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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Bias for EH 7868? posted by ScottK1 on June 11, 2006 at 10:19:30:
Check the value of the 7868 grid leak resistors. Both Fisher and Scott ignored the published limit for "fixed" bias 7591 family tubes. They got away with it using what is now known as NOS. Current production tubes meet all spec's, but are not tolerant of excess.Fisher used a 12AX7 twin triode wired common cathode and "concertina" phase splitter to drive the "finals". A simple modification is to replace the 'X7 with a 12DW7 (1/2 'X7 & 1/2 'U7). The 'U7 section makes a BETTER splitter, as it has higher gm and lower Rp (both DESIREABLE in this role) than a 'X7 section does. 12DW7s are available in both current production and NOS. Talk the mod. over with your tech.
Eli D.
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Follow Ups
- Re: Bias for EH 7868? - Eli Duttman 10:44:57 06/11/06 (0)
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