G7 power supply - polarization voltage (too high) ?

[ln76d]

Yesterday i've built PSU for Gyraf G7 mike.
My polarization voltage (RMS) is circa 185V?!?
I measured it without load. With load it should be little bit lower i think, but is it possible to get lower about 25V?
Am using custom toroid transformer with 135VAC and 9VAC on secondary windings and selenium rectifiers on both.
Two 470uF capacitors, 10k resistor and two 100k resistors for the pattern switch.
Any ideas what could increase polarization voltage that high? Is it possible that selenium rectifiers could increase voltage more than silicon rectifiers?

 

[RuudNL]

The unloaded voltage is correct.
(135 V x 1.41 = 190 V.)
With a load this voltage will be lower.
You can always increase the value of the resistor between the two reservoir capacitors, to get the voltage (under load) you want.
A safe value for the polarisation voltage is 60 V (maybe 70 V, but be careful!)

 

[ln76d]

Thanks RuudNL again 

That's good to know!
What resistance i should use to make a dummy load for measurement (or should i use RC load)?
For now i didn't finish microphone and this will take me a while.
I'll use teflon CK12 and 80V should be ok for it

 

[RuudNL]

So you are aiming for 160 V?
Say the microphone (anode) current is 1 mA. (Maybe 0,85 mA is more realistic.)
A resistor of 180 K.Ohms should be a reasonable dummy load.
160 V / 180 K = 0,88 mA = 150 mW dissipation in the dummy resistor.
With the dummy in place, you can experiment with the 'voltage drop resistor'.
Simply add 1.1 K.Ohms for every volt you have too much! (Ohms law!)
Dissipation in the 'drop' resistor is low, because the drop voltage is relatively low.