capsule to tube capacitor: voltage (how much "headroom")?

Help Support GroupDIY:

xeawr

Well-known member
Joined
Dec 14, 2016
Messages
108
Location
Germany / Austria
Hi there fellas!

I ordered a 1nF 160 VDC polystyrene capacitor to use it between the capsule and 12AT7 of my tube mic. The power supply puts out 155 VDC, that means I have a "headroom" of 5 VDC for the cap.

So, question: Safe to use it? (Or better go with a polypropylene 250 VDC cap?)

Yay or Nay?
 

Khron

Well-known member
Joined
Apr 8, 2010
Messages
1,892
Location
Finland
Is that capacitor (between the capsule and the grid) going to be between the positive end of the power supply and the tube's grid? My guess is "no".

Do keep in mind the capsule is a capacitor itself, and is unlikely to see (or withstand) much more than 60V or so between the backplate and either diaphragm, so... Short answer, even 160V is a wee bit overkill (imho).
 

xeawr

Well-known member
Joined
Dec 14, 2016
Messages
108
Location
Germany / Austria
yes exactly. From the capsule to grid. The cap marked in red in the Apex 460 schematic:

apex460-mic-lg.jpg


I wasn't sure how much voltage there is actually left after the 1G resistor and I assumed it must be quite a lot because in all Chinese tube mics this cap is rated between 250 VDC and 600 VDC... (showing my lack of EE knowledge here :) )

(and I always wondered where people get their high voltage polystyrene caps from )
 

RuudNL

Well-known member
Joined
Apr 26, 2009
Messages
2,407
Location
Haule / The Netherlands
The voltage over C3 is half the supply voltage.
The grid of the tube is at ground potential.
The voltage difference is the voltage over the coupling capacitor.
So 160 V is more than enough!
 

xeawr

Well-known member
Joined
Dec 14, 2016
Messages
108
Location
Germany / Austria
RuudNL said:
The voltage over C3 is half the supply voltage.
The grid of the tube is at ground potential.
The voltage difference is the voltage over the coupling capacitor.
So 160 V is more than enough!


Thanks RuudNL, much appreciated!!!
 

Latest posts

Top