xeawr

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

Re: capsule to tube capacitor: voltage (how much "headroom")?
« Reply #1 on: June 10, 2020, 10:46:43 AM »
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).
Khron's Cave - Electronics - Audio - Teardowns - Mods - Repairs - Projects - Music - Rants - Shenanigans

xeawr

Re: capsule to tube capacitor: voltage (how much "headroom")?
« Reply #2 on: June 10, 2020, 12:16:21 PM »
yes exactly. From the capsule to grid. The cap marked in red in the Apex 460 schematic:



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

Re: capsule to tube capacitor: voltage (how much "headroom")?
« Reply #3 on: June 10, 2020, 12:29:45 PM »
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!
There is a solution for every problem!

http://www.vansteenisaudio.nl

xeawr

Re: capsule to tube capacitor: voltage (how much "headroom")?
« Reply #4 on: June 10, 2020, 04:55:16 PM »
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!!!


 

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