voltage rating question for signal-tone caps

[tubeswell]

Doing my vibrato unit (which utilises the hammond triple array saturable reactor) at the mo'. Couldn't find any 150pF caps in 500V (or even 250V). Did get some 120pF caps at 3kV.

How critical are to 150pF value in this circuit?

I have got some 50V caps at 150pF. Seeing as how the 150pF caps in this circuit are on the other side of coupling caps from the HT supply, I reckon I can get away with 50V caps for the 150pF, unless anyone here has some important advice you feel you want to tell me. See schematic

TIA

Cheers

Pete

[Tiny_Daddy]

Mouser has them: 5982-15-500V150  for 56 cent.

[HotBluePlates]

You can get by without the full 500v rating, because as you said, there are coupling caps on either side to block the dc.

But the question is, do you design for what will work or for the worst case scenario? If the plate coupling cap fails, the 150pF cap will essentially see all the B+ across it, finding ground through a 10k cathode resistor.

True worst-case assumes the B+ will not be dropped on its way to feed the stage involved (perhaps because of another failure, such as an open from B+ to the LFO circuit). 330v implies 33mA through the 10k resistor. That current will have to flow through the saturable reactor winding on its way to the resistor. Can that wire actually support 33mA? I don't know, but I wouldn't want to chance it myself (it's pretty fine wire).

So a 500v rating probably isn't needed. But it's a standard part, and affords protection under the worst likely scenario.