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are also called pentodes. A pentode, unlike a triode, is fairly immune to variations in plate voltage, which makes it an excellent candidate for building a current regulator. By adding a cathode resistor and an RC network to grid 2, an effective current regulator can be made out of an EL34 or 6550 or KT66.
The output tube could be either a pentode or triode connected EL34 or 6550 (or even 300B). The transformer would have one end of its primary connected to ground and the other attached to the plate of the output tube through a 4-20 µF capacitor (polypropylene or oil). The secondary would be mistapped: the 16 ohm tap would now be an 8 ohm tap; the 8 ohm, a 4 ohm; the 4, a 2 ohm. This would be necessary to give the triode connected output tube a better load to work into. The single triode connected EL34 per channel will only have half the B+ voltage available to it, but it could be run at up to twice the current of the same
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tube in a stock ST-70 and will probably only be capable of putting out 5 watts or so, but potentially 5 great watts.
The input stage would remain unchanged except for the elimination of the plate resistor and its coupling capacitor on the Split-Load phase splitter. Remember, only one phase leg is needed. Of course, a better circuit could be used, for example a 5751 per channel or a 6SN7 Grounded Cathode amplifier directly cascading into a second Grounded Cathode stage.
One tricky part is supplying voltage to the heaters of the current regulator tubes. As their cathodes will be about 200 volts above ground potential, some precautions must be taken not to exceed the cathode-to-heater voltage limit of the tube. The best choice would be use one of the 6.3 heater winding for the top tubes and the other for the bottom tubes and reference the top heater winding to about 230 volts.
//JRB
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