AKG solidtube power supply help

[Khron]

Maybe a safety feature to cut the HT rail? What do you think?

... In case the heater rail goes WAY overvoltage - that zener there is 24V (BZX55C24), so that'll drag down the B+ only if the heater voltage goes over 25V or so (zener's 24V plus 0.6V to get the transistor to conduct plus some drop across that 1K base resistor).

 

[rock soderstrom]

According to the AKG P820 tube microphone schematic, which is pretty much the same as the Solitube power supply without the pattern voltage part, the R7 on the emitter to ground is a 100 ohm resistor.

Thanks for confirming! I was a bit confused by the E sign, but it seems to have been relatively common for ohm values in the past.

I think you are right, the transistor ZTX857 might be to "bleed" the HT/B+ rail to ground when no load is connected to the power supply. Then the "unloaded" heater rail will show 15VDC instead of 12VDC for the 12AX7 and drain 3VDC through the 12V zener diode, which then triggers the transistor ZTX857 through the base connection to let the connected HT/B+ rail at the collector pass through the emitter to ground. Maybe a safety feature to cut the HT rail? What do you think?

I'm very unsure how this works, I was thinking about it in bed last night, but I didn't really come to any conclusions. I don't think that the missing tube load would relieve the power supply to 15 V, because the LM317 would keep its set voltage even without the tube, the LED would be sufficient as a base load, I think...

 

[rock soderstrom]

... In case the heater rail goes WAY overvoltage - that zener there is 24V (BZX55C24), so that'll drag down the B+ only if the heater voltage goes over 25V or so (zener's 24V plus 0.6V to get the transistor to conduct plus some drop across that 1K base resistor).

(Edit: ohh, its a 24v Zener...) That could be, but why do they use a transformer with a secondary voltage of 23V for a 12V heater? Strange...

 

[HerbertR]

(Edit: ohh, its a 24v Zener...) That could be, but why do they use a transformer with a secondary voltage of 23V for a 12V heater? Strange...

In the P820 Schematic this Zener is 12V. In the Solitube PSU it is a 24V Zener
Sorry I have overseen the 317 Voltage regulator. Then my "unloaded" conclusion makes clearly no sense......