Fisher's Original Bad Boy: The 50A Power Amplifier

Discussion in 'Fisher' started by dcgillespie, Jan 12, 2018.

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    When the 50A showed up on the 1952 audio scene, everybody took note. In one fell swoop, Avery both broke long established rules, and could arguably be credited with starting the first High Fidelity power war. Frank McIntosh had recently introduced the world to his innovative 50 watt Unity Coupled amplifier design, when along comes Avery with his 40 watt 50A, leaving everybody else trying to catch up.

    But Frank's design used a pentode output stage -- which was well known to not be the way to achieve audio nervana with the high fidelity crowds of 1952. Against that, Avery's design used a triode output stage that was producing 4 times the power that everybody else was getting out of their triode amplifiers. With triodes and high power, Avery had the wind at his back, whereas Frank's pentode design faced a head wind.

    And what's not to like? Big beefly transformers, lots of tubes, and a chassis layout with a terminal board and military grade wiring harness that just screams "can't touch this"! The kind of stuff that Tim Taylor would drool over. The thing is simply beautiful to look at when operating, even if it's not producing a sound. When it is producing sound, it produces that immediate if almost patented midrange clarity and presence that the model is famous for.

    Built like a tank, the design of the 50A was certainly innovative for its day: Dual heavy duty rectifier tubes for a 40 watt amplifier with a real choke input power supply gives the 50A a level of B+ regulation like no other in its power class: The B+ supply drops a mere 20 volts from zero to full sustained power output -- which it can do all day long if required. Compare that to a popular single rectifier tube 120 watt amplifier of today. It uses the popular Williamson front end with a single preamp stage tacked on in front of it, and a driver stage whose tubes glow blue, and can easily drive a speaker all by itself. Think of the snob appeal: I mean after all, does your amplifier have driver tubes that glow blue? :) Global NFB that includes all but the preamp stage rounded out the design. It was truly a unique effort that includes 6 glorious stages of vacuum tube amplification (three of which are push-pull), the likes of which was never seen before -- or since. Compare that to those today of the less is better crowd, who would (if they could) have their CD player directly drive a SE DHT output stage!

    Avery's secret to getting all the extra power output was that the beefy driver stage allowed the ability to drive the output stage into Class AB2, which has the effect of strapping afterburners on to a conventional triode output stage. When Class AB2 commences, power output rises significantly beyond the normal power levels achievable. Now just as Frank faced a head wind due to his use of pentodes, Avery also would have faced a stiff headwind were the fact of his amplifier operating in Class AB2 widely known, as that mode of operation has never been equated with high fidelity performance (i.e., low distortion) either -- unless it is designed very carefully. The good news is that it's precisely because Avery did design it very carefully with the heavy duty power supply and driver stage, that the amplifier does in fact perform very admirably for high fidelity service. And, because Avery was ever the marketer, he chose not to advertise the fact that his amplifier used a mode of operation generally reserved for a steelyard PA system but rather, heralded the 50A as the first home high power high fidelity all triode amplifier, since he knew that the tecknoids of the day would all be drawn in by the well known siren call of an all triode amplifier, and likely had little knowledge of Class AB2 operation, or the design concerns surrounding it anyway. Smart move.

    My current client has sent me two of these beasts for electrical restoration and performance assessment -- one being an early A, and the other a somewhat later AZ. In assessing the results of my archeological dig into both units, it appears that my work represents the third layer of recapping and service these two amplifiers have seen -- though its clear they never knew each other in their former life. In any event, two layers of some rather junky work had to be removed from both units before any serious restoration work could begin. As well, the AZ had clearly been either the recipient of some mad professor's efforts to address some of the design's deficiencies, or the result of junior trying to make a guitar amplifier out of it after dad updated his system. I mean, this thing had a completely different OPT installed on it at one time (looks like Acro), and has various holes drilled all over the chassis to accommodate (apparently) the various different circuits that were tried out in it. What was left of the original beautiful Fisher wiring was sad.

    The well known voltage surge of these amplifiers at turn-on is now taken care of with the use of modern 5V4GA rectifier tubes (the original 5V4G tube had very poor dependability), and can caps from Hayseed Hamfest specified at 600 vdc for the first can, and 550 vdc for the rest. The extra safety margin on the can caps is really icing on the cake, as between the 5V4GA rectifier tubes and a CL-80 current limiter, there is no voltage over-surge at turn-on.

    In terms of the actual project, I'm rounding third for home, with a lot of mop-up details to attend to, as now, both amplifiers have finally been stirred from their extended multi-decade hibernation (both still had all original can caps installed dating from mid '51 to mid '54, and anywhere in between), and had a visit to the listening room for the first time today. Along the way, I've developed a lot of info that the other 50A fanatics out there might find useful, and can offer some options for improved performance along the way as well. For now however, a few pics are offered. The rest will be added to the thread as the details become finalized.

    Dave

    As received, with neither apparently having received any love in a very long time.......
    SAM_2307.JPG

    Underside of the AZ. Apparently, after all the Frankenstein experiments were done, it was hap-hazzardly "restored" back to the original circuit at some point (although not correctly), with of course much the beautiful Fisher wiring basically trashed.
    SAM_2300.JPG

    Underside of the A, in progress.
    SAM_2321.JPG

    In the listening room operating together for the first time.......
    SAM_2332.JPG

    My 202-T is enjoying some time with Fisher equipment older than it is........
    SAM_2333.JPG
     
    Crestwood23, rogerfederer and jgannon like this.
  2. Jailtime

    Jailtime Standin' on a corner Subscriber

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    Wow, beautiful amps and nice work! I didn't know much of anything about the 50A, interesting to read about them. Does the 50A share much with the later 200A?
     
  3. vendo81

    vendo81 AK Subscriber Subscriber

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    Very nice Dave. What are you using for output tubes?
     
  4. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    :lurk: This will be a great thread.......Glad you got the cabinet for the 202-T :)
     
    Last edited: Jan 12, 2018
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Al -- It's a McIntosh cabinet. Matt does superb work. Having one built was about the only way I was ever going to find one for this unit in my lifetime..........

    Jail -- The 200 was the last iteration of the original 50A design. While the two designs were near identical in many ways, there were certainly some differences too, in that:

    1. The preamp stage was eliminated.

    2. The 12AU7 AF Amplifier/Phase Inverter tube was changed to a 12AX7 -- with the added gain this provided necessary to compensate for the loss of the preamp stage.

    3. The bias regulator 12AU7 tube section was electrically stiffened by connecting the other half of this tube (originally used as the preamp stage) in parallel with it, which made for an effective improvement in bias regulation during Class AB2 operation.

    4. The unit now used GZ34 rectifier tubes. The 5V4GA had already been released as an excellent replacement for the problematic 5V4G, but since the GZ34 was still superior yet and available (it had been used in some versions of the 55A as well), Fisher never again returned to using the newer 5V4 after having so many problems with the original G version of the tube.

    5. Output tubes were now EL34s (also used in some versions of the 55A), but this was maining a marketing choice, as the EL34 was not really a good choice for Class AB2 operation. Fisher therefore took measures (in the 55A as well) to significantly limit how far into Class AB2 the amplifier could be driven with these tubes -- all in the name of protecting them from melt down. Using the EL34 however was apparently important to Fisher, and indicates he had a read on the fickle portion of the audiophile crowd of that day as well.....

    6. The biggest difference with the 200 is that the output stage no longer operates in triode mode, but in pentode mode. The inherent power increase this mode of operation provides means little class AB2 operation is really required to produce a slight power increase over that of the 55 watt 55A series. The 200 is rated for 60 watts.

    Vendo -- The output tubes used/shown are yet another variation of the 6L6GC family, the 6BG6GA. This tube has 6L6GC guts in it, but brings the plate connection out to a plate cap terminal. Hence, the tube must be used with an adapter that provides for the plate cap connection.

    Dave
     
  6. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    And Dave scores another GRAND SLAM! Gotta love those 6bg6's. DROOOOOOOOOLL!
     
  7. gadget73

    gadget73 junk junkie Subscriber

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    Very cool. Good thing the AZ's original transformer wasn't lost so it can at least be put back right.

    original tubes in this were what, 6L6GC then? Or were these a 1614?
     
  8. larryderouin

    larryderouin You can be sure if it's Westinghouse??????? Subscriber

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    1614's per the Sam's Schematic.
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    OPPORTUNITIES

    For all of its high flying design/build qualities, opportunities exist to put a polish on the 50A that brings it to new levels of performance and capability in the modern audio age. None are particularly complicated, yet all are effective in result. One is brought over from previous development work on a 55A, one is brand new, and one -- its need depending on whether you execute one of the modifications -- corrects a rather bone headed build mistake on Fisher's part. But first, some mop-up comments regarding the power supply and related topics.

    1. For typical AC line voltages of today of around 121-122 vac, a single CL80 should be installed between the AC receptacle and fuse post to minimize the turn on current surge from all the cold heaters. So installed and with today's line voltages as indicated, the heater voltage at the output tube socket terminals is about 6.45 vac which is excellent for performance and tube life.

    2. Regardless of what rectifier tube type is inked onto the chassis, the optimum rectifier tube to use when using any of the 6L6 family of tubes is the 5V4GA. This tube will allow maximum usable B+ voltage without over-dissipating the 30 watt versions of the 6L6 family of tubes, provides excellent regulation, eliminates the turn on surge, and is extremely dependable.

    3. If GZ34 rectifier tubes are used, then the amplifier is capable of supporting and maximizing performance from output tubes as large as KT-120 tubes.

    4. When replacing the selenium bias rectifier with silicon, do not add any extra resistance for compensation.

    5. With the scenario described in #1&2 above, B+ at the output of the choke is typically 440-450 vdc when the output tubes are drawing the optimum quiescent level of current. With the 6L6 family of tubes, this will be a cathode current 120 mA. Screen grid current under this condition is < 2 mA per tube, and plate dissipation is just under 26.0 watts per tube. 30 watt versions of the tube take this in stride, and show no color in the plate, although the output tubes do need to be well matched. Power output will reliably be 45-50 watts RMS at low and midband frequencies. Power output does drop off slightly at high frequencies due to the winding capacitance within the output transformer.

    MODIFICATIONS

    1. Ground Loop Elimination: If you do nothing to these amplifiers, do this: At V1, pin #6 is grounded to the same ground tag (located at the V1socket front mounting rivet), that the cathode resistor from pin #3, and the ground terminal of the input jack are grounded to. The problem with this is that pin #6 of this tube is the plate element of the section acting as the bias voltage regulator. Because the regulator is passing about 5 mA even under quiescent conditions, and is fed by a half-wave rectifier, it injects the noise of this half-wave current flow into the audio ground of the input tube -- the most sensitive ground point in the amplifier. When the level control is advanced, the noise is minimized. But when the level control is turned full down or nearly so, the buzzing sound of half-wave rectification comes through loud and clear, and particularly so when high sensitivity speakers are used. The answer is to remove the ground connection to pin #6 from that ground point, and then ground pin #6 to one of the ground tabs over at C-15 (bias filter cap). This corrects a well defined ground loop built into the unit from the factory. When corrected, the reduction in noise is immediate and quite obvious, making for a very black background. If you perform the Mosfet modification coming up, the ground loop issue will be corrected automatically during the course of executing that modification.


    2. Mosfet Bias Regulator: This was developed during my work with the 55A, and since that unit uses the same bias regulator circuit as the 50A/AZ (the 55A is the successor of the 50AZ), that makes it a drop in fit for the 50A as well. In the 50A, the updated regulator allows the driver stage to develop less output for a given level of Class AB2 drive by working to reduce bias creep to an absolute minimum. This allows the driver stage to operate with less output and distortion, which is the stage where the most distortion is generated within the design. But there's an additional benefit as well.

    Class AB2 bias creep not only allows the driver stage to be more effective, but it also means that the output stage can idle at a notably lower quiescent current as well. Bias creep in a Class AB2 output stage is always in the direction of pushing the stage towards cut-off during AB2 operation, which increases distortion. To compensate for this, the quiescent current must be set higher so that when the creep commences, it creeps towards the lowest distortion operating point. But if the creep can be minimized, then no compensation is required. The quiescent current can be set lower, and the lowest distortion operating point is maintained at all power levels.

    Now granted, a lower quiescent current means slightly more bias voltage for the driver to overcome. But the increase in bias voltage due to a lower quiescent current represents only a fraction of the bias voltage saved due to elimination of the stock design's bias creep. So even with the reduced quiescent current afforded by the new regulator, there is still a net reduction in required drive voltage from the driver stage. Between the reduced driver stage output, and holding the output stage operating point much more consistently at the lowest distortion operating point, the new mosfet regulator arrangement - all else being equal -- drops all distortion by about 33%.

    It was mentioned that in the stock 50A with 6L6 family tubes, the lowest distortion quiescent current operating point will be found to be at 120 mA. However, with the Mosfet bias regulator installed, a new, and lower distortion operating point will then be found at just 95 mA of quiescent current, dropping the plate dissipation in the output tubes significantly, and making just as significant an impact on improving tube life as well.

    So the new regulator lowers distortion, reduces output stage quiescent current, improves tube life, and promotes cooler overall operation. And, because the regulator's dynamic action doesn't even exist electronically below about 20 watts RMS of power output, it is also completely neutral with respect to any audible characteristics. There are simply no down sides to the mosfet bias regulator modification -- only benefits.


    3. Negative Feedback and Stability: This is an aspect of the design that Fisher clearly struggled with, with at least three different versions of NFB circuits used through out the production life of the 50A/AZ. It is also what caused the Naval Research Laboratory in April of 1953 to write the following about the 50A when they tested several different popular premium power amplifiers of the day:

    "The Fisher Model 50-A Amplifier

    This amplifier (Figure 34) was somewhat of a disappointment inasmuch as about the
    only manufacturer's advertised specification which was met by the amplifier tested was its
    power rating (Figure 21). It will deliver the full 40 watts before any sharp increase in
    distortion. Frequency response and transient response were not good at either the high or
    the low frequencies."


    Indeed! And there were/are in fact such problems with the design -- which will be addressed and resolved, next time.

    Dave


    Ground loop hum: The short lead grounding pin #6 from the stock bias regulator to the same point where the input jack, volume control, and cathode resistor of V1-A is grounded to is a text book recipe for generating ground loop hum. It does. Correct by removing this connection and grounding pin #6 over at a ground tab on the Bias Can Cap. Or much better yet:
    SAM_2334.JPG

    The new Mosfet Bias Regulator installed. The installation inherently corrects the ground loop hum noted, and provides all the benefits noted in the discussion above.
    SAM_2335.JPG
     
  10. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Great Thread Dave and thanks :lurk:
     
  11. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Thanks for another great thread.
     

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