Univox Amp Question

[LPDude2]

These are great amps! I am a BIG Univox tube amp fan.

The ONLY mod I would make to this amp (aside from recapping it, which is not a mod, it's maintenance) is right at the input. There is a .01uF series cap and 4.7M resistor to ground on the first stage, providing a "contact bias" scheme. This scheme is fine until you hit it hard with either powerful humbuckers or booster of some sort. The first stage then becomes prone to "blocking distortion", which is a condition where the capacitor cannot discharge fast enough and the grid of the tube builds up an increasing DC sharge as a result. Eventually, the tube gets driven into cutoff, and the signal gets "blacked", hence the name. It's a real nasty, raspy sound that results.

It would be best to remove the cap, add a jumper in it's place, then open up the cathode connection to ground and add a 1.5K resistor paralleled with a 22uF cap between the cathode and ground. This more ubiquitous setup makes the amp more user friendly, and just sounds better.

What type of cap should I use for the 22uF?

[Jazz P Bass]

A 22uf/ 50 volt axial lead electrolytic will do just fine.

[LPDude2]

A 22uf/ 50 volt axial lead electrolytic will do just fine.

Great. That's what I needed to know. Thanks!

[LPDude2]

I made one mistake by omission with the bias mod: R4 (4.7M), stays where it is (from grid to ground), but get changed to 1M.

I have one more question. What wattage resistors for this mod?