Home |
Search |
Today's Posts |
#1
Posted to rec.audio.tubes
|
|||
|
|||
Triode or pentode with local NFB?
The recent thread about the SET 300B based amplifier got me to question: Why
triode? 1) Obviously, a triode 300B boasts low distortion say 2% at 6...8W output and reasonable damping WITHOUT GNFB, but the penalties a low plate efficiency, complexity of filament supply, cost, possible self-destruction if shorted load, full volume and fixed bias. 2) A common pentode / beam tetrode class A1 SE (say, two 6L6 or EL34 in parallel) design needs GNFB to achieve the same benchmarks, but more efficient, cheap and will not blow the tubes. A penalty here is probably the GNFB loop which encompasses the OPT and would require shelving or other elaborate compensation, which only Mr Turner can properly master to achieve stability over the wide range of the load impedance. 3) Yet there is another possibility -- use pentodes (or beam tetrodes) with local feedback. In effect the goal here is to reduce the gain (from G1 to plate) of the stage to about the typical mu of a rival triode. In other words, replace the internal electric field local NFB in a triode by an external local feedback around a pentode. One way is to use cathode feedback with the cathode winding having about 1/4 of number of turns of the plate winding. A drawback here is a special transformer to be made. Another simpler method is to arrange a resistive local feedback. Throw say R1=470K from the output tube plate to grid, through a DC blocking cap of course. Feed the input signal to the grid via a series R2=150K resistor. Thus the output stage will resemble an inverting op-amp, the gain of which is defined by the ratio of R2 / R1. This is because an inherent gain of a pentode from grid to plate is high. Say for two EL34s in parallel it can be up to 50. (More practical to throw R1 from the output tube plate to the driver tube plate without a DC blocking cap.) Thus the local feedback can reach 15...20dB. As a result: a) Drive voltage will be comparable to 300B; b) Distortion will be same if not lower than with 300B; c) The local NFB will be unconditionally stable, since it does not include OPT; d) Output resistance referred to plate (damping) will be (1/Gm) * ((R2 / R1) + 1). With a paig of hi-gm tubes as EL34s it will be even lower than with 300B; e) Will yield higher efficiency with the same DC input power; f) No real need for resonance damping circuits across OPT primary; g) No GNFB will be needed; h) Not prone to self destruction with shorted load. A drawback though is a low input impedance (about R2). However, with a cathode follower in the driver stage it will not be an issue. I am wondering if such "op-amped" pentode stages are common. What am I missing? Regards, Alex |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Pentode gm wired as a triode | Vacuum Tubes | |||
Pentode in Triode Mode | Vacuum Tubes | |||
Using power triode or power pentode wired as a triode as a split-load phase splitter tube? | Vacuum Tubes | |||
Using power triode/pentode wired as triode as a split load phase splitter tube? | Vacuum Tubes |