Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

fisher 800c modifications
fisher 800c modifications
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 15th May 2007, 09:58 PM   #1
slhoward is offline slhoward  United States
diyAudio Member
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.
  Reply With Quote
Old 15th May 2007, 11:25 PM   #2
Eli Duttman is offline Eli Duttman  United States
diyAudio Member
Join Date: Apr 2004
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.
Eli D.
  Reply With Quote


fisher 800c modificationsHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Fisher 800c static,crackle skiman762 Tubes / Valves 2 9th April 2009 03:59 PM
Fisher 800c neg bias problem Glowin Plates Tubes / Valves 10 26th September 2008 08:39 AM
fisher 800c intermitant hum slhoward Tubes / Valves 1 28th May 2007 04:16 PM
Distortion in Fisher 800C Glowin Plates Tubes / Valves 1 2nd November 2005 06:57 PM
Fisher 800C FM Stereo problem Glowin Plates Tubes / Valves 5 2nd April 2004 10:46 AM

New To Site? Need Help?

All times are GMT. The time now is 01:17 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio