Microphonic

[ray_moth]

Microphonics seem to vary greatly by manufacture. For instance, I have a pair of GE 6AU6s that are very sensitive to being touched and give a tremendous boingggg! sound. On the other hand, my pair of RCA 6AU6s are only slightly microphonic.

 

[@mot]

My friend has built amp that after some time starts oscilating high frequency tone (very quiet but heardable). When he knocks input tube (ECC82) it stops for some time (5-10 minutes) and then goes again. Is this caused only by tube or it can be something else?
Thanks

 

[reaction]

Could be a resonating inner construction...

 

[Sherman]

[B...Hi, you mean the o-ring is placed in between the tube and the socket? That dampens the tube-socket contact right?

But basically the vibration from the chasis to the socket is not dampened right? And the vibration can reach the tubes through the pins i guess.. [/B]

No, the tube dampers are placed on the tubes themselves. I have two on each tube approximately 1/3 and 2/3 of the way up the tube from the base. The difference was immediate and noticeable when tapping the tubes. Much less microphonics. However when the amp is just sitting there and playing I can't say I notice anything in particular.

I suppose one could place one at the base by the socket. At first blush that does seem that it would help prevent vibrations from the chassis from moving the tubes too much.

 

[J.R.Freeman]

Interesting stuff! I've been day-dreaming of a 12AX7 preamp, and have been wondering about microphonic effects. Do any of you know of measurements of microphonic response, with a signal path of speaker to air to tube?

Jim