Having participated in some efforts recently to DC compensate the SET transformer, so the air gap could be eliminated, I have worked up some new circuits that are analogs to SET design.

These provide the same single triode output tube gain, but provide either twice the power, or even 4 times the power (see next post), while preserving zero DC flux in the output transformer. This means that an ordinary P-P type transformer can be used, which will provide better bandwidth (ie. better Bass, and the option of NFB if wanted), and the cost and weight are less with more power output.

This uses the technique of complementary current to drive the other winding of a P-P transformer. Your first impression will be that this is just some sneaky version of P-P, but a closer look is required. The pentode is used as a programmable current source. With no plate feedback effect ( as with a triode output ), its high impedance output has no say or control of the output voltage. The triode is calling the shots here.

See figure. The constant current source in the "tail" guarantees that the currents in each half of the circuit add up to a constant. This means that for every change in current the triode commands to one winding, an equal but opposite change in current occurs in the other pentode/ transformer half. Since the other winding section is inverted in phase with respect to the output, this just means an exact doubling of the current change as far as the output is concerned. So the output current changes will simply be doubled by this exact duplication. Like having double the gm and wattage rating of the triode.

(using P = I squared R might lead one to think this would quadruple the output power. But, since the triode now sees a higher impedance, 2x, due to the pentode's assistance, we would use .7 times the turns on each half of the xfmr to get the same impedance back...., so power ends up being just doubled. Not bad though, twice the power output from your favorite triode. )

The pentode grid bias gets adjusted for equal DC idle current in the xfmr halves, so no DC is present flux wise in the xfmr. The 1 Ohm resistors are for setting bias. The pentode could be replaced with an N channel Mosfet, since it is just acting as a programmable current source. (You will notice that the pentode, or Mosfet, are operating in grounded grid mode, so the same current goes in as goes out, so no sonic effects on the signal) The triode's cathode sees 1/gm of the pentode (or Mosfet) as a cathode resistance to ground, so the Mosfet will reduce this effect dramatically.

One can still use a partial cathode feedback winding, or distributed loading, for the triode if desired, just put the winding in series with the triode's cathode to CCS connection point.

Don

These provide the same single triode output tube gain, but provide either twice the power, or even 4 times the power (see next post), while preserving zero DC flux in the output transformer. This means that an ordinary P-P type transformer can be used, which will provide better bandwidth (ie. better Bass, and the option of NFB if wanted), and the cost and weight are less with more power output.

This uses the technique of complementary current to drive the other winding of a P-P transformer. Your first impression will be that this is just some sneaky version of P-P, but a closer look is required. The pentode is used as a programmable current source. With no plate feedback effect ( as with a triode output ), its high impedance output has no say or control of the output voltage. The triode is calling the shots here.

See figure. The constant current source in the "tail" guarantees that the currents in each half of the circuit add up to a constant. This means that for every change in current the triode commands to one winding, an equal but opposite change in current occurs in the other pentode/ transformer half. Since the other winding section is inverted in phase with respect to the output, this just means an exact doubling of the current change as far as the output is concerned. So the output current changes will simply be doubled by this exact duplication. Like having double the gm and wattage rating of the triode.

(using P = I squared R might lead one to think this would quadruple the output power. But, since the triode now sees a higher impedance, 2x, due to the pentode's assistance, we would use .7 times the turns on each half of the xfmr to get the same impedance back...., so power ends up being just doubled. Not bad though, twice the power output from your favorite triode. )

The pentode grid bias gets adjusted for equal DC idle current in the xfmr halves, so no DC is present flux wise in the xfmr. The 1 Ohm resistors are for setting bias. The pentode could be replaced with an N channel Mosfet, since it is just acting as a programmable current source. (You will notice that the pentode, or Mosfet, are operating in grounded grid mode, so the same current goes in as goes out, so no sonic effects on the signal) The triode's cathode sees 1/gm of the pentode (or Mosfet) as a cathode resistance to ground, so the Mosfet will reduce this effect dramatically.

One can still use a partial cathode feedback winding, or distributed loading, for the triode if desired, just put the winding in series with the triode's cathode to CCS connection point.

Don