500-B tuning eye not moving

[Dave451]

I just built and put an IBAM in my 500B I am restoring. When they say 'fixed bias' in the 500B, they mean it--no adjustment whatsover! I couldn't bear that and had good luck putting Dave G's IBAM in my 400, so I built and installed one in the 500B as well. Here's my thread if you want to look, Steve:

http://audiokarma.org/forums/index.php?threads/fisher-500b-re-hab.756462/

Dave

 

[RS Steve]

I just built and put an IBAM in my 500B I am restoring. When they say 'fixed bias' in the 500B, they mean it--no adjustment whatsover! I couldn't bear that and had good luck putting Dave G's IBAM in my 400, so I built and installed one in the 500B as well. Here's my thread if you want to look, Steve:

http://audiokarma.org/forums/index.php?threads/fisher-500b-re-hab.756462/

Dave

I have six of the boards built, but haven't had the spare time to get all the connections soldered up. I tried a couple times and developed brain freeze, I'm trying to digest an understanding of the circuit as I go, which will take a bit longer than I have been able to devote. I just had too many irons in the fire of late, but I'm getting ready to give it my full attention.

 

[Dave451]

Happy to step you through it if that's a help at all, since it's still fresh in my mind--just let me know. What kind of output tubes are you running in the 500B? With the Tung Sol Russian re-issues, I had to adjust the value of the 10K resistors to get the right bias range for the tubes.
Dave

 

[RS Steve]

Happy to step you through it if that's a help at all, since it's still fresh in my mind--just let me know. What kind of output tubes are you running in the 500B? With the Tung Sol Russian re-issues, I had to adjust the value of the 10K resistors to get the right bias range for the tubes.
Dave

I actually had to sort through all my spares and settle on a mix of 1 Sylvania pair, and 1 GE pair. I got the bias to be around .37 and .38, the new Tung-Sols were running between .67 and .71.

 

[Dave451]

That makes sense. The Tung-Sol's, in my experience, are hotter tubes and need a more negative ('higher minus') bias to keep the quiescent current at the proper level. The vintage or NOS tubes need a little more positive bias ("less minus") voltage on the grids. If you're using Dave G's IBAM circuit, then the 10K resistors are probably right with the old stock tubes. I had to change the 10K to 15K to get the right spread for my new Tung Sol 7591A's. The higher these resistors are, the more negative ("higher minus") is the bias voltage. With the original bias set up, the TS tubes would run very hot, as you see in the numbers.