Peculiar Fisher circuit

SpreadSpectrum · 15th March 2008, 08:39 PM

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?

sorenj07 · 15th March 2008, 08:47 PM

http://www.imageshack.us/
thumbnail for forums 1.

SpreadSpectrum · 15th March 2008, 08:50 PM

Oops forgot to attach the image.

SpreadSpectrum · 15th March 2008, 09:00 PM

That's kind of embarrassing. "Hey everybody, look at my circuit that you can't see.."

tubewade · 15th March 2008, 09:40 PM

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

SpreadSpectrum · 15th March 2008, 09:46 PM

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.

SY · 15th March 2008, 10:14 PM

Besides increasing bias slightly, they also provide some ripple cancellation.

SpreadSpectrum · 15th March 2008, 10:52 PM

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.

ray_moth · 16th March 2008, 03:05 AM

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).

Miles Prower · 16th March 2008, 03:17 AM

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.

15th March 2008, 08:39 PM	#1
SpreadSpectrum diyAudio Member Join Date: Nov 2007	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?

15th March 2008, 10:14 PM	#7
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	Besides increasing bias slightly, they also provide some ripple cancellation. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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15th March 2008, 08:47 PM	#2
sorenj07 diyAudio Member Join Date: May 2006	http://www.imageshack.us/ thumbnail for forums 1.

15th March 2008, 09:00 PM	#4
SpreadSpectrum diyAudio Member Join Date: Nov 2007	That's kind of embarrassing. "Hey everybody, look at my circuit that you can't see.."

15th March 2008, 09:40 PM	#5
tubewade diyAudio Member 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

15th March 2008, 09:46 PM	#6
SpreadSpectrum diyAudio Member 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.

16th March 2008, 03:05 AM	#9
ray_moth diyAudio Moderator Emeritus 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).

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