[sdiy] Tube microphonics: 12SQ7GT & 12AV6, 6SN7GT & 6CG7/6FQ7, etc.

[anthony]

I have been thinking about tube microphonics lately and was wondering what 
aspects of the physical parts of the tube, like length of getter supports, 
getter position & shape, wire thickness, placement of mica supports and so 
on.

I have read that many people have found the 12AV6 & 6AV6 to be microphonic 
and a few others have complained about 12SQ7GT's being microphonic - which I 
found hard to believe at first because of the superb performance of the 
single triode preamp based on a National Union 12SQ7GT and a Tung-Sol 
50L6GT. The plate of the NU triode had a seam/fin on just one side, which I 
would think would lend itself to being non-microphonic. The 6AV6 I would 
guess is often microphonic because of the diode plates which look like a 
tuning fork. Then I got a "grab-bag" of various 12SQ7GT's on eBay. It was a 
real gamble, but all but one of the 5 were golden; the one I set aside (a 
Philco branded one) I did only because the gain was markedly less than the 
others. I also discarded the original tube I used because the others 
outshone it, none displaying the dysphonic scratchy distortion that I was 
afraid was due to a design limitation to using a plate resistor half the 
size that is usually used when the tube is a preamp tube in a common radio. 
I wanted rockin' gain, dammit! And this NU tube, which probably was a good 
one when it was new, was just shagged out. The weird thing was that one of 
the 12SQ7GT's (a Westinghouse branded one) looked just like a 12AV6, but 
stuck in an octal envelope. My guess is that it was of more recent 
manufacture for the purpose of replacing old tubes and was probably never 
used as an OEM part and using the 12AV6 innards was economical because 
they're supposed to be electrically identical to the 12SQ7 even though they 
don't look anything alike - except for this one Westinghouse tube. I 
auditioned it, expecting it to be a microphonic turd, but was surprised to 
find it one of the best tubes. The others ranged from definitely used to 
NOS-OOBFLT (NOS out of the box for a long time). So I examined to 
12AV6-looking Westinghouse 12SQ7GT and noticed that it looked like they took 
pains to make it a decent tube.

So that made me wonder what to look for in a tube. When buying tubes on eBay 
that I can't inspect up close, I usually just go for brands I know will be 
good like RCA, Tung-Sol, Mullard, etc... But some tubes are made by these, 
but branded for others. Like the Silvertone branded 12SQ7GT that looked a 
lot like the old NU, but there was another NU tube that looked different: it 
had two fins instead of just one on the triode plate. And I don't know if 
National Union made their own 12SQ7GT's or not...

This brings me to the 6CG7/6FQ7's that I have that I want to use in an amp 
design that originally used a 6SN7GT. The datasheet for the 6CG7 says it's 
electrically identical to the 6SN7, but they look very different. Of the 4 
6CG7/6FQ7's that I have the one that looks most promising is an RCA with a 
side-getter. And the relatively worst-looking one is another RCA of more 
recent vintage with a top-getter with a long support wire.

Is there anything to plate geometries tha contributes to a tube's 
microphonics?

AA