Fisher 200 EL-34 monoblocks with interstage transformer

WopOnTour

Hoarder Extraordinaire
I'm soon going to be restoring a beautiful pair of Fisher Model 200 mono-blocks (see circuit below)
I realize it was much more common "back in the day" to use an interstage output coupling topology as these amps employ. My question would be, would there be any sonic advantages to removing the interstaging and go to strictly capacitor coupling? (the plan would be to use audiophile grade Teflons or PIOs)
Anyone with experience with these amps?
WopOnTour

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I am waiting for a response too. I am having trouble visualizing what is exactly the reason for the transformer unless they are using it as the phase inverter (most likely). It is kinda fuzzy on this laptop. While I am no expert, I usually find that when I restore something with good equivalent replacement parts, it seems to work very well. I figure that the Fisher engineers (and Scott engineers etc.) pretty much knew what they were doing.

Example: My Sansui 1000A had modifications to the bias pot arrangement. Extra resistors to "increase" the amount of negative bias. Replacing the leaky paper in oil capacitors, putting all resistors back to factory original values allowed bias adjustments to be right where they were designed to be.

Think about this too. Major modifications MAY affect the value of the amp. That is usually not my main concern. If so, all of my Scott amps would be worthless because they don't have leaky Pyramid and American Radionic Ceracap capacitors in them...
 
The cathodes of the 6CL6's are cap-coupled to the grids of the EL34's and return to ground through the transformer on the primary side.

The secondary side of the transformer is tied into the bias circuit of the amp.
 
Push-pull cathode follower, coupled with a 1:1 transformer with caps providing a high-frequency path around it (minimizing the phase shift that would occur otherwise). Seems odd for pentode output - if you triode wired the EL34s it would make a LOT of sense, since voltage swing will be greater and grid current must be supplied if you drive the outputs into AB2. Look at the bias supply - it also comes from a cathode follower, so grid current is expected..

The circuit is similar to the earlier Fisher 50A which used 6L6s in triode, class AB.
 
Thanks guys
I guess I really should start by leaving it all 100% stock and THEN perhaps look for ways to improve it, but only IF REQUIRED.
It's going to need a complete recap of the PSU (these amps still have the 4 original paper covered cans, so my plans are to "stuff") I was looking to decide what to do about the "interstage" arrangment as the 20uf coupling caps are going to be quite pricey and if the interstage was considered inferior in sonic performance to single cathode coupling, I wouldnt bother with them and just rewire for a more conventional coupling circuit.

i.e. But it appears there's a concensus that it would generally provide superior range/response performance then? or potentially hinder it somehow? I'm just no that familiar with this topology and was hoping to find someone here that might be. Are there other non-Fisher amps that emply this? Someone told me there's a McIntosh amp designed like this?
The tube generated bias, 70V speaker outs, and damping factor adjustments are also interesting...
Seems odd for pentode output - if you triode wired the EL34s it would make a LOT of sense, since voltage swing will be greater and grid current must be supplied if you drive the outputs into AB2. .
Thanks Tom, actually my plan was to mount a triode<>pentode switch on the back of each amp.Thinking also of perhaps adding some 1 or 10 ohm resistors in the cathode circuits, (and ditching the phono test plug ground path) so you can more easily rebias for triode/pentode switch-hitting...

WOT

<EDIT> I guess the 20uf coupling caps won't be huge $$ after all. I assumed they were film caps but they are electrolytic (dont see that often) so I'll likely just use Black-gates or Jensens and then Russian PIOs as couplers between the 12AU7 and 6CL6
 
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Push-pull cathode follower, coupled with a 1:1 transformer with caps providing a high-frequency path around it (minimizing the phase shift that would occur otherwise). Seems odd for pentode output - if you triode wired the EL34s it would make a LOT of sense, since voltage swing will be greater and grid current must be supplied if you drive the outputs into AB2. Look at the bias supply - it also comes from a cathode follower, so grid current is expected..

The circuit is similar to the earlier Fisher 50A which used 6L6s in triode, class AB.


Thanks Tom for the explanation. Don't see that type of circuit very often do you? Is it something that was popular say, 50 years ago?


FWIW, I really think you may want to leave it stock and just renew those components that are leaky. Filter capacitors come to mind and usually coupling capacitors are no good. But I would really keep it as designed.

I say this not to tell you that you shouldn't change anything. It is your amplifier to do with whatever you like. I just think the outcome would be spectacular if it were back to original, as designed, condition.

Just an opinion, not suggestion. Please keep us informed!
 
I just looked at the schematic again. I had not looked far enough back in the circuit. The phase inverters are way, way back! Then, the signal path is parallel through a few sections of 12AX7. Is this unusual or did the HiFi manufacturers often do this?
 
The Williamson has the phase splitter a stage back from the power section. Unity coupled amps also had it way back there.
 
Thanks. I am more familiar with what I would call conventional design. I lead a sheltered life, I usually fool with Scott.
 
It is probably the rarest and certainly the most badass amp Fisher made.

I'd truly want to hear them in their original configuration for a good long time before moving onto modifications.

What a monster!

200.jpg
 
I too say keep it stock. Replace all the caps and leave it be. A damn nice piece of history there. John
 
Glad I am not the only one that feels that way. But again, it is yours, you should be able to do what you want. Now if it were mine..... ;)
 
No way would I modify the circuit. That configuration allows some serious AB2 drive capabilities without the bias dangers associated with a standard direct coupled cathode follower, and without the phase shifts of a plate-driven interstage transformer. Removal of the transformer would be a serious downgrade.
 
Many would say that going from transformer coupling to RC coupling would be a step down in quality. That is a very cool and interesting amp. I also vote to leave topology alone until and unless you experience any real short comings in it. I would wager that if it is restored to its original glory that you would have a very dynamic amp. I eagerly await listening test reports after you get it finished.

mike
 
I thought my SA 300 was cool but you win...that's an awesome pair of amps! I had my 300 restored without modification....I know they're different but if the 300 is anything to go by, Fisher knew what he/they were doing. Keep it original is my vote...
 
Good Grief!

I had a pair of those amps when I was in high school in the early 70's.

I knew they were nice, but I was not that knowledgable and did not realize I had a killer pair of mono-blocks. I bought them needing repair which I did to make them work.

Since transistor amps were new at the time and the rage I had a hard time selling them when I joined the Navy.

When I got out of the Navy and inquired about them from the friend of mine who bought them(for a really cheap price!!!), he mentioned that someone told him the amps were worth a fortune and he wanted $2000.00 to buy them back with broken tubes and sitting at the bottom of his closet with crap all over them.

Suffice to say, that was the last time I saw him as he pissed me off being too greedy to feed his bad(and illegal) habits he had acquired over time.

Those amps were pretty looking though, mine were in nice cosmetic shape when "I" had them.

Regards, Ron
 
I too say keep it stock. Replace all the caps and leave it be. A damn nice piece of history there. John
Many would say that going from transformer coupling to RC coupling would be a step down in quality. That is a very cool and interesting amp. I also vote to leave topology alone until and unless you experience any real short comings in it. I would wager that if it is restored to its original glory that you would have a very dynamic amp. I eagerly await listening test reports after you get it finished.mike
No way would I modify the circuit. That configuration allows some serious AB2 drive capabilities without the bias dangers associated with a standard direct coupled cathode follower, and without the phase shifts of a plate-driven interstage transformer. Removal of the transformer would be a serious downgrade.
I thought my SA 300 was cool but you win...that's an awesome pair of amps! I had my 300 restored without modification....I know they're different but if the 300 is anything to go by, Fisher knew what he/they were doing. Keep it original is my vote...
Forgot to add:Don't modify that circuit. It was very well designed!
Ron
Thanks Mike, Ron, jon, John and my other brother John... ;)
That appears to be the consensus, and so it shall be!
The only reason I questioned it in the first place was that I dont recall seeing this type of interstage layout on anything else, and given the model year (~1958-59?)I assumed it might be a mid-fi arrangment or some kind of "patch-work" for a component/circuit mis-match...

Really eager to get started on these, as I've never seen these Fishers before anywhere
I hope they sound as good as they LOOK!
It is probably the rarest and certainly the most badass amp Fisher made.
I'd truly want to hear them in their original configuration for a good long time before moving onto modifications.
What a monster!
Yep, that's them soundmotor (I've added some pics in my original post as well) The little white thing you see standing like a tube is actually a ceramic wire-wound screen resistor.

I'll tell you one thing, these things are HEAVY!! (~60lbs each!)
The underside is beautifully laid out out within what appears to be some sort of copper-alloy chassis (will post better pics sometime soon) there's a few minor scratches here and there but the brass cages and face-plates are near perfect!

WopOnTour

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The underside is beautifully laid out out with a copper lined inner chassis (will post better pics sometime soon) a few minor scratches here and there but the brass cages are near perfect!

Yes please, love to see some more pics. Very interesting circuit.:yes:

Thanks for posting, and congrats on your find.

Jeff
 
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