The early 1950's. The hifi bug was well on its way to becoming an epidemic, with the stage being set for a scene that more and more amplifier manufacturers would jump into. And, then as now, the arguments raged on as to what approach produced the best sound. Today, much of the discussion is all about boutique component selection, tube rolling, and burn in time. But back then, such discussions were hardly a thought or consideration. Back then, it was all about the heady discussions of pentode or triode design, or how the Circlotron, Unity Coupled, or Ultra-Linear configurations could best be utilized. Class of operation was always at the top of the "discussion" list, with Class A, and AB1 both having their followers, and the high power/low power camps both chiming in right on queue. During those times, there were no wimpy discussions about whether point to point or tag board construction sounded best. It was all about serious real man discussions, whose outcomes formed the basis of entire companies or product lines. Brands became known for the design approaches they championed, many of whom lived or died by the decisions they made. It was an exciting time in the world of high fidelity, and truly the genesis period of this hobby we all know and love.
With Fisher's company freshly reorganized as Fisher Radio Corporation after the war, they jumped into this fray with their own signature design around 1953. In keeping with Avery's burning desire to be unique if not the first with the products his company produced, he introduced the model 50-A power amplifier (series). And what an amplifier it was. The concept of dollar per watt hadn't been conceived yet, but with this amplifier, one of Fisher's first "firsts" must surely have been realized: the concept of pound per watt. As a single chassis device, this mono basic power amplifier achieves a one-to-one ratio, tipping the scale at 50 pounds, producing an advertised 50 watts of power. This then was no chickified model designed to win over the hearts of the fairer sex, but an all out 1953 execution of Tim Taylor's "mo' power, mo power!" concept. With two massive transformers as an anchor, this amplifier is the antithesis of a chassis in search of some transformers to build an amp on. Rather, this is a couple of transformers that happen to have a chassis strapped to them!! It's a beautiful thing to behold, if not hernia inducing. To be sure however, for those of us with grand kids, this ain't your dad's old hifi amp......... it's YOUR grandfather's!........so that should help to keep things in perspective.
As to its design, it was, and remains truly unique in equipment designed for the home high fidelity setting. The power supply is within the conventional realm for its day, but massive. Dual heavy duty rectifier tubes, a (swinging) choke input filter system, and vacuum tube bias regulator circuit get things powered up. The phase inverter and pre-driver small signal circuits borrow heavily from Williamson, and like that design, there is a global NFB loop that (in part) encompasses these stages. But after that, any resemblance to any other design of the day for high fidelity applications ceases. Following the pre-driver stage, two triode tubes (or triode connected pentodes) operate as push-pull Class A cathode followers, whose output is coupled to the output stage via transformer/capacitive coupling. This driver stage then allows the triode connected output stage to operate in class AB2, which is VERY much different than anything else Fisher -- or anyone else in this product category -- ever produced.
Briefly, early high fidelity design was all about low power triode output stage operation, as that was the only practical way to achieve a low output impedance, and low distortion at the time. Pentode operation produced more power, but had a high output impedance (and higher/odd distortion elements). Negative feedback could correct all of that, but proper application of it was still elusive at the time, so low power triode amplifiers and big high efficiency speakers ruled the day. Think about 7-10 watts RMS and K-Horn speakers.
It is against this backdrop then that the Fisher 50A did its cannonball leap off the high dive. Big power, low output impedance, flat response, and good distortion characteristics made for a package that others just did not have at the time. The fact that it could also serve as a boat anchor in an emergency was just an added plus. Over the course of about 4 years, the amplifier went through various versions, with the biggest changes being in the rectifier and output tubes used. Earlier on, there was also a change in driver tubes used, but for all those changes, the circuit itself changed very little if hardly at all. The change in tubes simply represented the use of bigger (higher dissipation) tubes as they came along. The 55-A -- which is the subject of this thread, is then (to my knowledge) the last of this breed, sporting big 5AW4 rectifiers and 6550 output tubes. That's about as big as it got back in that day.
With that background then, I now have a previously restored 55-A on the bench, with no other goals (at this point) than to answer a few basic questions:
1. How well is it operating?
2. Is it meeting published factory specification? and,
3. Are there any reasonable modifications that can be done to this amplifier today to improve its performance?
As a result, basic tests and baseline tests are now being conducted, and ideas formulated, that will all be presented in upcoming posts. But for now, pics include:
1. This is one heavy beast! Displaying the appearance that all Fisher power amplifiers did in the 50s, it is built into a very thick gauge, high brass content chassis, painted in Fisher Brown. From my past experience, these chassis can display poor ground integrity over time, interacting poorly with the dis-similar metals used in the various T-Strips, tube sockets, and ground connection points used. In this unit however, every ground connection was very much in tact -- a bonus!
2. Under the tube cover, where there is also a ceramic resistor mounted to prevent it's heat from building up under the chassis.
3. Underneath, there's plenty of real estate to house the rather simple circuit. As was common in the day with premium equipment, a tag board is used to mount many of the audio circuit components on.
4. Negative grid bias voltage is given as -42 vdc, which the output stage was carefully brought to while monitoring it's various quiescent operating parameters.
5. This bias voltage produced a total cathode current draw of 180 ma in the output stage, and also caused the meter to rise to the 0 Power Output point on the Power Monitor meter. Clearly, this is the intended quiescent bias set point.
6. At 90 ma per tube (about 84 ma per plate) and 420 vdc on the plate, this equates to 35.3 watts per tube, which is basically the dissipation rating for a 6550 tube (of any American manufacture), under the conservative Design Center rating system that was in effect at that time. However, at this dissipation level, neither output tube displayed any color in the plate, even when viewed through the lens of a time lapse pic.
If you ever wondered about some of the earliest efforts at Fisher when the hifi bug was in its infancy, this is it. If you're interested, you might check in and follow along!
More as it becomes available.
Dave
With Fisher's company freshly reorganized as Fisher Radio Corporation after the war, they jumped into this fray with their own signature design around 1953. In keeping with Avery's burning desire to be unique if not the first with the products his company produced, he introduced the model 50-A power amplifier (series). And what an amplifier it was. The concept of dollar per watt hadn't been conceived yet, but with this amplifier, one of Fisher's first "firsts" must surely have been realized: the concept of pound per watt. As a single chassis device, this mono basic power amplifier achieves a one-to-one ratio, tipping the scale at 50 pounds, producing an advertised 50 watts of power. This then was no chickified model designed to win over the hearts of the fairer sex, but an all out 1953 execution of Tim Taylor's "mo' power, mo power!" concept. With two massive transformers as an anchor, this amplifier is the antithesis of a chassis in search of some transformers to build an amp on. Rather, this is a couple of transformers that happen to have a chassis strapped to them!! It's a beautiful thing to behold, if not hernia inducing. To be sure however, for those of us with grand kids, this ain't your dad's old hifi amp......... it's YOUR grandfather's!........so that should help to keep things in perspective.
As to its design, it was, and remains truly unique in equipment designed for the home high fidelity setting. The power supply is within the conventional realm for its day, but massive. Dual heavy duty rectifier tubes, a (swinging) choke input filter system, and vacuum tube bias regulator circuit get things powered up. The phase inverter and pre-driver small signal circuits borrow heavily from Williamson, and like that design, there is a global NFB loop that (in part) encompasses these stages. But after that, any resemblance to any other design of the day for high fidelity applications ceases. Following the pre-driver stage, two triode tubes (or triode connected pentodes) operate as push-pull Class A cathode followers, whose output is coupled to the output stage via transformer/capacitive coupling. This driver stage then allows the triode connected output stage to operate in class AB2, which is VERY much different than anything else Fisher -- or anyone else in this product category -- ever produced.
Briefly, early high fidelity design was all about low power triode output stage operation, as that was the only practical way to achieve a low output impedance, and low distortion at the time. Pentode operation produced more power, but had a high output impedance (and higher/odd distortion elements). Negative feedback could correct all of that, but proper application of it was still elusive at the time, so low power triode amplifiers and big high efficiency speakers ruled the day. Think about 7-10 watts RMS and K-Horn speakers.
It is against this backdrop then that the Fisher 50A did its cannonball leap off the high dive. Big power, low output impedance, flat response, and good distortion characteristics made for a package that others just did not have at the time. The fact that it could also serve as a boat anchor in an emergency was just an added plus. Over the course of about 4 years, the amplifier went through various versions, with the biggest changes being in the rectifier and output tubes used. Earlier on, there was also a change in driver tubes used, but for all those changes, the circuit itself changed very little if hardly at all. The change in tubes simply represented the use of bigger (higher dissipation) tubes as they came along. The 55-A -- which is the subject of this thread, is then (to my knowledge) the last of this breed, sporting big 5AW4 rectifiers and 6550 output tubes. That's about as big as it got back in that day.
With that background then, I now have a previously restored 55-A on the bench, with no other goals (at this point) than to answer a few basic questions:
1. How well is it operating?
2. Is it meeting published factory specification? and,
3. Are there any reasonable modifications that can be done to this amplifier today to improve its performance?
As a result, basic tests and baseline tests are now being conducted, and ideas formulated, that will all be presented in upcoming posts. But for now, pics include:
1. This is one heavy beast! Displaying the appearance that all Fisher power amplifiers did in the 50s, it is built into a very thick gauge, high brass content chassis, painted in Fisher Brown. From my past experience, these chassis can display poor ground integrity over time, interacting poorly with the dis-similar metals used in the various T-Strips, tube sockets, and ground connection points used. In this unit however, every ground connection was very much in tact -- a bonus!
2. Under the tube cover, where there is also a ceramic resistor mounted to prevent it's heat from building up under the chassis.
3. Underneath, there's plenty of real estate to house the rather simple circuit. As was common in the day with premium equipment, a tag board is used to mount many of the audio circuit components on.
4. Negative grid bias voltage is given as -42 vdc, which the output stage was carefully brought to while monitoring it's various quiescent operating parameters.
5. This bias voltage produced a total cathode current draw of 180 ma in the output stage, and also caused the meter to rise to the 0 Power Output point on the Power Monitor meter. Clearly, this is the intended quiescent bias set point.
6. At 90 ma per tube (about 84 ma per plate) and 420 vdc on the plate, this equates to 35.3 watts per tube, which is basically the dissipation rating for a 6550 tube (of any American manufacture), under the conservative Design Center rating system that was in effect at that time. However, at this dissipation level, neither output tube displayed any color in the plate, even when viewed through the lens of a time lapse pic.
If you ever wondered about some of the earliest efforts at Fisher when the hifi bug was in its infancy, this is it. If you're interested, you might check in and follow along!
More as it becomes available.
Dave
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