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Single Ended with Slam
Here Mr. Rozenblit tries his hand at a parallel SE amplifier. This is not your grandfather's SE amplifier. The output transformer is not connected to the plate as it is in virtually every other SE amplifier, but at the cathodes of the output tubes, three 6550's all wired parallel. In other words, the output stage functions as a Cathode Follower and not a Grounded Cathode amplifier. All the benefits of a Cathode Follower buffer thus obtain, such as very low output impedance and linearity. Unfortunately, so too do the liabilities of a Cathode Follower, such as no gain and heater-to-cathode voltage variations that might exceed the tube's limit. The former liability is compensated by cascading Grounded Cathode amplifiers that easily meet the 200 volts plus peak voltage requirement for driving the output tubes to full output. Like the Super Compact 150 Watt Amplifier, an interstage Cathode Follower is used to extend the bandwidth. (This time the plate resistor of the previous stage matches the Cathode Follower's load resistor.) The ladder liability is addressed by connecting the output tube's heaters to a separate winding whose center tap is tied directly to the paralleled cathodes of the output tubes.
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The Cathode Follower output stage finds its own Cathode Follower driving its grids. I assume this Cathode Follower is needed to unload the last Grounded Cathode stage so that it can realize as much gain as possible or maybe the Cathode Follower is needed to allow Class A2 operation of the output stage. Here is a case where the triode connected pentodes are not the best choice. Maybe the amplifier would have been better off with the pentodes left as pentodes. Pentodes make good Cathode Followers. In fact, the Cathode Follower that feeds the output stage is a pentode based Cathode Follower. Yes, this would require some work to reference the grid 2 voltage source to the moving cathodes, but as Mr. Rozenblit does not share my distrust of zener diodes, a string of them could easily be tied to cathodes and then to the grid 2 pins with a resistor bridging this connection to the B+ to bias the zeners adequately. The main advantage to configuration lies in the elimination of the need for the interstage Cathode Follower.
There are a few more tricks to this amplifier that I will let Mr. Rozenblit explain to you in his book. (I hope you have discerned just how much I want everyone reading this journal to buy a copy of his book.)
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