The Fisher 100 Power Amplifier

audmod01

Super Member
I looked recently at the schematic of the Fisher 100 power amplifier. This unit uses EL37 push-pull output tubes and an unusual output transformer. Not only are the two plates of the EL37 tubes connected through a center tapped primary winding, there is another center tapped winding that connects the cathodes of the EL37 tubes together and to an RC network to signal ground. I have not seen any test reports on the performance of this amplifier, but am wondering if this design has some advantages over others that do not have the center tapped winding for the output tubes cathodes. Can anyone comment about its performance?
fisher 100 10001 19999 sm-03.jpg
 
Did you have anything specific you wanted to know?

The design is an ultralinear amp. I did find some stock/Fisher test reports for the 80-az (a very similar earlier model). I can attach them if you like. They're promotional material type stuff with graphs.

I have measured the performance on my pair of 100's (recapped). I believe it was ~24.5-25 watts per monoblock on mine.

Sound-wise, I personally think they are incredible. Extremely clear, extremely wide sound stage, etc. As for impedence, they seem to be happy to drive whatever I throw at them.

Dare I say, I think I may like them more than my 500b. Well, different anyways.

I am going to hook them up to a distortion analyzer at some point in the future. I'll be sure to post results when the time comes. If you want any particular measurements etc let me know and I'll try and get some for you :)
 
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Output stage cathode winding feedback does have it's advantages, but also has penalties as well. As Nate said, it is a form of the classic Ultralinear configuration, with Williamson himself dubbing it as "Super Ultralinear". Unlike classic UL however, Cathode Feedback (CF) impacts both the screen and control grids, where as strict UL only impacts the screen grid. Like conventional UL however, the CF winding effectively becomes part of the load that the tube works into, as well as an agent for FB. And as with UL, the greater the portion of that load that is attributed to the feedback winding, the greater the loss of power, since the dynamic screen grid voltage is diminished as the portion of the load that the feedback winding represents increases. Therefore, just as with triode operation, the connection has diminishing returns. Within the main of the audio bandwidth however, the use of CF is highly effective in reducing distortion, since in this case, distortion is not just reduced by the amount of the FB factor, but by the square of the FB factor thanks to the push-pull connection. Therefore, you get a lot from a little, which is a good thing too because a significant drawback of the concept is that the drive requirements for the stage increase rapidly as the amount of CF is used. Such designs therefore always have significant driver stages, and potentially extra gain stages as well to make up for the loss of gain in the output stage. In the 70-AZ, this concept was taken to the max, with the OPT in that model also employing classic UL screen taps as well, in addition to the CF winding.

The use of CF is a concept that requires an extremely well engineered OPT however. At supersonic frequencies, the phase shift in garden variety OPTs can cause such configurations to become unstable. This is the reason that many of the 80-AZ/100 units employ a 500 pF cap from the plate to cathode of one output tube, this to effectively improve the HF coupling between the plate and CF windings within the transformer, and help prevent parasitic oscillations from forming at high levels of power output.

Of the various examples I've come into contact with, the power output that Nate quoted is certainly accurate, and can go up or down a few watts depending on which output tubes are used. True EL37s will produce nearly 29 watts midband, and a very healthy 27 watts at 20 Hz. It is this end of the spectrum where the design really shines: At 1 db down from 30 watts (~23.5 watts), these units produce less than .5% THD at 20 Hz. Ditto for the midband as well. No Fisher receiver can produce that amount of 20 Hz power, let alone at such a low distortion level to boot.

The high end of the band however is where their performance is the weakest: At just 19 watts output, distortion at 20 kHz is over 3% (3.3% typical), with waveform compression becoming significant above that power level. Of course, there's little musical information requiring any significant power at this frequency anyway, so it is of little consequence -- but a fact of the design none the less.

IM distortion typically runs 2% at 26 watts equivalent power output. Frequency response is typically down 3 db by 36 kHz. Loaded stability of the design is good. Fisher used a number of wiring "gimicks" over the years to help promote stability in all of the "box car" amplifiers, most of which are not show on any schematic. As but two examples, many units had the control grid lead of one output tube wrapped around the plate lead of the other output tube. Somce examples used 10Ω plate suppressor resistors for the output tubes. In the Model 100, Fisher reduced the global NFB (from that of the 80AZ), but increased the standing current in the output stage to help get a handle on HF stability, without any significant reduction in distortion performance.

That's the high points that I've taken from my notes over the years with these amps. Between that and Nate's comments on their sound, that should give you a pretty good idea of their lab and listening room performance.

Dave
 
Nate and Dave;

Thanks for your comments!

That is exactly the kind of information I was looking for. Dave's explanation makes clear the effects of the cathode feedback along with the negative feedback from the speaker secondary winding. I had never looked that closely at the 100 or the 70-AZ or 80-AZ units, but it does not surprise me that those units feature the cathode feedback too. Am I correct that the major improvement is intermodulation distortion reduction?

A number of power amplifiers with push-pull outputs use tubes such as a 12AU7 which have lower impedance than 12AX7 tubes to drive more demanding output tubes. The Fisher 440 EL84 output stage uses 7247 tubes for driver/inverter stages. The 1/2 that performs the phase inverter function is the equivalent of a 12AU7 stage while the driver half is equivalent to a section of a 12AX7 according to one article I read.

Joe
 
I have one amp with similar cathode feedback windings, a Bogen DS-225. Output response is flat as a board until the transformer saturates right around 28 hz. Its got reasonably low distortion too, under 1% anywhere you'd actually be able to listen to it. The driver is a little less heroic on the Bogen, being one stage less than the Fisher 100, but its not exactly a feeble driver stage either. 1/2 a 12AU7 as a voltage amp driving a 12AX7 long tail pair inverter into a pair of 7355 tubes (think re-based 5881 / 6L6GB) to make an honest 22 watts per channel.

Its a very clean sounding amp, not tubey at all. Some people might not like it, but it really does a fantastic job of imaging and not coloring the sound.
 
I just picked up a pair of Fisher 100 monos.
I will Do the necessaries :)
I’m looking for feedback from those that have substituted the EL-37s with 6L6GCs ...???
Are there any circuit modifications needed to use the 6L6s? @dcgillespie

Thanks!
 
As long as you use 6L6s of the "GC" variety, no changes are needed. Modifying the unit to operate with less capable tubes of this family will reduce performance. Appropriate tubes for these amps besides the original EL-37s would be KT66s, 6L6GC, and 7581. Tubes to avoid would be the 6L6 up through and including the GB version, and the 5881. They simply do not have the necessary plate dissipation rating for the design of these amplifiers.

Dave
 
As long as you use 6L6s of the "GC" variety, no changes are needed. Modifying the unit to operate with less capable tubes of this family will reduce performance. Appropriate tubes for these amps besides the original EL-37s would be KT66s, 6L6GC, and 7581. Tubes to avoid would be the 6L6 up through and including the GB version, and the 5881. They simply do not have the necessary plate dissipation rating for the design of these amplifiers.

Dave
Perfect, thanks!
 
As long as you use 6L6s of the "GC" variety, no changes are needed. Modifying the unit to operate with less capable tubes of this family will reduce performance. Appropriate tubes for these amps besides the original EL-37s would be KT66s, 6L6GC, and 7581. Tubes to avoid would be the 6L6 up through and including the GB version, and the 5881. They simply do not have the necessary plate dissipation rating for the design of these amplifiers.

Dave
I just finished them up.
I going to sell them. They sound fantastic, but my MC-60s can push my B&Ws better...
here’s some photos:
4CEE283C-90DD-4233-AEEB-880DD54B5272.jpeg 1FBF12E3-97DE-4E23-B0B9-67A164B1AAB2.jpeg B9E456FE-3CCA-45D5-9E08-5D1A75DAC24C.jpeg

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8A039D14-A403-4915-ADD4-CA184EBFEC55.jpeg

The 6L6s sound fantastic, Maybe I’ll keep looking for EL-37s...
 
I just finished them up.
I going to sell them. They sound fantastic, but my MC-60s can push my B&Ws better...
here’s some photos:
View attachment 1570234 View attachment 1570235 View attachment 1570236

View attachment 1570237
View attachment 1570238

The 6L6s sound fantastic, Maybe I’ll keep looking for EL-37s...

I’m a bit late to respond but maybe you’ll catch this.

Nice to see another fully functioning set on here! How do you like them so far - do you still use them?

I don’t think I can ever part with these now personally.

A ‘number’ of years ago I think I changed the wiring in my model 100s slightly to use 6CG7s (in place of the 12AU7s). (I really wish I could find the info I referenced to do this...it might have been from the infamous @dcgillespie himself!).

I also snagged a 6CG7 -> 6SN7 adapter. I believe this is okay but would love to get an expert’s reassurance if possible.

It sounds so, so, so good with CBS Hytron black plate 6SN7s and (edit) Siemens 12AT7s. :)

I’m really open to tube suggestions, if anyone has them.


Haven’t used the amplifiers in a while, I should give them some time while the weather is still cold. ; )
 
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I’m a bit late to respond but maybe you’ll catch this.

Nice to see another fully functioning set on here! How do you like them so far - do you still use them?

I don’t think I can ever part with these now personally.

A ‘number’ of years ago I think I changed the wiring in my model 100s slightly to use 6CG7s. (I really wish I could find the info I referenced to do this...it might have been from the infamous @dcgillespie himself!).

I also snagged a 6CG7 -> 12SN7 adapter. I believe this is okay but would love to get an expert’s reassurance if possible.

It sounds so, so, so good with a 12SN7 CBS Hytron black plate 12SN7s and Siemens 12AU7s. :)

I’m really open to tube suggestions, if anyone has them.


Haven’t used the amplifiers in a while, I should give them some time while the weather is still cold. ; )

I absolutely love them! But I have tube amps coming out of my nose. I tried selling them on Bartertown but everyone thought I was overpriced at $1100 for the pair. So they sit on a shelf.
 
I have a pair of 100's and I have a guy that wants them. I don't want to sell mine. I will let him know about yours.

Larry D.
 
I also snagged a 6CG7 -> 12SN7 adapter. I believe this is okay but would love to get an expert’s reassurance if possible.

You want to use a 6SN7. 6CG7 and a 6SN7 are basically the same tube in different packages. 12SN7 is similar, but with a 12v heater vs a 6v heater. I'm guessing it won't work too well being fed half it's required voltage.
 
You want to use a 6SN7. 6CG7 and a 6SN7 are basically the same tube in different packages. 12SN7 is similar, but with a 12v heater vs a 6v heater. I'm guessing it won't work too well being fed half it's required voltage.

Ahh thanks for catching that - that was a typo. Meant 6SN7! Corrected above.

The 6SN7s really take the amps to another level, I think.

I absolutely love them! But I have tube amps coming out of my nose. I tried selling them on Bartertown but everyone thought I was overpriced at $1100 for the pair. So they sit on a shelf.

Well their loss : ). That’s a shame; I feel like these monoblocks would best most/any amplifier above that price. Regardless, I hope they go to a good home if you do sell them. (And if you are still trying to find a home for them in a couple of years; let me know, I’d probably be interested in them.)

If you ever get the itch to play with them, try substituting NOS Russian PIO caps, and upgrading the resistors (I can look at what I used if you like; might even have a few spares around).

If I had an endless supply of tubes and such I’d use them more frequently. Mine come out for special occasions when I want to kill my power bill and stay warm : ).

Kidding aside, they are really out of this world, at least for me; crazy (but fun) detailed, near holographic imaging, amazing frequency response.


I have a pair of 100's and I have a guy that wants them. I don't want to sell mine. I will let him know about yours.

Larry D.

Glad to see there are 3 of us in this club now! (Apologies if I missed anyone!) What do you think of yours? Any tubes you like in particular in these?

Maybe some other owners will pop out of the woodwork.

I might be persuaded to bring these to the upcoming AK meet in March if there’s any interest and room to play them.
 
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Ahh thanks for catching that - that was a typo. Meant 6SN7! Corrected above.

The 6SN7s really take the amps to another level, I think.



Well their loss : ). That’s a shame; I feel like these monoblocks would best most/any amplifier above that price. Regardless, I hope they go to a good home if you do sell them. (And if you are still trying to find a home for them in a couple of years; let me know, I’d probably be interested in them.)

If you ever get the itch to play with them, try substituting NOS Russian PIO caps, and upgrading the resistors (I can look at what I used if you like; might even have a few spares around).

If I had an endless supply of tubes and such I’d use them more frequently. Mine come out for special occasions when I want to kill my power bill and stay warm : ).

Kidding aside, they are really out of this world, at least for me; crazy (but fun) detailed, near holographic imaging, amazing frequency response.




Glad to see there are 3 of us in this club now! (Apologies if I missed anyone!) What do you think of yours? Any tubes you like in particular in these?

Maybe some other owners will pop out of the woodwork.

I might be persuaded to bring these to the upcoming AK meet in March if there’s any interest and room to play them.

when I had 604Es, the Fisher 100s almost matched the magic of my Marantz 8b.
 
Well now I have to listen to the 8b before I die. Added to the list : )

Mine are just powering a pair of ADS L980s.

On another note- a big thank you goes to Dave for all of the info above (and for all the invaluable info you continue to seemingly pull out of the woodwork!). I don’t think I properly thanked him before (as I was in the middle of moving last year around that time and other things took priority). Seriously, Dave, your info and contributions are invaluable.


I stuck CE caps in mine. Here’s a pic of one when I had them under the knife. The caps fit perfectly.
DC0BE7E7-6458-4BEB-AF83-896CA77EB28E.jpeg

I usually leave the screws out of the cage so I can take them off and let them cool a bit more when I use them. Here’s mine in their current spot (Currently my small apartment limits the space options, alas...)

A0584C1A-B2BA-4141-8711-E25795BE0416.jpeg
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Next on the list to fix are the RCA jacks, I think.
 
I've recapped mine and used Cardas Golden Ratio caps for couplers. They're dead quiet and sound great. I may try some Miflex copper caps in there and hear if there's a difference. For output tubes I'm just using some inexpensive Russian 6L6 military tubes. I do have some Tung-Sol KT77's and some GE 6L6GC's that I need to try out in them.

Larry D.
 
Those miflex caps look interesting, let me know if you do put those in. I’d love to know what you think.

You’re making me itch to switch out the auricaps in my 500b now : ).

My power tubes are also Russian, though I’m running NOS 1986 6P3S-E’s. I think they’re pretty darn good, especially considering the price at the time a few years back.

One monoblock came with a pair of the original Mullard EL-37’s in amazing condition.

Unfortunately, I don’t have a set for the other monoblock, though they do sound super nice.

If you get around to trying the other power tubes I’d also be interested to hear what you think. My experience has only been with the Mullards and the NOS Russian ones above.
 
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