Shock Mounting.

The picture PRE5.JPG shows details of using both 3M viscoelastic
material to shock mount the tube sockets and the use of EAR
Specialty Composites to very effectively control low level preamp
microphonics. The 3M material may be gotten from "Tube Dampers".
Normal "Tube Damper" operation is shown in PRE6.JPG.

The socket mounting overall looks like this.....

       /-\            /-\     <--- screw head
      -----          -----
      |   |          |   |    <---- chunk of viscoelastic material
      -----          -----
        ____########_____     <---- tube socket
      -----  ######  -----
      |   |  ######  |   |    <---- another chunk of visco mtl
      -----  ######  -----
  ~~~~~   ~~ ###### ~~   ~~~~ <---- Chassis
      -----   !!!!   -----
      |   |   !!!!   |   |    <---- another chunk of visco mtl
      -----          -----
       +++            +++     <---- Nut
        +              + 

EAR Composites is at 317 692 1111  or 302 738 6800. The material
shown in PRE5.JPG is "Isoloss" CF-42-50. (Pink). It is backed by
Isoloss CFNT-5-13-50 (yellow).


Results:

Quantatively, the Visco' mounting provided approximately 16 dB isolation, and the
tube dampers added another 4-6 dB isolation, totaling about 21 dB isolation. The
combination of the mounting AND the foam in the phono section provides slightly
over 30 (!) dB isolation. The result is you can pretty well pound on the chassis,
and the only microphonics you hear are the 6SL7 (predominates), followed by
the line amp 5842, peaking out at 4-6 kHz, with almost no contribution from the
phono section, even though it is at the lowest signal level. Also, the really nasty
very high frequency resonances in the 5842's seem to be tamed quite well.

The quantative measurements were taken with signal gen feeding a power
amp and speaker generating about 95ish SPL at the preamp face, and true RMS
meter reading the preamp's output with inputs grounded and gain cranked max.