Fisher's Original Bad Boy: The 50A Power Amplifier
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.
As received, with neither apparently having received any love in a very long time.......
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.
Underside of the A, in progress.
In the listening room operating together for the first time.......
My 202-T is enjoying some time with Fisher equipment older than it is........