Bottlehead shunt regulator upgrade kit

[shane]

Hi,  I'd like to ask if the AC impedance of the bottlehead shunt regulator is the Rp of the triode section, or 1/Gm of the triode section.

Thank you

[Paul Birkeland]

It's a bit more complicated than that, as the TL431 itself has gain that changes rather significantly with frequency.  We also sometimes throw a bypass cap in to send more of whatever is riding on the HV regulated rail back into the regulator, and that also alters the AC impedance a bit. 

IIRC, the AC impedance of something like the shunt regulator in the Extended Foreplay III without the extra bypass caps was a couple hundred ohms at 120Hz.

PJ could author a more thoughtful response on this topic but he's on vacation so I'll fill in as best as I can till the expert returns.

[shane]

Thanks for the reply.  If it's a couple hundred ohms, I guess that the impedance cannot be equal to the Rp of the tube.  Is it as simple as 1/Gm plus the AC impedance of the voltage reference in the cathode circuit?.  Thanks, Shane

[Paul Joppa]

No, it's primarily set by the transconductance of the chip, adjusted by the normalized voltage. In theory (and by the specs) that works out to around 35 ohms for a 350v regulated supply.

I don't recall making a measurement. In the Foreplay days we built several (6 IIRC) shunt regulators and did comparative listening to choose one.

[shane]

Thank you.