kidvybes,
I use 11 pieces of tube audio gear using 129 tubes of 30 types, so I've dealt with microphonic tubes often. My microphone preamp- a Peavey VMP-2, uses 6-12AX7s and 2-12AT7s**.
[**2- Sylvania square getter 12AX7WAs 1958, 2- Sylvania 3-mica 5751s, 2- GE two mica 5751s, 2-Valvo/Hamburg 6201s- no dampers]
The reason for tube dampers is to reduce the "microphonics" of a tube. Tubes are susceptible to mechanical vibration and if the structure vibrates, it may impart a ring or thunk to the sound. I have an Audio Research SP-10 preamp and I've tried tubes in that one (uses 12- 7308s/E188CCs) that were so microphonic that only touching the GAIN control produced a ring on the speakers.
The frame grid tubes like the 6DJ8 family are especially prone to microphonics. Sometimes you'll see tubes with an extra mica space- often military issue, and this is to try and reduce vibration inside. I choose triple mica tubes often.
The damping rings reduce the mechanical vibration and if you're lucky, they can make otherwise unusable tubes acceptable. There is however, a possibility the damping is a sonic penalty rather than enhancement when a tube is damped.
Microphonics only become a problem when they are robbing signal to put into the ringing. This is subtle. If you're brave, try very, very, very lightly tapping the top of a preamp tube. If it's audible in the speakers- try dampers.
Some people believe that tubes sound better when they are a little bit microphonic- these sound more open. In the the 6DJ8 family, the 6DJ8- which is the least regulated- is often preferred over the 6922 or 7308.
I've heard the Herbie's dampers (about $10 for a 12AX7 size one) are the hot models. These are like little clip on rings that have little balls on them that damp the tube.
Wirth microphonics, don't fix it unless it's really broken! Damping a tube that doesn't need it may dampen the sound. In all my gear, I'm only using dampers on six tubes- and these are all the first phono preamp positions.
Cheers,
Bambi B
