Fisher's First High Power Stereo Amplifier: The SA-300

Thanks for yet another great write-up. Perhaps, it's worth noting that the ARC amplifiers were not "modifications" as in the case of the SA-300 presented here. ARC's implementation of the cathode coupling/feedback seemed more like an after-thought rather than a proper design, since it could easily spec or use OPT with separate tertiary windings instead of relying on the secondary windings to do the job.

As for using "a well designed output transformer for the effective implementation of this type of NFB", while that is certainly preferable, nevertheless, the cathode NFB can work well enough even with less than stellar OPT's, e.g., Quad-II is probably the most famous amplifier that uses the cathode NFB or what Peter Walker called the "Acoustical Connection", yet Quad-II's OPT has generally been considered rather run-of-the-mill, perhaps even sub-standard, yet the amplifier works beautifully and is still much sought after today. So it still comes down to how the design is implemented, clearly, Peter Walker knew what he was doing... :)
 
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Thanks for your comments -- and the reminder of the Quad-II's use of this type of NFB as well.

The interesting thing about ARC however, is that the concept was used in virtually all of their designs. You'd think that if an afterthought, then future models would would have included a true cathode tertiary winding in the OPT, but that was never the case. Of course, the more you went that direction (more FB via a true cathode winding), the more the design would start to take on a Mac type look (loosing uniquity) -- and also suffer power output loses as screen grid voltage becomes more and more compromised, until you finally go all out with a 50% cathode winding and then use cross coupled screens to achieve a true Unity Coupled design.

Regarding the quality of OPT used, I think your comments are spot on -- as long as the secondary winding reflects the total HF characteristic of the transformer at at least the 4Ω and 16Ω taps. Several decades ago, my first experiments with this type of feedback were (unfortunately) based on the Acrosound series of transformers (TO-300 and TO-330), both of which will absolutely fail miserably if this type of FB scheme is employed with them, because with at least these specific transformers, each output tap presents a different HF response characteristic relative to the other. As a result, even when connected for negative feedback, the output stage becomes an oscillator. On the other hand, the Dynaco series of OPTs presents an identical response characteristic at each tap, and so works perfectly when this type of connection is used. Back in the 70's, Eisenson experimented with this type of FB with the ST-70 design, producing excellent results. Ultimately then I agree with you, that it can be even a mediocre OPT that can be used to good effect -- as long as the response characteristic is identical at at least the taps relevant to this discussion. But also.......

I've never had a Fisher model 70 amplifier come across my bench so as to be able to work with that design, but it employed both UL taps and a separate cathode tertiary winding. Fisher rather quickly dropped the UL taps (maybe to eliminate any patent rights issues), and used just the cathode tertiary winding in their later model 80 and 100 "Boxcar" designs -- amplifiers I have had significant experience with. These amplifiers went through a series of build "gimmicks" throughout their production life to help promote stability, in addition to additional capacitance that the output stage required to be connected between the primary windings to maintain stable HF performance within that stage. Tests showed that the cathode winding represented 20% of the total primary winding, that ultimately provides about 9 db of NFB on each side of the push-pull connection. This effective NFB of 18 db, coupled with 15 db in the outer loop had these amplifiers operating with over 30 db of NFB, yet for all of that feedback, mid-band THD was still a measured average of 0.44% at 1 kHz, while IMD clocked in at 2.1%. 20 kHz power output suffered a 20% loss over mid-band power output, and tipped the scales at 3.3% THD when power output at that frequency was just 66.3% of mid-band power capability. Clearly, output stage/output transformer issues are at play with this design, as the rest of the design (rather typical Williamson type front end) is capable of far lower distortion performance than the unit actually delivers.

This suggests that (again), while a mediocre transformer may in fact remain stable when partially cathode coupled, and even provide good benefits at mid-band frequencies, it still takes a very high caliber transformer to produce the benefits of push-pull cathode coupling across the full audio band of 20 Hz to 20 kHz. Clearly, with the distortion results produced in the modified SA-300, this was achieved with this combination of connection, tubes, and OPT, while it is highly questionable that the purposed designed transformers of the 80 and 100 amplifiers ever did (same transformer used in both models). It is quite possible then that for all practical purposes, the use of high Gm tubes, a quality OPT, and partial cathode coupling represents a very desirable (if not optimum?) mix of good effect (distortion reduction), good stability (as long as the secondary tap HF characteristics are all equal), and practical application (allowing the use of rather standard driver configurations), beyond which, a jump to a full Unity Coupled design may be the next most worthwhile step to take. No doubt that Walker knew what he was doing. But it just may be that in spite of having the design capability, Johnson recognized that partial cathode coupling using the output winding was the most practical way to go -- hence, no dedicated cathode tertiary winding (cost never hindered any ARC design!). With the results produced in this modified SA-300 project, it's hard to argue with that reasoning. :)

Thank-you again for your comments and input on this subject. If this project and discussion proves anything, it's that just as with output transformers and vacuum tubes, the application of NFB -- and how effective the results are it produces -- are as much a product of art and creative thinking, as it is design and engineering.

Dave
 
Thanks for sharing your finding on the Fisher 80 and 100 amplifiers, which are great reminders for not getting carried away with NFB, as it’s not a cure-all. Similarly, even though the Quad-II worked beautifully, its ultimate performance is still limited by the pityful OPT, as the LF output power and distortion does not match that of the mid-band - no amount of Acoustical Connection or gNFB could get around that.

As for the ARC and its reputation for the “cost is no object” approach to amplifier design and construction, the use of secondary winding for the cathode feedback nevertheless represents a cost-saving measure, and quite unfitting for the amplifiers of this price range... ;) Just me 2c.
 
Dave.

I have been studying the 3 schematics you posted to familiarize myself with the modifications. I have a couple questions.

On the EFB schematic, the 2 watt resistor at the top after the 410 V supply. I cannot make out its value. 15? 150? 1500?

On the Input and Inverter Driver Stages schematic, The arrow at the top with the 16 ohm label . Does that go to the 16 ohm speaker tap?

Can the three mods be done individually? If so I assume EFB could be done first, and then I am stumped.

Thanks,
Curt
 
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Hi Curt -- Sorry for the poor resolution of the schematics.

The resistor connecting to the Drain of the Mosfet is a 150Ω 2W piece.

Yes. The 3600Ω/270pF network connects to the 16Ω tap of the OPT secondary.

As to installing the modifications individually, I wouldn't advise that, as they all work together cohesively as a team. That is, individually, they wouldn't work well as designed when intermingled with the other elements of the stock design. For example, the EFB circuits are are not designed to be used with the partial direct coupling of the stock driver stage. It could be designed to, but is not since the partial direct coupling was one of the very elements of the stock design that was sought to be eliminated. If the EFB circuits were to be used with the stock driver stage, it would result in the output stage being severely under biased.

Similarly, if the new AF Amplifier and Phase Inverter stages were used with the stock power supply and output stages, it would almost surely result in instability, since most of the original stability circuits have been dispensed with in the new front end design since it operates with much less FB in the global NFB loop.

Now you possibly could implement just the output stage changes since the global loop of the stock design would automatically compensate for the reduced output stage gain that its new configuration produces. Likely however, HF performance would suffer since all the HF stability networks would still be in place while the amplifier was operating with reduced FB in the global loop.

So again, I would implement the complete modification in its entirety for best results. If you stumble along the way, take plenty of good pics, and I'm sure we can get you through it just fine.

Dave
 
My apologies for dredging up this old thread. I have to say, the performance improvements made to this vintage Fisher are really impressive. Dave, you deserve a lot of credit for the innovative concepts behind this work, not to mention that you must have the patience of a saint. There's no rulebook one can refer to for this sort of thing, particularly when the original design is created and optimized by someone else.

Dave, we've discussed pentode PP operation of the EL34 in a few e-mail exchanges. I'm the guy who's "re-creating" a few vintage amps that have the right output transformers, but the wrong (unobtainable) tubes. One thing about this article that caught my attention is your mention of 12% open loop distortion of this amplifier. I have to say, that's much worse than I would have ever suspected. Can you tell me the general operating conditions when you measured this? Also, assuming this was THD, did you happen to record the approximate spectral content of the distortion?

Thanks!
Jack
 
Hi Jack -- The 12% open loop was not a measured value, but a calculated one: 0.6% THD at 1 kHz (measured) X 20x distortion reduction from 26 db of NFB means that theoretically you're starting from 12% THD. Neither did I measure the spectral content either.

I would again emphasize that by itself, the EL34 is a very low distortion tube -- but when operated in medium power mode, must be very carefully supported by the power supply to achieve it benefits in this regard. The easiest, most economical, and most practical way to achieve those benefits in the 60s was to use a tapped screen grid design -- which is why those types of designs seriously outweighed straight pentode designs of the period by a longshot. The problems in that day of practically maintaining a proper relationship between the main B+ voltage, screen grid voltage, and control grid voltage in a pentode design -- when operating the tube at medium plate voltage and power output levels (versus high voltage/high power designs) -- was such that it really wasn't practical, or else needed the high NFB levels and all the additional design efforts to make a serious go of it as outlined in the thread -- which again, Fisher did. But also again, at what cost to tube life, heat production, power consumption, etc.? (square peg into a round hole) -- which were the motivating factors in developing the modification I presented to begin with. Taking those on however required addressing the big elephant in the room, which was how to efficiently operate EL34 tubes in pentode, medium power mode, and ultimately resulted in the modification presented.

Today, with the aid of SS devices and using a new approach (EFB™) to managing these tubes in this mode of pentode operation, open loop distortion can be reduced down to 1%. Applying the same 20X level of distortion reduction (rearranged into a local and global arrangement for better stability) then produced the final (measured) 0.05% THD achieved.

Dave
 
Hi Dave, I appreciate your clarification on this point. It’s perfectly reasonable of course to calculate open loop distortion in that way. The resulting number doesn’t speak well of Fisher’s design decisions, but like you, I try not to judge too harshly using hindsight. Technical data published for the EL34 indicates good linearity in the context of pentode power tubes, and considering the great success of 6L6 variants in this mode, I have to wonder if Fisher got caught with their pants down after committing to the EL34. Voltage stabilization in that era would have been costly and unorthodox in audio products, so perhaps this is all responsible for Fisher’s eventual use of so many band-aids. I have to say, continuing to use the EL34 in multiple models seems like stubbornness, but there must have been considerations of which I’m not aware. Thanks again!
 
Just completed a SA-300 mod/rebuild using the techniques here.

sa300_mod_underneath_10-20-2023.jpg

All the mods Dave mentioned are done here, though I laid out some components a little differently, due to the parts I had available. Essentially the same values, just a few things arranged physically a bit differently.

I installed a new set of Tung Sol EL34B output tubes- and everything came right up, and was easy to bias and DC balance. Bias was rock-solid with the EFB and the EL34B tubes, once set.

Fired it up, and drove it with a 60Hz sine wave from a signal generator- with both channels driven together, it immediately belted out 25VRMS at the 16 ohm taps, loaded, on both channels, with no obvious sign of clipping or other distortion. That's 39 watts per channel, both channels driven! Pretty impressive!

Sound is also very good- very fast, very good bass control, very dynamic. And with the improved power supply filtering (the addition of the new cap right off the rectifier, and the choke), it's absolutely silent. NOTHING can be heard from my bench speakers, with the inputs shorted.

Looks (and more importanly, SOUNDS) like this is going to be one heck of a performer.

Regards,
Gordon.
 
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