problem with 12AE6A buffer

[ionomolo]

I've done a tube buffer for solid state amps using a 12A6A (millet-like) mu follower biased at 700 uA/18V using grid feedback and i got some weird stuff.

At first everything is fine and the sound is exellent, but after a while i start to hear the radio and some cracking trough it, and the sound degrades a bit. I belive this is instability due to a too long feedback loop (7 cm including parts length). Am i right? Has somebody gone through something similar?

EDIT: Feedback is done by anode -> 1uf -> 100K -> Grid -> 100K -> Input

Bias is done in the cathode using a 5K potentiometer paralleled with a 470 uF cap

[pointy]

if you are getting radio and static the odd photon, phone call from mars etc............ .
may be it would be best to put the whole lot in an earthed metal box.
ie. like a valve power-amp base.

or like a bread-board with a cake-tin over the top.......... me amigos!

[ionomolo]

Originally posted by pointy
if you are getting radio and static the odd photon, phone call from mars etc............ .
may be it would be best to put the whole lot in an earthed metal box.
ie. like a valve power-amp base.

or like a bread-board with a cake-tin over the top.......... me amigos!

Thanks, this will help make the amp even quieter.

The big problem however is already solved, it was the heather voltage that was floating with respect to the cathode.