SOT: 12ax7 option and a question about hiss

[brett]

Hi.
I just tried a 12ax7WB, the Sovtek high-gain version of the 12ax7 in the pre-amp stage of my Firefly. It produced excellent results relative to both a large-plate Ei 12ax7 and an old (??Sovtek) regular-plate 12ax7 that I borrowed out of my Musicman.  I feel that it might be worth considering it for use a range of amps.

By the way, when I have the volume tuned way up in my Firefly, I get quite a lot of hiss (waterfall).  Interestingly, the hiss reduces (virtually dissapears) when I play.  As the envelope of a note decays, the hiss returns.  ??  What is the hiss likely to be???  Thermal noise???  I used metal film resistors (But the volume pot is a 1 M job).  Could the hiss be background radiation from the big bang??  Should I surround my 12ax7s with foil??

thanks for any help

[brett]

Bump.   Anyone
What is likely to be the main source of hiss in a valve pre-amp??

thanks

[puretube]

well, the reason, that the noise disappears, *can* be masking...

the reason, that the noise appears, can be a lot of sources:

e.g. cable/shielding/powersupply/circuit-topology/tube/the pot....

using a metal device (earthed...) around the tube, has almost never hurt
an (audio) preamp tube...

is the filament supply DC or AC?

tried the 12AX7LPS?