Paul Ruby mod in Vox Night Train

[Chuck H]

Can you show us a diagram or schem of what you did?

IIRC the cathode voltage will rise to the onset of clipping and though it continues to rise the amp will be in cutoff at the clipping voltage. So the zener voltage needs to be higher than that. Without a scope you'll just need to get a couple of zener values to try. If you have 12V now then maybe 15V and 17V.

Also, FWIW, in my home experiments the added cathode zener offered a hair more improvement when used with the PR mod. And it makes choosing the PR mod zeners easy since they would be one or two volts higher than the cathode zener voltage. For example, you could use a 13V cathode zener and 14V PR zeners and the amp is certain to be in cutoff when the PR mod zeners conduct.

EDIT: It should be like this:

[Silvio55]

Yes thats what I did, I soldered one end of the zener to the potīs wiper and the 1N4007 to ground,

[Chuck H]

And each pair of diodes has the cathodes facing each other? That's correct then. See my edits above.

[Silvio55]

Yes, the cathodes facing each other, I double checked that before and after soldering.

[frus]

Chuck,
I was wondering about your cathode zener fixed bias. Why use the resistor and capacitor at all then? To allow the bias to "breathe" just a little bit (1 V or something), or is there another reason?

[Chuck H]

There are two reasons. First, I do want a little "breath" when the amp is clean. I size the cathode zener so it starts to conduct a little past clipping. And second, I think the resistor continues to carry some of the load and so helps to keep the zener from unsoldering itself, which I've heard they can do because they tend to be smaller for their dissapation and get very, very hot.

EDIT: Just thinking out loud now... But if a Zener is used exclusively to elevate the cathode wouldn't it only act as an AC bypass on the negative half of the wave form on AC below the zener voltage. In which case at least a bypass cap is needed. So if anyone wants to try this, that is, a zener and capacitor only cathode bias with no resistor, care should be taken to locate the two components away from each other since the zener is likely to get very hot. Corrections on AC bypass theory happily accepted.