I built the PSU using Gyraf's board. Wired it up and checked some
voltages - and found that the supply is rather low under load.
I got about 176 Volts without the mic connected, and was down to 136
V with the mic in the loop. Heater supply dropped from 6.3V
(set whilst unloaded) to 6.08V. Here are some voltages:
Jakob Erland suggested "Yep, 136V is a bit on the low side, allthough
the mic will work fine anyway.
The 6V at the heater is good. We like to be running a little under-voltage
here. The simplest way to raise voltage
would be to lower the resistance of the resistor between the
two 470u/200V electrolytics. Try paralleling the 10K
with another 10K on top of it. That should do it. Another solution
would be to use a different secondary transformer."
This is what I got....First attempt in red, revised voltages in green....
"Unloaded" meand the PSU alone, "loaded" means with the (capsule-less) microphone connected.
This is a bit better: I now have 144V on pin 2 and 144-72-0V on the polarity selector.
I think this is close enough, and the mic certainly works quite happily at these values.
You can get all of the PCB artwork and schematics from the Gyraf
site.
I have spotted a couple of small errors on the component overlays (I used revision 2 artwork)
(A) The electrolytic capacitor has the positive leg towards the
tube-end of the board on the top layout,
but towards the transformer end on the lower layout:
(B) Also, the F+/F- supplies swap over.....
