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07-06-2023, 11:25 AM
Trans amp Part-3
It should be obvious that the trans-tube amp concept has similar requirements to any feedback-controlled circuit. The most important being that the gain element within the feedback loop should have as much gain as possible.
A typical power tube is either a triode, tetrode or pentode. A triode has only one connection option to the load via its plate, so the triode's theoretical maximum voltage gain is its mu. A tetrode can be tied to the load in three ways: strictly via the plate, with the screen tied to a fixed voltage; wired using an ultralinear tap for the screen; or with the screen and plate linked for triode performance. A pentode is wired similarly to the tetrode as its suppressor grid is typically tied to its cathode.
The triode voltage gain is lowest. Its plate impedance is also lowest of the three wiring modes.
Ultralinear allows local feedback over a portion of the transformer primary, which reduces the plate impedance of the tube compared to tetrode/pentode mode but not as low as triode mode. Voltage gain is higher than triode mode.
Tetrode/pentode mode has the highest output impedance and the highest voltage gain.
The open-loop distortion of the three modes are from lowest to highest: triode, UL then tetrode/pentode.
Low-z drive into the OT is preferred whenever possible, which is why hifi tube users prefer triode output stages when minimal use of feedback is desired. The low-z drive helps the OT perform better and not having it within a feedback loop means that it has the least impact on the sound. The lowest impedance drive possible without hybrid circuitry is cathode drive. This connection requires a very high drive voltage to the power tube, which adds zero voltage gain of its own and actually introduces a slight voltage loss, and then the drive stage design becomes crucial to low overall THD.
The trans-tube circuit uses feedback up to the drive point of the OT and can reduce the drive impedance to cathode-drive-like values, which is phenomenal as we still retain the voltage gain of the tube. Is there any advantage to enclosing the OT in an overall feedback loop? There could be, but the stability of both the inner loop (trans stage) and outer loop have to be carefully dealt with. The trans-tube approach allows overall open-loop functionality and stability but at low-THD.
The Gm element adds its own THD to the mix, and this will be relatively benign if the element itself is linear and/or simple. Menno experimented with opamps and found that the harmonic profile of the THD and IM was fatiguing despite there being relatively lower numbers than for the plain tube circuit. Obviously that approach could be greatly improved upon were one motivated to do so. Since the trans-tube amp is a hybrid already, how far you go with that is up to each hobbyist. Menno did retain opamps for the bias servo loop, but went discrete for the Gm element and grid buffer.
It should be obvious that the trans-tube amp concept has similar requirements to any feedback-controlled circuit. The most important being that the gain element within the feedback loop should have as much gain as possible.
A typical power tube is either a triode, tetrode or pentode. A triode has only one connection option to the load via its plate, so the triode's theoretical maximum voltage gain is its mu. A tetrode can be tied to the load in three ways: strictly via the plate, with the screen tied to a fixed voltage; wired using an ultralinear tap for the screen; or with the screen and plate linked for triode performance. A pentode is wired similarly to the tetrode as its suppressor grid is typically tied to its cathode.
The triode voltage gain is lowest. Its plate impedance is also lowest of the three wiring modes.
Ultralinear allows local feedback over a portion of the transformer primary, which reduces the plate impedance of the tube compared to tetrode/pentode mode but not as low as triode mode. Voltage gain is higher than triode mode.
Tetrode/pentode mode has the highest output impedance and the highest voltage gain.
The open-loop distortion of the three modes are from lowest to highest: triode, UL then tetrode/pentode.
Low-z drive into the OT is preferred whenever possible, which is why hifi tube users prefer triode output stages when minimal use of feedback is desired. The low-z drive helps the OT perform better and not having it within a feedback loop means that it has the least impact on the sound. The lowest impedance drive possible without hybrid circuitry is cathode drive. This connection requires a very high drive voltage to the power tube, which adds zero voltage gain of its own and actually introduces a slight voltage loss, and then the drive stage design becomes crucial to low overall THD.
The trans-tube circuit uses feedback up to the drive point of the OT and can reduce the drive impedance to cathode-drive-like values, which is phenomenal as we still retain the voltage gain of the tube. Is there any advantage to enclosing the OT in an overall feedback loop? There could be, but the stability of both the inner loop (trans stage) and outer loop have to be carefully dealt with. The trans-tube approach allows overall open-loop functionality and stability but at low-THD.
The Gm element adds its own THD to the mix, and this will be relatively benign if the element itself is linear and/or simple. Menno experimented with opamps and found that the harmonic profile of the THD and IM was fatiguing despite there being relatively lower numbers than for the plain tube circuit. Obviously that approach could be greatly improved upon were one motivated to do so. Since the trans-tube amp is a hybrid already, how far you go with that is up to each hobbyist. Menno did retain opamps for the bias servo loop, but went discrete for the Gm element and grid buffer.
