I may get a whole lot of blow-back on this one, but here goes: I've never been particularly fond of the Fisher SA-300. Besides the SA-1000, it was the last of the high performance stand alone stereo basic amplifiers Fisher produced, serving as big brother to the SA-100 during its five year run. But whereas the measures taken to achieve high performance in the SA-100 and SA-1000 all work together in harmony to achieve that end goal, those applied to the SA-300 more resemble management of medication side effects, where one medication is taken to address the side effects of another medication, etc. In other words, the measures applied are collectively working as much to contain the side effects as they are to actually improve the performance of the amplifier.
The finished product all works well enough as designed: The amplifier will meet its distortion specification (.5% THD at 1 kHz within 1 db of rated output) and power rating (35 watts RMS per channel) well enough when only one channel is driven (typical of how amplifiers were rated back in the day), and even meet its noteworthy hum and noise specification (100 db below rated output) in Channel B, and come darn close to it in Channel A, where EMF radiation presents challenges in meeting the specification for that channel. But the high tube killing quiescent current draw (78 mA / tube) and resulting heat produced (continuous operation at 110% of rated plate dissipation/tube) in achieving these specifications are simply unprecedented. Add to this the heat dissipated by the heavy duty power supply employed to support this beast and you've got all the makings of a true space heater. With the cost of tubes and AC power (both to operate the amplifier and cool the room where the amplifier resides) ever on the rise, the SA-300 is not an amplifier to turn on and forget about in today's audio environment. Quite simply, for all the design work that went into it, the SA-300 comes off as incredibly inefficient and extraordinarily complex, producing performance results no better than other competing designs that are simpler and more efficient. Put in more classic terms, if a camel is a horse designed by committee, then the SA-300 is Fisher's camel. But how did it get that way?
No doubt, as with any product development, there was likely a tug of war going on between marketing and the engineering department, with the bean counters acting as judge and referee. And as usual, marketing probably ended up joining forces with the bean counters to rule the day, leaving the poor engineers to figure out how to pull it all off. The engineers were not completely silent however, as the SA-300 ended up being engineered like no other Fisher product before it, or after. The conversation must have gone down something like this:
Marketing: We want a new 60-70 watt basic stereo amplifier developed around the EL34 tube, because it's the hot European tube (pun intended) that everybody is designing with right now.
Engineering: Well to operate that tube efficiently in that power range, it needs to operate in Ultra-Linear or tapped screen operation mode.
Bean Counter: Oh no. We're not going down that road again. We did that with the 70 series amplifiers. We don't want to pay any more licensing or transformer development costs for something that is not an original Fisher concept to begin with.
Marketing: Fine. But this new amplifier must use the EL34 one way or another to be competitive.
Engineering: But we've already used that tube in the 55-A series, and can't claim it's first use anyway. If we can't go the UL route, then the 6L6 would be a far better way to go.
Marketing: No, no. We've already used various versions of that tube before in the 50-A and 70, 80, and 100 amplifiers -- they're just considered too out of date and passe' now compared to more modern tubes. It must be the EL34.
Engineering: OK -- but given the restrictions, it's going to take a lot of engineering hi-jinks to make a competitive product out of a non-UL EL34 amplifier specification wise.
Marketing: We nearly live and die by specifications -- we've got to have respectable specifications!
Bean Counter: Hi-jinks equals high cost. Will it cost more than if we had to go down the UL route?
Engineering: You tell me. You're the Bean Counter -- but probably not. Likely just more R&D time up front, and then the build will take a little longer on the line.
Bean Counter: That's it? I like it! -- A fixed cost that ultimately goes away, with just a little more build time.
Marketing: OK, it's settled then. EL34s with engineering hi-jinks. You guys figure it out. Who's up for lunch?
And so the SA-300 was born. Obviously none of us were there to know how the conversation actually went, but given the square peg into a round hole outcome that the design represents, the above offering just might be closer than you think.
Now by itself, the EL34 is a wonderful tube. In true Class A triode mode, it barely produces any distortion at all. In UL mode, it is very efficient, producing both high power output and low distortion in very economical circuits. And in pentode mode, it can even keep up with the power output capabilities of the 6550. But good as the EL34 is, it's characteristics make for a double edged sword when it comes to design considerations.
As a high Gm tube, the EL34 is a quite sensitive which is nice because it eases drive requirements. But in addition to high control grid sensitivity, that also makes it sensitive to any changes in operating supply voltages presented to its screen grid as well. Further, because it is a true pentode design without aligned grids, the change in screen grid current from quiescent to full power conditions is quite large. Collectively then, that's the rub. The EL34 itself is capable of operating with low distortion with the operating parameters Fisher used. But operating them that way in practical real world installations can get them into trouble real quick. Here's why:
In medium voltage pentode circuits (as Fisher used here), the screen grids are basically powered from the same voltage source as the plates are, this to achieve maximum power output. To deal with the large change in screen current, any dropping resistance used for filtering purposes in the screen circuit must be kept small. As a result, for minimum hum, maintaining a tight DC balance is critical. But the primary concern, is that with an operating voltage so close to and powered from the plate supply voltage, any sag in the plate supply voltage produced by a practical power supply design will translate directly to a sagging screen grid voltage as well -- which in the EL34, causes distortion to increase quite significantly -- and is exactly what Fisher engineers were having to deal with given the design restrictions they had. Fisher being Fisher however, they were bent on producing good performance results anyway, and so the engineering hi-jinks began. So just what are they?
That is discussed in the next installment.
Dave
The finished product all works well enough as designed: The amplifier will meet its distortion specification (.5% THD at 1 kHz within 1 db of rated output) and power rating (35 watts RMS per channel) well enough when only one channel is driven (typical of how amplifiers were rated back in the day), and even meet its noteworthy hum and noise specification (100 db below rated output) in Channel B, and come darn close to it in Channel A, where EMF radiation presents challenges in meeting the specification for that channel. But the high tube killing quiescent current draw (78 mA / tube) and resulting heat produced (continuous operation at 110% of rated plate dissipation/tube) in achieving these specifications are simply unprecedented. Add to this the heat dissipated by the heavy duty power supply employed to support this beast and you've got all the makings of a true space heater. With the cost of tubes and AC power (both to operate the amplifier and cool the room where the amplifier resides) ever on the rise, the SA-300 is not an amplifier to turn on and forget about in today's audio environment. Quite simply, for all the design work that went into it, the SA-300 comes off as incredibly inefficient and extraordinarily complex, producing performance results no better than other competing designs that are simpler and more efficient. Put in more classic terms, if a camel is a horse designed by committee, then the SA-300 is Fisher's camel. But how did it get that way?
No doubt, as with any product development, there was likely a tug of war going on between marketing and the engineering department, with the bean counters acting as judge and referee. And as usual, marketing probably ended up joining forces with the bean counters to rule the day, leaving the poor engineers to figure out how to pull it all off. The engineers were not completely silent however, as the SA-300 ended up being engineered like no other Fisher product before it, or after. The conversation must have gone down something like this:
Marketing: We want a new 60-70 watt basic stereo amplifier developed around the EL34 tube, because it's the hot European tube (pun intended) that everybody is designing with right now.
Engineering: Well to operate that tube efficiently in that power range, it needs to operate in Ultra-Linear or tapped screen operation mode.
Bean Counter: Oh no. We're not going down that road again. We did that with the 70 series amplifiers. We don't want to pay any more licensing or transformer development costs for something that is not an original Fisher concept to begin with.
Marketing: Fine. But this new amplifier must use the EL34 one way or another to be competitive.
Engineering: But we've already used that tube in the 55-A series, and can't claim it's first use anyway. If we can't go the UL route, then the 6L6 would be a far better way to go.
Marketing: No, no. We've already used various versions of that tube before in the 50-A and 70, 80, and 100 amplifiers -- they're just considered too out of date and passe' now compared to more modern tubes. It must be the EL34.
Engineering: OK -- but given the restrictions, it's going to take a lot of engineering hi-jinks to make a competitive product out of a non-UL EL34 amplifier specification wise.
Marketing: We nearly live and die by specifications -- we've got to have respectable specifications!
Bean Counter: Hi-jinks equals high cost. Will it cost more than if we had to go down the UL route?
Engineering: You tell me. You're the Bean Counter -- but probably not. Likely just more R&D time up front, and then the build will take a little longer on the line.
Bean Counter: That's it? I like it! -- A fixed cost that ultimately goes away, with just a little more build time.
Marketing: OK, it's settled then. EL34s with engineering hi-jinks. You guys figure it out. Who's up for lunch?
And so the SA-300 was born. Obviously none of us were there to know how the conversation actually went, but given the square peg into a round hole outcome that the design represents, the above offering just might be closer than you think.
Now by itself, the EL34 is a wonderful tube. In true Class A triode mode, it barely produces any distortion at all. In UL mode, it is very efficient, producing both high power output and low distortion in very economical circuits. And in pentode mode, it can even keep up with the power output capabilities of the 6550. But good as the EL34 is, it's characteristics make for a double edged sword when it comes to design considerations.
As a high Gm tube, the EL34 is a quite sensitive which is nice because it eases drive requirements. But in addition to high control grid sensitivity, that also makes it sensitive to any changes in operating supply voltages presented to its screen grid as well. Further, because it is a true pentode design without aligned grids, the change in screen grid current from quiescent to full power conditions is quite large. Collectively then, that's the rub. The EL34 itself is capable of operating with low distortion with the operating parameters Fisher used. But operating them that way in practical real world installations can get them into trouble real quick. Here's why:
In medium voltage pentode circuits (as Fisher used here), the screen grids are basically powered from the same voltage source as the plates are, this to achieve maximum power output. To deal with the large change in screen current, any dropping resistance used for filtering purposes in the screen circuit must be kept small. As a result, for minimum hum, maintaining a tight DC balance is critical. But the primary concern, is that with an operating voltage so close to and powered from the plate supply voltage, any sag in the plate supply voltage produced by a practical power supply design will translate directly to a sagging screen grid voltage as well -- which in the EL34, causes distortion to increase quite significantly -- and is exactly what Fisher engineers were having to deal with given the design restrictions they had. Fisher being Fisher however, they were bent on producing good performance results anyway, and so the engineering hi-jinks began. So just what are they?
That is discussed in the next installment.
Dave
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