Posted by 2wo [ 71.242.1.162 ] on January 29, 2006 at 19:22:49:
In Reply to: Re: Bias supply posted by manualblock on January 26, 2006 at 08:59:33:
Don’t lose heart.
A P-P 2a3 is a good thing and this circuit look like a winner.
It may look complicated at first glance, with it’s pos and neg supply’s, grid choke and feedback.
But there is no reason why it could not be built in stages.
Start with the basic front end, hold off on the CCS for the moment. Leave out the grid choke and feed back for now and use cathode bias for the first cut.
This way we don’t need a neg power supply to get music going. and we have something that looks like the attached. I did not try to redesign the input section for single supply operation. Dougles is the pentode expert. This may not be Douglas’s amp, but the bones are there.
My apologies to Douglas for diddling with his design, he puts this stuff in for good reasons but how good does an amp sound that doesn’t get built?
At the last NE Bottlehead meet, I got to listen to Zacster’s 6b4 amp. Which is a cathode bias with a Curcio Front end, built on a ST70, no feedback.
It sounded outstanding. Has me dragging my ST70 out of the dungeon, for yet another flogging.
I think this is a good approach. Listen for a while, then add the feedback circuit or the grid chokes.
Next we need to address the Neg supply, not a big deal by it’s self. Just leave a bit of room under your chassie for a 3"x3" or so transformer. Now we can add the CSS (BTW Jim McShane Has a kit of CSS parts for the "bargain basement amp that they are working on over at the DEC forum. They lifted it form Guienevere).
With the Neg supply in place you can try fixed bias for the outputs if you wish.
I like cathode bias but if you have a neg supply it is easy enough to try it.
This way you can try one thing at a time and see if it is your cup of tea, with no waste save for a few resistors.
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