• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Tubecad hybrird feedforward feedback shunt regulator


2008-01-09 10:38 pm

as I am looking for some 300V tube shunt regulator I have come across tubecad´s hybrid one. I have few questions about it:

1) The 380 ohm resistor should be calculated as R=1/gm. Shouldn´t be the resistor halved, because there are two triodes in parallel?

2) Can I use lower value capacitor on the output RC filter, say 1uF?

3) Will be OPA134 opamp ok to use?



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    ss series-shunt regulator 12au7 2.png
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1. Resistor is based on the gm (gain) of the tube(s), which doesn't change by paralleling them. The Rp will change, but the gm won't. Since only gm is used in the formula, the resistor value is correct (assuming the gm of the actual tube is within spec).
2. Possibly OK. It's used as a dampener to control ringing, so it may or may not work with only 1/10 the suggested value.
3. Should be fine. It's a good op-amp, fast without being a pain to stabilize. I'd actually recommend something slower like a 741 - this is primarily being used to filter out lower frequency noise so speed isn't needed, and there's no provision for a power supply bypass cap (which would be a negative in this circuit), so a lower performance opamp may be needed. Either way, I'd build in a a socket so you can switch out opamps if needed.


2008-01-09 10:38 pm
Yes, I will use socket for opamp, will try OPA134 and TL071.
This shunt regulator will be used after maida regulator to clean maida´s noise.
I am curious if it improves sound, maybe lower noise, but higher output impedance.

It´s a pity John Broskie doesn´t post specs about this regulator - PSRR, output impedance and noise.