Here's the overall topology of the 6V6 P-P amp I propose, using the Russian 6P1S copy of the 6V6. The amp will be powered with a DC-DC converter run with a 24V switching adapter. That same adapter will provide DC power to run the filaments of the 6P1S and the 8CG7 front end (with suitable dropping resistors). Obviously, the reference designators still need sorting out, as well as do the gain resistors for the folded cascode stage (since they participate in the output stage bias) and the cathode resistors for the 6P1S outputs. The screen voltage for the output stage is on the low side as a consequence of the design of the DC-DC converter. Output transformers will be the Hammond 1650E, as I have a pair. I'm not sure whether I'll use the screen taps or just leave them hanging and use straight pentode mode. Oh, and I realize in retrospect that the filament dropper resistors for the 8CG7s are used twice - that will get sorted. Matched 6P1S are winging their way here as I write this - how well-matched remains to be seen. Oh yeah, the RET shown on both pages should in fact be -BIAS. That value also remains to be determined. Obviously, the amp will need some time with bench supplies to get the various biasing resistors and supplemental supplies sorted out,
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Here is the amp with fungoes (mostly) fixed. Values for output cathode resistors and folded cascode gain resistors will still need to be determined via bench work. This is obviously going to be a work in progress, as I haven't even started on the PCB layout yet. Since there is no power transformer, I'm expecting a fairly light and compact package with tubes on top and output XFMRs and DC-DC converter hidden beneath with some shielding to prevent noise pollution.
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One thing I forgot in the schematic is something that's needed for every amp with a folded cascode VT diff front end - that's an extra transistor to disable the folded cascode PNP transistors until the tube in the diff stage is fully heated up. This avoids a situation where there's a huge positive voltage on the output tube grids due to feedthrough into the PNP folded cascode stage. One NPN transistor of sufficient voltage rating gets the job done. I'll add it to the next set of circuits I post.
The updated schematic shall be appreciated. I am watching your design and build carefully and may try to build it. I have a nice stash of NOS Svetlana 6P1P-ev tube acquired before the Russia/Ukraine war that I would like to put to good use. You design intrigued me.
Couple of questions: Did you mean to include the power supply also in your post #44, or why is there a duplicate schematic? I could not spot any differences. I was also curious why you chose the 6P1P screen voltage at 150 Vdc The datasheet shows 250V max for both anode and screen. Is it to control the maximum screen dissipation below 1.3 watts? Or a voltage doubler PS that makes 150 Vdc convenient to get with B+ of 300V?
Couple of questions: Did you mean to include the power supply also in your post #44, or why is there a duplicate schematic? I could not spot any differences. I was also curious why you chose the 6P1P screen voltage at 150 Vdc The datasheet shows 250V max for both anode and screen. Is it to control the maximum screen dissipation below 1.3 watts? Or a voltage doubler PS that makes 150 Vdc convenient to get with B+ of 300V?
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I'm choosing a plate voltage of 300VDC - the configuration of the DC-DC converter I'm using to power the amp gives me 150VDC as a consequence of its design. I figure 150V on the screens will also limit dissipation when the amp is driven hard. If I include the power supply in my schematics, I'll just draw a little box with 24VDC going in, and appropriate bias voltages coming out. The DC-DC design is proprietary, and I wind my own transformers for it.
You of course must realize that a folded cascode circuit is essentially a current divider. I can divide that current whichever way I please. A 6V6 also is not exactly the most difficult output tube to drive.... If you can drive 7591As with wimpy 12AX7s and equivalent, I see no reason not to be able to do the same with 6V6s (they are pentodes, after all) and a hearty tube like a 6/8CG7.
Have started collecting parts for my 6v6 push pull. Thought of using a Hammond choke, 193j, as well as a 193c per channel further out in the power supply. A pair of Hammond 1650E output transformers should also be included. Going to buy a Hammond 370HX or similar as well. A two millimeter aluminum plate with a wooden frame around it?, maybe?