To see if particular NOS caps have enough V rating for PI grid input

[pjd3]

Hello,
Wasn't quite sure what to title this one.

While rifling through the stash for a "Vibroverb normal channel - blackvibe 6L6 converted to Single channel Pro Reverb) for components I cane across a good handful of NOS Sprague's and Mallory dipped film capacitors. many of the caps are the 6PS NOS caps of various values and voltage ratings.

I an using a schematic of a "single channel Deluxe reverb with no tremolo" that I found that I' using as a guide for what will be a "single channel Pro Reverb with no tremolo" amp. I already have the transformers and chassis there to go, gust need to build up ab AB763 type board

The question I have is specifically targeting what the minimum voltage rating should be for the PI input coupling caps. As I look at the circuit, I'n curious if they need to be at the 450 to 600volt rating that nearly all other caps are best rated at - there is the resistor tree bridge (for voltage dividing the absence of the normal channel and then the .001uF to the other triode of the PI doesn't appear to be facing large voltages like other coupling caps in the circuit. I have a number of 200 volt rated NOS .1uF caps and wondering if I can get away with using those in only the PI input.

An I terribly off on this? and should keep the voltage rating of those caps as high as the rest of the coupling caps in the AB763 circuit? I will be using a PT with a HT of 354-0-354 (Bassman PT) @ 250nA

It seems that folks think favorable on these NOS Sprague 6PS film caps, especially for vintage amps, and I wouldn't mind giving then a home in this amp.

Thanks!
Best
PD3

[Stevem]

Caps also have a ac voltage limit.

The audio output of any tube gain stage is ac.

Even though a 400 volt D.C. Rated cap is overkill that's what I would use if I had them.

[sluckey]

The question I have is specifically targeting what the minimum voltage rating should be for the PI input coupling caps.

We need to see your schematic to give you a specific answer. It's common to have up to 80VDC on the PI side of the cap. And most Fender circuits have zero volts on the other side of the cap. So, a 100V cap could be a minumum voltage for those amps. However, some circuits may have the PI input cap connected directly to the plate of the previous preamp stage. In this case the PI cap voltage rating must be high enough to safely block the dc from the previous plate.

So, show us your circuit and we can give you a specific answer. Having said that, a typical cap value might be .001µF or .01µF. Those values are very available in 500V or 600V ratings and the price difference between those and a 200V cap is pennies. I try to keep a full line of Xicon and Mallory 150s that are all rated for 630V (rather than maintain a stock of mixed voltages) and that's what I use for all my PI input caps.

[pjd3]

https://www.tdpri.com/threads/deluxe-re ... d.1003900/

Thank you for stopping by. The above link goes right to the schematic I've been looking at. I'm fine with ordering up some nice new 450-630v caps but, I'd be happy to install these NOS Sprague and Mallory 200volt caps just in that place if 200 volts would cover the expected range of voltage. It seems like many amp builders will even go out of their way to get a hold of NOS caps like these so, I didn't feel too bad about installing those "if" they were up for the voltage range of the PI input coupling caps. It seemed as though those particular caps may not be seeing the amount of voltage that other coupling caps in the circuit.

and certainly, if you see anything "off" in the schematic that comes up in the link I opened with, please feel free to advise! It looks to be pretty on to me.

Thanks guys,

Pjd3

[sluckey]

Based on what I said above, do you think a 200V cap is sufficient?

[pjd3]

Hi Sluckey,

Yes. If there is only up to 80V on the PI side and perhaps zero on the other side, then sure. 200 ought to be sufficient, at least for the AC signal range. There is a .1uF coupling cap between the previous stages plate and the dividing resistor network (blocking the high DC V) before it hits the PI input coupling cap so, I'm led to think that 200V will cover that fine.

I'll make sure I inspect that more than I have to learn something and to be sure.

Thank you Sluckey,

Phil D