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In Reply to: RE: Even my ten-year-old says they're yucky. posted by Triode_Kingdom on September 06, 2014 at 23:51:57
The 12AX7 PI in Fisher receivers have miller capacitance issues. Perhaps better drive capacity would come forth with 12AT7s, but overall stage gain is on the edge with the lower gain.
Follow Ups:
Fisher provided an excellent answer to the WRETCHED performance of the 'X7 triode in "concertina" phase splitter service. The X-100 used the 'U7 section of a 12DW7/7247 in the splitter role. "Borrowing" X-100 circuitry avoids the possibility of overloading the power trafo filament winding(s).
Getting the 'X7 splitter out is truly important in 7591/7868 units, where the O/P tube grid to ground resistance limit is exceeded. Current production tubes are intolerant of the liberties Fisher took.
Eli D.
The X-100 shows a 12AX7 as the PI. The X-202 uses a 7247, but in the tone control section.
Double check your schematic. The X-100 schema I have shows the 7247 as the PI. The KX-100 uses a 12AX7. And I've sold 12DW7 (7247) tubes to Fisher owners.
Edits: 09/10/14
Thanks. HI-FI engine appears to have the KX-100 schematic sitting in for the X-100
The 12DW7 (7247) does not exactly sound great if one believes in tube rolling, but is very convenient. The rare CBS is best.
Perhaps a 12AX7 & Brimar CV4003 sharing each channel- PITA to implement with two sockets.
While not nearly as bad as the LUDICROUS/INSANE price NOS TFK ECC803S stock commands, I find the price of Mullard made 12DW7/7247 tubes to be painful.
FWIW, AA sponsor Jim McShane's price for current production JJ 12DW7s is $13.50 each. Yes, a lot of JJ's production is "shaky", but their 12DW7 is pretty darned good.
Eli D.
"their 12DW7 is pretty darned good."
Thanks for mentioning that. I have an amp out in the garage that can use a pair.
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