Tube pre-amp: coupling caps voltage

[Thomeeque]

Hi!

What voltage capacitors should be used for coupling between triode stages¹ with "standard" supply voltage (around 300V). I have always expected, that it should be at least triode plate voltage (e.g. 135V), but now I see some JCM900 schematics where they use 50V caps (if I read it right) - so is 50V enough?

Thanks, Tomas

¹ As "coupling cap" defined for example here: http://www.diystompboxes.com/wpress/?page_id=10

[d95err]

Let's consider the maximum voltage that can appear on a typical coupling cap. You have the plate voltage of the triode, e.g. 150V. Then you have the signal voltage. Lets assume the biggest possible peak signal output of the stage is 80V. Then you need the coupling cap to be able to handle 150 + 80 = 230V. However, make sure to always use a big margin for cap tolerances. I would never use coupling caps with a lower voltage rating than the B+ voltage.

What JCM900 schematic seems to use 50V caps? That's either wrong, or it's for some specific caps that don't have full signal or plate voltage.

[MartyMart]

+1 ... coupling caps in my Epi VJ's that I modified were 400v rated !
I think the originals were 250v IIRC
MM.

[d95err]

I think this is what you're reffering to:
http://www.schematicheaven.com/marshallamps/jcm900_25xx_45xx_50w.pdf

Yes it does say non-elec caps are 50V (but read the fine print) unless marked:
* D  Ceramic Disc
* R  MCT Radial

It does not specify voltage ratings for the ceramic caps, but that's usually fairly high, and I assume the MCT caps have a high voltage rating.

Since it's partly a hybrid amp, lots of places in a JCM900 are fairly low voltage. I guess that's where all the low voltage caps come in. The signal voltage is limited in some places. E.g. the zener diodes on the input makes sure the signal voltage doesn't get too high in the first part of the preamp (so if you push a JCM900 with a booster, you get zener diode distortion rather than more tube distortion...).

[Thomeeque]

OK, thanks for replies, now it makes sense to me - my JCM900 schematics (paper-only) are missing those magic words unless marked, that confused me.