Capacitors voltage tolerance

[Enzo]

Juan, diagramm beat me to it, but considering only the circled caps, just the 250pfs need voltage ratings B+ high unless something fails ahead of them.

[tubeswell]

To break it down even further, when you first fire up the amp, there is a brief period of time (few seconds) where the tubes heaters haven't warmed up enough to get the tubes conducting any current, but where the B+ DC voltage has risen to the maximum it can get to.

At that point in time, any other parts of the circuit (i.e.: other than the tubes) that form a circuit between the B+ rail and the ground return, will 'experience' the voltage drop between the B+ and the ground. For each gain stage, those 'other parts' of the circuit typically are the plate resistor, the coupling cap that is attached to the plate, and the grid load resistance (e.g.: volt pot, tone stack pots, or grid leak resistor etc) of the next stage's grid. However since the tubes are not yet conducting, then the resistors attached to their electrodes (i.e. the plate and grid-load/leak resistors) won't be conducting either, so the DC voltage at either end of the resistors is the same. So both ends of the plate resistor will be at B+ potential, and both ends of the grid-load/leak resistor will be at ground potential.

Since the coupling cap is typically between the plate resistor and the following stage's grid leak/load resistor, and since neither of those resistors will be conducting DC current during these brief few seconds that the tubes are waiting to reach conducting temperature, then the entire DC voltage drop between the B+ and the ground then occurs between one side of the coupling cap and the other side of the coupling cap. So the coupling cap should be rated to withstand this peak voltage differential.

The way the power supplies of most tube guitar amps are designed, the B+ rarely gets above 500V maximum. In some amps, the maximum B+ never get above 400V. Depends what you've got.

[J M Fahey]

Fine with me, just wanted to state something that "for us" is obvious, maybe not so for others.
Personally, I believe in "better safe than sorry".
And the *wording* of the phrase was no *that* clear:

more specifically on your vibrolux schem...only the 2 250pf capacitors need to be b+ rated.

"Schem" is *all*; writing "among the circled capacitors" would, yes, have clearly restricted the discussion only to them.
Peace.

[diagrammatiks]

Fine with me, just wanted to state something that "for us" is obvious, maybe not so for others.
Personally, I believe in "better safe than sorry".
And the *wording* of the phrase was no *that* clear:

"Schem" is *all*; writing "among the circled capacitors" would, yes, have clearly restricted the discussion only to them.
Peace.

yup your right of course.