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In the Forum: Audio Discussions
In the Thread: How to get a LOT of SET power.

Post Subject: No go!Posted by N-set on: 4/1/2011

This whole 833 A2 concept, given the constraints,
is a no go. When driven positive, the grid equivalent resistance drops
down to 1/transconductance level. For 833, being a high-gm tube,
this can be as low as 100R. In order not to distort significantly the driver
should have the output impedance in the 10R region, and at the same time
mu of 35 to maintain the 2 stage topology. Clearly this is impossible to fullfill.
Some of Romy's constraints must be relaxed if one wants to keep 833.

1) active: 3 stage with the stepdown IT to 833; 2 first stages can be DC coupled;
6E5P input would pose much more relaxed conditions on the driver EL34, 845, GM70, whatever,
but increased complexity

2) passive version: step-up input transformer ->driver ->stepdown to the output tube.
this would pose harder conditions on the pre as the seen input impedance will drop, but at least at the ULF the
parasitic capacitances, amplified by the step-up, do not matter;
one may actually try to use the input trans as a part of the input filter; getting one milion
henrys should not be very problematic as there would be no DC; big nanocrystaline cores with
mu=80.000 are cheap now; also the choice of the driver is a bit more relaxed now as one can adjust the gains.

3) resign from A2, move to A1 at 2.5 to 3kV @ 10-15k load, probably very linear
but "only" 70-80W + pain in the ass power supply;
eventually parallel 833 to get into the 150W range and resign from heating in your house during the winter.

At the moment 2) seems to me as the most elegant to implement, but I haven't thought it well over...

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