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May 20, 2019, 11:41:31 am

Author Topic: Bottlehead shunt regulator upgrade kit  (Read 412 times)

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Offline shane

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Bottlehead shunt regulator upgrade kit
« on: October 07, 2018, 04:21:44 pm »
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

Offline Paul Birkeland

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Re: Bottlehead shunt regulator upgrade kit
« Reply #1 on: October 08, 2018, 05:38:34 am »
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.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline shane

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Re: Bottlehead shunt regulator upgrade kit
« Reply #2 on: October 14, 2018, 04:12:00 pm »
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

Offline Paul Joppa

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Re: Bottlehead shunt regulator upgrade kit
« Reply #3 on: October 14, 2018, 08:03:52 pm »
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.
Paul Joppa

Offline shane

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Re: Bottlehead shunt regulator upgrade kit
« Reply #4 on: October 15, 2018, 03:55:40 pm »
Thank you.