by 1973shovel Sat Jun 22, 2013 11:58 pm
Hi Peter,
It took me some time to remember where I read that the ST-35 sounded worse using standard fixed bias. Tonight I remembered, it was by a well regarded tube guy, Gary Kaufman, on the Audio asylum. Here's a link:
ST-35 fixed biasKaufman's quote: "As you've guessed, the bias is not adjustable - the ST35 uses cathode/self bias. I've tried individual bias resistors in an ST35. Although it sounds like a good idea, it sounded really poor IMHO. I'd stick with the original design."
In another post in the same thread, Mr. Kaufman continued: "I tried individual bias resistors (I seem to recall using 360 ohm) per tube, and also later per channel. Using one bias resistor per channel wasn't bad, but no improvement over the single resistor. Using a separate resistor per tube (with individual bypass caps) really sounded "unfocused". I have no explanation, frankly I would have expected the opposite."
Mr. Kaufman made this observation in 2001. After reading Dave Gillespie's explanation of why Mr. Kaufman's ST-35 conversion to standard fixed bias "sounded really poor", I sent Gary Kaufman a link to Gillespie's article, but never got a reply.
I hope you read Gillespie's findings on the Z-565 Dynaco transformers, and why the EFB works so well with them. Both from a measured as well as a subjective standpoint.
To me, having to purchase a matched quad of output tubes is a small price to pay for the sonic bonus the EFB brings to the ST-35 and SCA-35. And, of course individual EFB tube biasing can be achieved by using a more complicated circuit. This is my plan when I build a larger chassis version of the ST-35, with an improved input stage.
Enjoy your reading!