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Old 15th May 2007, 09:58 PM   #1
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Join Date: Mar 2007
Thumbs up fisher 800c modifications

I purchased my fisher 800c from the fisher doctor so it has most of the modifications associated with making the unit dependable and bringing the fm end up to current specs. This includes cathode resistors in the output tubes and upping the bias voltage etc.

Are there any modifications that would make the tone of this receiver even better. I have already installed Auri caps, as the coupling capacitors. I want to keep the tone control section intact.
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Old 15th May 2007, 11:25 PM   #2
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Location: Monroe Township, NJ
Fisher "cheated" and used 330 KOhms as the 7591 grid leak resistor value. That's in excess of the published 300 KOhm max. Current production (EH/JJ) 7591s meet, but don't exceed, spec's. Problems with current production tubes in Fisher built units are all too common. Don't say, I'll use NOS. NOS 7591s are VERY scarce and fast becoming "unobtanium". IMO, something MUST be done to ensure reliable operation with current production 7591s.

The heart of the problem is the use of a 12AX7 section as the "concertina" phase splitter. That's a "bean counter" decision, if ever I saw 1. The 12AX7 driver tubes need to be replaced by 12DW7s (1/2 'X7, 1/2 'U7). A few caps and resistors get changed along with the tubes. Sorry, bigger coupling caps between splitter and "finals" are necessary in combination with smaller grid leak resistors. Save the Auricaps for another project. My preference is SCR (France) made Tin Foil and polypropylene film. Costly boutique stuff is (IMO) a waste of money in an amp with loop NFB.

If you can source and afford TFK or Mullard made 12DW7s, by all means use 'em. IMO, the practical choice is pre-NATO action EI made 12DW7s, which strongly resemble TFK and they are affordable.
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