Peculiar Fisher circuit - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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

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
Reply
 
Thread Tools Search this Thread
Old 15th March 2008, 08:39 PM   #1
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Default peculiar Fisher circuit

I have attached an image of a post tone control gain stage and the phase inverter for a Fisher X-101-B. I was curious what the purpose of R1 and R6 are.

It seems like they would reduce degenerative feedback (by a controllable amount) in the gain stage across all frequencies equally, which would be an advantage over bypassing with a cap I guess. Its less efficient on the power, though.

Why would you do this in a cathodyne?

Anyone else ever see this before?
  Reply With Quote
Old 15th March 2008, 08:47 PM   #2
diyAudio Member
 
sorenj07's Avatar
 
Join Date: May 2006
http://www.imageshack.us/
thumbnail for forums 1.
  Reply With Quote
Old 15th March 2008, 08:50 PM   #3
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Oops forgot to attach the image.
Attached Images
File Type: jpg fisher.jpg (34.7 KB, 384 views)
  Reply With Quote
Old 15th March 2008, 09:00 PM   #4
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
That's kind of embarrassing. "Hey everybody, look at my circuit that you can't see.."
  Reply With Quote
Old 15th March 2008, 09:40 PM   #5
diyAudio Member
 
tubewade's Avatar
 
Join Date: Jun 2007
Think nothing of that. Happens to all of us at one time or another.

Those resistors will shift the operating point of the valve. They will raise the voltage at the cathode making for greater reverse bias. I've seen this setup used in some circuits that have lower than optimum B+. I, too, would be interested in what other effects this might have on the operation of those stages.

I have a Harman Kardon and a Motorola amplifier that both use a similar configuration.

Wade
  Reply With Quote
Old 15th March 2008, 09:46 PM   #6
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Well, in this amp there is more than adequate B+ for 12ax7s. Why not just use a more optimal Rk? I'm thinking they were looking for less cathode degeneration here.

The cathodyne stage is what's got me here. It seems like they just made it more complex for no real reason. I can't think of a benefit for R6.
  Reply With Quote
Old 15th March 2008, 10:14 PM   #7
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Besides increasing bias slightly, they also provide some ripple cancellation.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 15th March 2008, 10:52 PM   #8
diyAudio Member
 
SpreadSpectrum's Avatar
 
Join Date: Nov 2007
Quote:
Besides increasing bias slightly, they also provide some ripple cancellation.
Maybe that was their reason for it.

They eliminated these resistors in the X-101-C.
  Reply With Quote
Old 16th March 2008, 03:05 AM   #9
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Yeah, that's probably it. The cathodyne is notorious for its poor PSRR (a fact that seems to have concerned John Broskie quite a lot).
  Reply With Quote
Old 16th March 2008, 03:17 AM   #10
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally posted by SpreadSpectrum
The cathodyne stage is what's got me here. It seems like they just made it more complex for no real reason. I can't think of a benefit for R6.
Yeah, that is very strange. It looks like the designer didn't know that a cathodyne is inherently self balancing. Sure, they didn't have laser trimmed, precision, +/- 1% metal films back in "the day", but still, looks like way too much trim pot there for balancing the +/- 10% C-comp resistors used quite commonly then.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote

Reply


Hide 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
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fisher X-101-D bsgd Tubes / Valves 6 27th April 2009 08:05 PM
fisher xp 60c tim614 Multi-Way 0 6th July 2008 02:51 PM
FS: Fisher KX-100 speaker Swap Meet 1 12th May 2007 10:14 PM
very peculiar psu problem Jaap Tubes / Valves 13 10th February 2007 07:48 AM
Peculiar Sovtek EL34 bevaviour ray_moth Tubes / Valves 12 15th April 2005 03:04 PM


New To Site? Need Help?

All times are GMT. The time now is 11:01 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2