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In Reply to: RE: Sherwood S-5000 posted by fredtr on December 20, 2016 at 16:18:52
I see the S5000-II, which uses 7868s. The 7868 is electrically equivalent to the 7591.
Sherwood uses a 12AX7 in the unit, instead of the 7199 found in the S5000. Fisher used the 'X7 in their later units too. Fortunately, Fisher shows the way it should be done in the early X-100, which uses the 7247/12DW7.
Heed the 7591 data sheet. Use 270 Kohm grid to ground resistor, most definitely not a larger value. Current production tubes do not tolerate liberties, with that value.
Look for some space inside. Full pentode mode "finals", like the S5000-II uses, work best when g2 B+ is regulated at a fraction of anode B+. "Shoehorn" a Maida style regulator in, if you can.
Eli D.
Follow Ups:
Thank-you for the g2 B+ suggestion, I will give it a try. This has been causing me to scratch my head for a while. I keep thinking that I missed something or screwed something up, but I haven't been able to find it.
I'm only using old stock tubes but I had changed the grid resistors when I had first gone through the amplifier. But thank-yo for the reminder.
Read the LENGTHY thread. Then, pay the man his money.
Eli D.
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