Microphonics?

[Carlp]

OK, so I built a "parts bin" El Cheapo (Eli Duttman's design) using some JJ Electronics 12AT7s and decided to buy a few high-temp silicone O-Rings to see if I could hear any difference. On a quick listen, I don't hear much. If anything, I maybe hear a little roll-off or muddying of high frequencies but little else.

So...what's the scoop with microphonics? I know the T7s probably aren't as susceptible, and I will try them on the AX7s I'm using for a phono stage under construction (once I get it going). But is my experience to be expected if they are used on a tube with little in the way of microphonics? Could they actually degrade the output?

More importantly, anyone care to describe in very simple terms what microphonics sound like?

thanks,
Carl (cart before the horse...)

[m6tt]

The JJ 9-pin twin triodes are generally very solidly built compared to most NOS...I've had some old GEs that made a particular amp 'sing' for a while, until they started breaking down internally and crackling. I am fairly sure that for some electrode designs, or older microphonic tubes the dampers might help. Since pentodes have more electrodes they're also more often microphonic (in my experience). The dampers would really shine for say ef86 or many 12ax7 (especially older and/or used tubes).

[SY]

Microphonics sound just like what the name implies- if you tap the chassis lightly anywhere near the tube (or lightly tap the tube itself), you'll hear an amplified version of the tapping through the speakers. With some tubes, the microphonics are highly resonant, so it will sound almost like tapping a bell or tuning fork. Ting! Ting!

Use a plastic chopstick or something else nonconductive to try this!

Two other notes: in a power amp, the JJ ECC81 will normally show very little microphony; low level stuff like phono stages is where the real issues are. And microphony can be much worse in tubes that are oscillating at RF, in which case damping the tube isn't fixing the underlying problem.

[richwalters]

Originally posted by SY
Microphonics sound just like what the name implies-

Use a plastic chopstick or something else nonconductive to try this!

I do the tapping test with that wooden/rubber ended suction tool often used by motormechanics for lapping valves in. One then hears the tube guts and not the tool.

A sort of acoustic defining spec was introduced by Philips and others but doesn't lend itself to todays standards. (look at Franks data sheets of ECC83/ EF86 etc).
B9A Pentodes such as 7199 and ECF series can have notorious and persistent metastable electron noises. Throw in dodgy mechanical internals, then all the ringing is complete and a way out is to use'm as triodes = i.e Low gain and lower noise.
In saying this, the lovely high mu GE 12BY7's can be ringy but suprisingly the Svetlana EF86 is also quiet. There is no excuse for a single grid electrode tube to be ringy.
I've hulled out alot of good tubes because mechanical QC was so poor and not one tube damper cured anything. In fact those silicon thingy rings look darned awful.

Technically, a sensible way of using "microphonic tubes" i.e for a power amp is to design for low input line sensitivity, i.e +6dBu (cum 1.5V) or even lower sensitivity (as I do) then the s/n ratio also improves. The preamp can easily give more. One wonders why so many yesteryears designs claimed full power out for only 100mV input or lower, is asking trouble from hum and microphony.
The original Mullard 20W amp without global nfb claimed only 6.5mV input for full output, ideal for guitars when compared to 220mV with 30dB feedback. That's only one example.

richj

[Carlp]

Wow, thanks for some great responses. My computer has been on the fritz so only just saw these. I THINK I understand what microphonics sound like now. I'm working on a phono stage (Fred Nachbaur design) and will test them out there. For now, just trying everything (cheap or free) I can so I learn.

So might tube dampers negatively affect output if there isn't any microphony going on? Just wondering if I could be hearing something or just making it up...

Carl

[SY]

No, all they'll do is slightly quiet mechanically-induced noise. To really get the ringy-dingy out, you'll want to look at some flexible suspension methods. There's a nice section on this in Morgan Jones's "Building Valve Amplifiers," which you ought to own anyway.