Modding GT-2B tube mic = New Passive Tube PSU won't output 6.3v for heater

[elskardio]

Hi Guys,

I'm currently modding a GT-2B microphone. It's the same schematic as the Apex 460 with a CDDA topology.

I built a new passive PSU based on Matador's Universal PSU schematic.

With the mic connected, I can adjust B+ for 120v but the Heater voltage won't go higher than 5.9v. With the tube removed, I have full adjustment on the heater up to 11v.

I've attached both schematic (mic and psu).

If someone have any idea what could cause the problem, please share! The tube is an EH 6072A.

Thanks

 

[rock soderstrom]

but the Heater voltage won't go higher than 5.9v.

5.9V is actually ok but if you want 6.3V, bridge one of the two resistors (R7 or R8) in front of the regulator.

Edit:
You use both triode systems, your tube draws 0.35A. This current leads to a voltage drop of 1.645V across R6 and R7 each, which together with the voltage the LM317 needs to regulate and the losses across the rectifier give the 5.9V you have measured.

V=R x I=4.7R x 0.35A = 1.645V

 

[elskardio]

You use both triode systems, your tube draws 0.35A. This current leads to a voltage drop of 1.645V across R6 and R7 each, which together with the voltage the LM317 needs to regulate and the losses across the rectifier give the 5.9V you have measured.

Yes I totally missed that one!
The PSU was designed for a C12 clone which use only one side of the tube.
Thanks

 

[TLRT]

You need a higher voltage secondary transformer, I Guess the factory mains transformer is rated 9.5v. lm317 can handle 1A I think,so that part is ok. I usually test those scenarios with a huge adjustable isolation transformer that has voltage and current meters,so very easy with this tool to find out the requirements.

 

[rock soderstrom]

You need a higher voltage secondary transformer, I Guess the factory mains transformer is rated 9.5v. lm317 can handle 1A I think,so that part is ok. I usually test those scenarios with a huge adjustable isolation transformer that has voltage and current meters,so very easy with this tool to find out the requirements.

...or halve one of the resistors before the LM317. You can probably even bypass it, you have enough filtering there and your microphone has cathode bias which is not as sensitive as heater based fixed bias.