While I wait for the SE OPTs from Edcor to be delivered for my SEUL build, I might as well get going on the design of another amp. I acquired these Fisher TA-800 OPTs a while back and I've been sitting on them for over a year now wondering what I can build with them. These are smallish OPTs, they look like the same size as you'd find in a Fisher X-100C or X-100D. They measure out at 6.2K primary, and I think they are rated at 25 watts output.
I am targeting 15 watts/channel for this amp, for one thing, because it will be going to my daughter who lives in a small apartment, so we don't want to blast out the neighbors.I also want common tubes to be used so my daughter can easily replace them if necessary in the future.
After some preliminary pencil to paper analysis, I narrowed down the tube choices to 7591 or 7868, EL34, or 6L6. Building around a 7591/7868 would be expected (aka boring), so that tube was eliminated early on. It's been years since I've built around the EL34, and I'd love to try that tube again, but to dumb it down to deliver 15 watts output with these OPTs, I'd need to run it PP class A with 240V plate and 250V screen. That doesn't seem too wise to run the screens at a higher voltage than the plate, so the EL34 was eliminated.
That leaves the trusty workhorse 6L6. To keep the power down in the 15 watt range, I'd also need to run it in class A PP, with 350V delivered to the center taps, and 250V screens. Perfect! The output stage tubes are biased at ~345V plate, 16V differential between grid and cathode, and 70 mA quiescent current.
Running a PP amp in class A is pretty much ideal as far as power supply requirements are concerned, since class A PP output stage will draw constant current from the power supply under any power output conditions. Therefore there is no need to worry about regulation. Furthermore, in a class A output stage, I can run with cathode bias and not worry about AC bypassing the cathode resistor. That makes for a more simple design.
But I wanted some small amount of DC balance adjustment capability. I was going to extract it from the cathode voltage and feed a few volts of it to the grid to allow some DC balance adjustability, but then I realized I have a spare 5V secondary winding, so why not use it to generate a bit of DC bias I can use to balance the stage?
The screens,will get their pseudo-regulated 250V supply from a simple zener shunt mosfet setup.
For the voltage amp and inverter/driver stages, there were two choices I considered: a Dynaco ST70 like pentode/triode frontend, or the Mullard 5-20 like voltage amp and cathode coupled differential pair. I debated on this extensively, but ultimately decided on the Mullard style frontend because the tubes used (12AX7 and 6SN7) are more readily available than the 6U8 I would have used if going with the Dynaco style frontend.
Normally I would direct couple the 12AX7 voltage amp to the 6SN7 inverter, but in this case I didn't quite have enough voltage delivered from the power supply to do it and still bias the two stages where I wanted. I could shoehorn it, but it requires that the 12AX7 idle at a lower current (0.5 mA) than I wanted. So I opted for capacitor coupling and in doing so, I can now run the 12AX7 at pretty much any quiescent point I want. I chose 1 mA, 1V on the cathode, and 137V across the tube. That's a nice linear place on the 12AX7 plate curves. I'll then need to live with the extra time constant and phase shift that capacitor coupling introduces when feedback is applied.
I targeted 0.35 to 0.4V sensitivity. I will need 18 dB feedback to get the sensitivity up to 0.35V, which I believe the Fisher OPTs should be able to handle.
Anyway, that's the design I've got so far. Nothing's been built or even prototyped yet. Pencil to paper says it should perform well. New avenues for me to explore on this build is the class A PP output stage, the combo fixed and cathode bias scheme, and the Fisher TA-800 OPTs.
Should be a fun ride.
Proposed schematic attached.
I am targeting 15 watts/channel for this amp, for one thing, because it will be going to my daughter who lives in a small apartment, so we don't want to blast out the neighbors.I also want common tubes to be used so my daughter can easily replace them if necessary in the future.
After some preliminary pencil to paper analysis, I narrowed down the tube choices to 7591 or 7868, EL34, or 6L6. Building around a 7591/7868 would be expected (aka boring), so that tube was eliminated early on. It's been years since I've built around the EL34, and I'd love to try that tube again, but to dumb it down to deliver 15 watts output with these OPTs, I'd need to run it PP class A with 240V plate and 250V screen. That doesn't seem too wise to run the screens at a higher voltage than the plate, so the EL34 was eliminated.
That leaves the trusty workhorse 6L6. To keep the power down in the 15 watt range, I'd also need to run it in class A PP, with 350V delivered to the center taps, and 250V screens. Perfect! The output stage tubes are biased at ~345V plate, 16V differential between grid and cathode, and 70 mA quiescent current.
Running a PP amp in class A is pretty much ideal as far as power supply requirements are concerned, since class A PP output stage will draw constant current from the power supply under any power output conditions. Therefore there is no need to worry about regulation. Furthermore, in a class A output stage, I can run with cathode bias and not worry about AC bypassing the cathode resistor. That makes for a more simple design.
But I wanted some small amount of DC balance adjustment capability. I was going to extract it from the cathode voltage and feed a few volts of it to the grid to allow some DC balance adjustability, but then I realized I have a spare 5V secondary winding, so why not use it to generate a bit of DC bias I can use to balance the stage?
The screens,will get their pseudo-regulated 250V supply from a simple zener shunt mosfet setup.
For the voltage amp and inverter/driver stages, there were two choices I considered: a Dynaco ST70 like pentode/triode frontend, or the Mullard 5-20 like voltage amp and cathode coupled differential pair. I debated on this extensively, but ultimately decided on the Mullard style frontend because the tubes used (12AX7 and 6SN7) are more readily available than the 6U8 I would have used if going with the Dynaco style frontend.
Normally I would direct couple the 12AX7 voltage amp to the 6SN7 inverter, but in this case I didn't quite have enough voltage delivered from the power supply to do it and still bias the two stages where I wanted. I could shoehorn it, but it requires that the 12AX7 idle at a lower current (0.5 mA) than I wanted. So I opted for capacitor coupling and in doing so, I can now run the 12AX7 at pretty much any quiescent point I want. I chose 1 mA, 1V on the cathode, and 137V across the tube. That's a nice linear place on the 12AX7 plate curves. I'll then need to live with the extra time constant and phase shift that capacitor coupling introduces when feedback is applied.
I targeted 0.35 to 0.4V sensitivity. I will need 18 dB feedback to get the sensitivity up to 0.35V, which I believe the Fisher OPTs should be able to handle.
Anyway, that's the design I've got so far. Nothing's been built or even prototyped yet. Pencil to paper says it should perform well. New avenues for me to explore on this build is the class A PP output stage, the combo fixed and cathode bias scheme, and the Fisher TA-800 OPTs.
Should be a fun ride.
Proposed schematic attached.
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