Home
AudioAsylum Trader
Tubes Asylum: Re: Conrad Johnson PV10A w/phono; why so hard to get 12AU7s?!? by tvr2500m

Questions about tubes and gear that glows. FAQ

For Sale Ads

FAQ / News / Events

 

Re: Conrad Johnson PV10A w/phono; why so hard to get 12AU7s?!?

207.172.134.244


[ Follow Ups ] Thread:  [ Display   All   Email ] [ Tubes Asylum ]
[ Alert Moderator ]

So this is the problem - "Then, the 12AU7s went bad -- very noisy and microphonic." ??? Is the noise happening with the phono and line stages, or just with one stage?

What you're experiencing, I'm fairly certain, is what a lot people experience with some of the CJ zero NFB circuits. The tubes haven't gone bad, the preamps aren't tough on tubes in that they run the tubes hard and consume them quickly. The circuits provide opportunity for oscillation and the tubes ring or sing at more or less volume, which you can hear through the speakers of the system the preamp is connected to. Again, the tubes haven't gone bad, so don't throw them away!

I have a PV-12A (with a phono stage that sits unused). Speaking with a number folks about this, including the person who was key in designing these preamps, it all comes down to the value of a couple of resistors, one on each tube. These resistors are the "grid stoppers" (Bill Thalmann, the person in large part responsible for the design of these preamps, called them cathode dampers). The value was selected for sonics. Increasing the value of this resistor will eliminate the ringing/singing. I have a schematic of a PV-12 that highlights this resistor. The same resistor would be easy to find in the PV-10.

I do get ringing every once in a while - fortunately not too often. Some tube types and brands are much more prone to ringing than others, but some tubes can ring while others don't even among tubes of the same type and brand (the RCA 5963s black plates can be a very nice sounding tube, but they all seem to ring; this shouldn't be a surprise since as computer tubes I don't think they were designed for applications where low noise service was important). And you never know when ringing will start or stop. Even new tubes selected for low noise can start singing at some time, or can just stop. Tube dampers don't fix this. With tubes that do ring, I will/can accept some amount of ringing, but sometimes the ringing is more than I'll accept. You can also try gently tapping the tubes. Sometimes this will stop the ringing, sometimes for just a little while, sometimes for good. There are many tubes I've found that I've never had ring.

Feel free to contact me off-Aslyum if you'd like to chat more about this.

Cheers,

- SJ


This post is made possible by the generous support of people like you and our sponsors:
  Atma-Sphere Music Systems, Inc.  



Topic - Conrad Johnson PV10A w/phono; why so hard to get 12AU7s?!? - ARC 12:11:29 04/14/06 ( 6)