Help me identify capacitor values

[thebadtall]

Hello, I was reading a tube tone control schematic,

On the center, for the tone control capacitor, 47nf 4.7nf 22nf and so on , can I use 100v capacitors , or it is too low ?

What is the voltage that goes through them ?

Thank you.

 

[erikl]

All these tone control nodes are on grid potential of V2/2 - on both capacitor nodes. So I think a max. operating voltage of 100V is more than enough!

The 100nF anode caps should stand ≧ 300V, of course.

 

[thebadtall]

thank you, ive just figured out.

i have made this preamp, and run it at 180v, but all caps are 100v.
so anode cap has serious problem.

when i apply >200v the sound changes completely.

i measured the tone ctrl voltages and are less than 200mv

anode caps are arcotronics mkp. what will happen to them due to overvoltage?

 

[FoxyRick]

anode caps are arcotronics mkp. what will happen to them due to overvoltage?

Within 20% of the rated voltage:

L_E = L_N * (V_N/V)⁸

Where:
L_N is the expected life at the rated voltage
L_E is the expected life at the actual operating voltage
V_N is the rated voltage
V is the actual operating voltage

More than that and I would imagine rapid dielectric breakdown and subsequent reduction in capacitance as the foil is destroyed along with the polypropylene dielectric. Possibly (but at those voltage, not likely) a short.

 

[thebadtall]

the Le is hours?

i have only 100v caps, but i have plenty.
if i put 2 in parallel and in series with another 2 in parallel , will it be 100n 200v ?

 

[FoxyRick]

Hours, yes, although if you use years for Ln then Le will be years also. Note that if you just put V at just 20% more than Vn in that equation, it reduces the life expectancy to about 23%. A sobering thought!

If you put caps in series to increase voltage handling, you generally need resistors in parallel with each cap (or pair in this case) to ensure that the voltage is shared equally. Like a voltage divider with the centre joined to the junction between the caps. I'm not sure how that would work out in this instance though; possibly not well.

 

[thebadtall]

ok, i will buy some higher voltage caps, thank you