capsule voltage/dropping resistor in Gyraf G7?

[erikx]

I have what might be a basic question here... I'm looking over the G7 schematic (http://www.gyraf.dk/gy_pd/g7/gic_s.gif) and the polarization voltage isn't entirely making sense.

The schematic puts +160V on pin 2, which then hits a 10K resistor, followed by a 2u2 capacitor paralleled to ground. I'm assuming this is noise filtration, but it seems like there should also be a voltage drop across the 10K resistor.  When the voltage hits the 470K/470K divider though, it appears to still be 160V: the resistors in the divider are equal and the output voltage is 80V.

This voltage (taken after the 10K resistor) is also the B+ to the tube. Suppose the tube is biased to 1.5mA. (This should be close enough for argument's sake---I'm assuming the current draw of the capsule is negligible, yeah?)  1.5mA across 10K should yield a drop of 15V, correct? It seems like the voltage to the backplate would then be (160 - 15)/2 = 72.5V.

There's probably something here I'm just not thinking about right, but I can't figure out what...

Thanks,
-E

 

[Tim Campbell]

There has been a revision to this.  Almost no capsule like to see such high polarization voltage.  Most prefer 60/120v and so there has been an update to drop the volage to 60/120v.

 

[erikx]

Thanks---yeah, I've seen that. I guess I was bracketing that issue and asking a more basic question for my own edification:

Say, for argument's sake, it was built as drawn in the schematic. It seems like the voltage should drop over the 10K resistor, but as drawn, it doesn't. I'm just wondering if I'm misreading it.

 

[moamps]

Your calculation is correct, only IMO the anode current is less (about 1mA ) so the voltage drop on 10k resistor is smaller.Voltage biases difference of few volts will not affect significantly the polar characteristic and output level of the microphone.

 

[erikx]

Ok, thanks---makes sense. Just wanted to make sure I wasn't losing my marbles!
-E