The amount of thought that Mr. Rozenblit has put into making this amplifier speaker safe is refreshing. His explanation of his choice of part values and topology is required reading for anyone contemplating building any tube OTL amplifier.
   The circuit comprises a series connected output stage with one bank of triodes sitting on top of the other bank. The input stage is a Grounded Cathode and the phase splitting and additional gain is provided by a Long Tail phase splitter. The balanced signal is brought to the output tube via two Cathode Followers. The connection here is not directly made, but  purposely capacitor coupled. As mentioned earlier, he has considered the potential fault conditions very carefully.
    The amplifier's design is straightforward except for the terminating the Cathode Follower's load resistor into the output. This is the critical design trick to the amplifier, as it is equalizes the gains between top and bottom tubes and the basis for Mr. Rozenblit's patent. In all honesty, I cannot say I understand why this should make that much of a difference, but until I test bench the circuit, I will refrain from commenting.
    Voltage feedback is used. The feedback ratio is surprisingly low for an OTL amplifier and comes in at 22 dB. The resulting output impedance is a low 0.4 ohms.

Grounded Grid Preamp
    This circuit is a truly minimalist line stage amplifier. It derives its name from the fact that the triode that provides the gain for the circuit receives its input from its cathode being coupled to a Cathode Follower rather than its grid. Still, I prefer to use the name "Common Cathode amplifier" as  that is how the circuit is described in many of my beloved electronic books from the 40's and 50's. (Two articles on this circuit topology and it modification are available at the GlassWare site.) Furthermore, "Grounded Grid amplifier" is usually used to describe an amplifier with only one triode that receives its signal directly at its cathode and hence has both a very low input impedance and low input capacitance.
   Mr. Rozenblit seems to have a penchant for full-wave voltage doublers. Two are used in this preamp, one for the positive rail voltage and one for the negative rail voltage. Contrary to some of the buzz going around today, there is nothing wrong with these power supply configurations. This power supply circuit doubles the voltage that a full-wave bridge rectifier circuit would provide, but with only half the output current capability. The ripple is at the same 120 Hz frequency as in the full-wave bridge rectifier circuit and the transformer core is not partially magnetized as it is in the half-wave voltage doubler circuit. Shunt

A greatly simplified representation of the Grounded Grid Preamp

pg. 13

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