Expanding questions about tubes in a preamp

As others have shared, there are types of noise that are intrinsic to the component and the tube itself. As geoff points out all tubes have some degree of microphonics and the gain of the component in which the tubes are used has a huge effect on this phenomenon. Placement or isolation of a component also can have positive results. I agree with ieales that damping, isolation or relocation of the component are where you should start. Hiss is usually a component issue. I do not like dampeners for numerous reasons not least of which is mentioned by Ralph at Atmasphere. 

Gents, how the hell does adding damping rings change the sound of a tube.

Get two 6550’s one GE NOS green label and one Chinese junker, flick each and listen to the Chinese one ring like crystal and the GE just go thud. That’s the reason.
It’s all about airborne feed back from your speakers, if your tube pre and power amps were in another room there’d be far less concern.
This is why the flick test in my first post will give you a fighting chance against micro-phonics in tubes.


Cheers George

Describe in detail what extraneous noise you are experiencing.

If it's "hiss" from the tweeters, then dampers are not the solution (cabling type and placement of the cables/gear would be as well as shielding unused inputs with shorted plugs).

I posted to your prior query and will add that your unusual preamplifier (which may be modified - due to it using a 12ax7 in place of a 12au7, which was standard in the original design) now uses two extremely high gain tubes (12ax7/EF86).

The original 12au7/EF86 design makes more sense (on paper anyway;-).

As far as micro-phonics go the 86 tube is notorious for such.

I'm wondering if the 12ax7 should be replaced with a 12au7 and if the 12ax7 was substituted in error @  one point and then remained.

DeKay

DeKay

Thanks for your clarification - probably the route I will go will be to
change to 12au7s

Can you suggest changing the EF86 to ?

Not certain if it’s OK to swap 12au7 for 12ax7 in the event your unit has been modified.

Maybe someone here knows.

My only familiarity with 86’s is from re-tubing a couple of vintage Vox guitar amps for a neighbor.

It seemed that he went through 86’s quite often as he had accumulated lots of duds.

I also recall 86’s with shiny/sparkling plates and others with normal dull plates, but don’t recall particular brands (other than the shiny Telefunken I had on hand).

Have you tried contacting the designer of the preamp?

Anyway, what type of noise are you experiencing (gently tapping on the unit will tell you if it’s microphonic)?

If this is it, then gently tapping on either/both tubes will tell you which one (or both).

DeKay