Am I right saying that bootstrapping the plate of the driver through a cap from the cathode of the output stage doesn’t affect the Zout of the amp, whilst bootstrapping from the plate of the output stage increases the Zout?
Zout is always affected, at the point where the (voltage) feedback is taken. Negative feedback reduces Zout, positive feedback increases it.
Fair enough, but one could argue that bootstrapping is classically meant as voltage sensing, series applied positive polarity feedback, so feedback from a cathode isn't automatically voltage sensing. Nonetheless, this circuit acts exactly like a "Schade" feedback with polarity reversed, which was my point. An analogy would be to the bootstrapping in McIntosh output stages, voltage sensing, series applied feedback from the opposite polarity OPT winding.
All good fortune,
Chris
Indeed. What I meant is that in this specific case, bootstrapping from the source of a mosfet source follower, that has a very low impedance compared to the output tube it is driving, will not affect the Zout of the amp. I mean, it will do it but by a negligible amount.
https://www.mouser.it/datasheet/2/308/1/FQP9P25_D-1809667.pdf
The gFS of the FQP9P25 (this is what is used as cathode driver, but there are no LTSpice models for it) is 5.7 S, so its Zout is 0.17 Ohm.
The rp of the tube above is around 5.000 times more.
The gFS of the FQP9P25 (this is what is used as cathode driver, but there are no LTSpice models for it) is 5.7 S, so its Zout is 0.17 Ohm.
The rp of the tube above is around 5.000 times more.
I would like to thank you all.Thinking about all those hints you gave me, I developed this circuit I'm finally happy with and that I will build.
I'm placing the order to mouser to get the parts I'm still missing.
The series of resistors R8 + R7 + R4 + R15 are a voltage divider to give negative voltage feedback plate-to-grid to the output tube to triodize its curves à la Tubelab UNSET, and positive voltage feedback from output tube's plate to driver's cathode to gain some more sensitivity à la McIntosh.
Then the bootstrapped driver plate resistor through the source follower that is driving the output tube from the cathode, in order to have CCS-like load and maximise its gain while reducing the distortion. Current is set around 9 mA as per Tubelab's SSE driver.
Then there's the global negative feedback through the LM4562 that can drive up to 26 mA, whilst this application needs only 10 mA.
I've used this feedback in my PP amps, and I'm very happy with it (in my two PP amps I apply the feedback to the second grid of the LTP).
Shortly, U2 is used to set the amplitude of the output of the amp at the same amplitude of the input signal and send it to the non-inverting input of the OPAMP, while the input signal is applied to the inverting input. The result is a non-inverted distortion of the amp. R3 sets the gain we apply to the distortion, and then we apply only the amplified distortion to the bottom of the voltage divider R8 + R7 + R4 + R15, in order to correct the input stage without affecting the sensitivity of the amp.
The other advantage of this feedback, is that it reduces the ripple and noise of the amp, so the PSU can be cheaper and simpler.
I will report the results. I expect them to be not so far at 1 and 10 Wrms, then most probably the OPT won't be able to apply the feedback at 20 Wrms.
https://sklep.toroidy.pl/en_US/p/TTG-KT88SE-Tube-output-UL-transformer-3kOhm-KT88-300B-SE/566
I'm placing the order to mouser to get the parts I'm still missing.
The series of resistors R8 + R7 + R4 + R15 are a voltage divider to give negative voltage feedback plate-to-grid to the output tube to triodize its curves à la Tubelab UNSET, and positive voltage feedback from output tube's plate to driver's cathode to gain some more sensitivity à la McIntosh.
Then the bootstrapped driver plate resistor through the source follower that is driving the output tube from the cathode, in order to have CCS-like load and maximise its gain while reducing the distortion. Current is set around 9 mA as per Tubelab's SSE driver.
Then there's the global negative feedback through the LM4562 that can drive up to 26 mA, whilst this application needs only 10 mA.
I've used this feedback in my PP amps, and I'm very happy with it (in my two PP amps I apply the feedback to the second grid of the LTP).
Shortly, U2 is used to set the amplitude of the output of the amp at the same amplitude of the input signal and send it to the non-inverting input of the OPAMP, while the input signal is applied to the inverting input. The result is a non-inverted distortion of the amp. R3 sets the gain we apply to the distortion, and then we apply only the amplified distortion to the bottom of the voltage divider R8 + R7 + R4 + R15, in order to correct the input stage without affecting the sensitivity of the amp.
The other advantage of this feedback, is that it reduces the ripple and noise of the amp, so the PSU can be cheaper and simpler.
I will report the results. I expect them to be not so far at 1 and 10 Wrms, then most probably the OPT won't be able to apply the feedback at 20 Wrms.
https://sklep.toroidy.pl/en_US/p/TTG-KT88SE-Tube-output-UL-transformer-3kOhm-KT88-300B-SE/566