How quiet is quiet?

[tbbenton]

I do see your reasoning, but the current available to charge that cap will be 23.5ma, 90ma won't pass through the top CCS.  Also the cathode voltage and anode voltage are both zero until the shunt regulator starts conduction, so the reference terminal is free to vary a bit.  This has worked on the shunt regulators in many Extended Foreplay III's.  If we had a fet in there instead of a tube, then I might be pretty worried. 

Good point on the current source.  How much current does the tube sink?  Assuming worst case that 23 mA runs through the 2.49k R3, then there's about 60v on the 431, which is way above maximum rating of 37v.  This capacitor bypass is bad news.  I tried it and fried my Eros.

[Caucasian Blackplate]

Hmm, that is not good news!  PM me and I can ship you another board.  I will have to take some measurements in my FP-III and Eros related circuit to see why this might happen...  What was the exact failure in your Eros?

[Paul Joppa]

Just a note - the Eros does not have any cathode followers, all the stages are isolated from power supply noise by current sources. Making the shunt regulator even quieter won't affect the noise, which comes from other sources - mostly the EF86's thermal noise floor.

[tbbenton]

Hmm, that is not good news!  PM me and I can ship you another board.  I will have to take some measurements in my FP-III and Eros related circuit to see why this might happen...  What was the exact failure in your Eros?

Snapping sound and a flash of light, not in that order, of course.  All LEDs dead.  The resistors all still measure good, but replacing the 431s alone (from above - didn't remove the pc) didn't bring it back to life.  I will need to perform further tests.

[tbbenton]

Follow up:   My TL431s were indeed toasted and the LEDs on the shunt reg board were all shorted by the event.  Thanks to Paul, I received and installed a new shunt reg board and my Eros is back in fine form.  I am happy to have things working again and I have no plans to monkey any further with the power supply.