. That's why people still use them although they're obsolete for 60 years. And if you insist on tap tap tapping your tubes in an otherwise quiet room, you'll drive yourself crazy. Unless you're already crazy. As in neurotic - "imma keep probing until I find something to complain about then eat up your time whining about it."

[xtian]

Tried to find a 6SL7 preamp tube for my client's Alamo Amp 3--his was microphonic, and sounding bad when playing palm-muted low notes. I had two Russian equivalents in a box, but they were worse. So I bought him a $40 TungSol reissue...and it was only slightly, marginally better than his original.

Sorry, venting.

[Leo_Gnardo]

Tried to find a 6SL7 preamp tube for my client's Alamo Amp 3--his was microphonic, and sounding bad when playing palm-muted low notes. I had two Russian equivalents in a box, but they were worse. So I bought him a $40 TungSol reissue...and it was only slightly, marginally better than his original.

Sorry, venting.

I'd vent too, about a rattly premium price $40 currently made 6SL7. Can't say I've been attracted to pay big bucks for Mike Matthew's so-called "premium marque" but still made in Russia on the same assembly line tubes.

When stymied by this kind of malarkey, I've built adapters. Groove used to offer them ready made but quit long before Fender bought 'em out. I wire a 9 pin noval socket into an octal header, making the necessary connections, then fill the intervening space with clear epoxy or acrylic. Voila' now you can use 12AX7, 12AY7, 5751, whatever. But the attrition rate was high - I overfilled 2 out of 3 that I made, with plastic goop rising into the noval and blocking 9 pin base from entering the contacts fully. WOOPS! I was looking forward to not needing to do this again, but your experience tells me I probably will. The lengths we have to go to, to keep crustomers happy and ourselves - sorta - sane.

[drewl]

It varies with the amp also.

A tube that's microphonic in one amp, may not be in another.

[J M Fahey]

Just thinking aloud and in no particular order.

1) I measure microphony, at least close enough to find the best one in a set, by using my "simple 12AX7 tester" by touching tube tip with one of those "sausage shaped personal massagers for tension relief" if you catch my drift, and watching signal on a scope, which can be compared tube to tube.
Sinewave is too low frequency to be displayed properly as such but peak amplitude can easily be seen and compared.
PS: One of these days I am going to open an old cellphone battery, guess there is s small thin vibrator motor inside, could be kludged into a microphony exciter.

2) servicing is servicing, customer in general does not want mods (unless Jimmy Guitar God uses them and he read about it on a Forum) but in my own builds *all* my tube amps have either a FET first stage with some 10-12dB gain which in due time lowers the microphony problem by the same amount OR input jack goes straight to a TL072 set up with 20dB gain which is even better, and only then visits a "glass walled house".

Life´s too short for unnecessary suffering and in 99% of cases ¨*first* gain stage offers little to no advantage or "tube sound".

[J M Fahey]

Here is a useful tool for the workshop full of valves that are possibly noisy.
This simple circuit will detect any noise and a gentle tap with a biro will show up microphony.
Select diferent bases as required. This is just the basics. Anode resistors 100k
Cathode 1k8
Grid 10k
Heater smoothing 6k8u
HT smoothing 100u
the rest is nominal.
I hope you find it useful.

Hey!!!! LOVED it!!!!

[drewl]

in 99% of cases ¨*first* gain stage offers little to no advantage or "tube sound".

BLASPHEMY!
May the tube gods smite thee as thou hast not been smotten before!

[Leo_Gnardo]

1) I measure microphony, at least close enough to find the best one in a set, by using my "simple 12AX7 tester" by touching tube tip with one of those "sausage shaped personal massagers for tension relief" if you catch my drift, and watching signal on a scope, which can be compared tube to tube.
Sinewave is too low frequency to be displayed properly as such but peak amplitude can easily be seen and compared.
- - - - -
2) servicing is servicing, customer in general does not want mods (unless Jimmy Guitar God uses them and he read about it on a Forum) but in my own builds *all* my tube amps have either a FET first stage with some 10-12dB gain which in due time lowers the microphony problem by the same amount OR input jack goes straight to a TL072 set up with 20dB gain which is even better, and only then visits a "glass walled house".

Life´s too short for unnecessary suffering and in 99% of cases ¨*first* gain stage offers little to no advantage or "tube sound".

Ohhh, man, what customers I have, who watch me work on their amps sometimes - they would be howling like rabid hounds when they see me probe the innards of their amps with a "personal relaxation device" such as you mention Juan. Imagine the scandal! (The old man's lost his mind for sure! Lemme tell you what he did...) But - Juan I think you are on to something. Ohhh my, will have to search the catalogs now for "calibrated vibrators." Gotta have fresh batteries too or it's not calibrated. Going to need an extra for the field repair kit. Definitely easier to implement than a calibrated piano hammer style tube tapper like I described in an earlier post.

FET front end? People pay what, $50,000 on up for some Dumble amps that have an FET first preamp stage. So... heresy it's not. Let's remember, whatever noise is generated by the first stage goes on to be further amplified by subsequent stages. Like it or not, a quiet transistor first stage pre isn't poison, though it may upset the population of tube purists. AND how many guitarists (lots & lots of them) now use a - transistorized - foot pedal preamp, Klon and all its clones for instance, but then they patch into the normal guitar input. Any advantage there might be a tad less noise because they've dialed down volume controls on their amp because the signal at the input jack is that much stronger.

Also, Mesa tried it, late 70's maybe early 80's with their "FETron" preamp modules that fit into cylinders and mounted in 9 pin sockets. Didn't go over so well for them. And New York City amp guru Harry Kolbe installed his 2-FET plastic magic cube into amps throughout the 80's. I own a 100W Marshall head that has one. Squawky, dry, noisy, not what my ears want to hear. What the promoters say, there's an ass for every seat - some nostalgic Kolbe fan will inevitably pop up and buy that thing I hope when it gets listed on Reverb. If not I'm really gonna hate having to de-modify that amp to stock but dammit if that's what it takes I'll have to do it, phooey!

[Helmholtz]

AND how many guitarists (lots & lots of them) now use a - transistorized - foot pedal preamp,..

Good point!

[Enzo]

And a tube too microphonic for V1 might be perfectly fine as a phase inverter or reverb tube or whatever.

[Mick Bailey]

+1. Changing the order of premap tubes can sometimes be a cheap and easy fix for a noisy amp.

[35L6]

Don't caps and transistors make noise when you tap on them ?