Half-power the simple way?

There's about three things you need to do and watch out for if you want to do this. First is you'll need to split the 4 tubes into two pairs with each pair getting it's own cathode bias resistor/cap circuit. This is really easy for the most part. It really depends on what type of chassis room you have to do with. One pair will alsways be on, and the other pair will get switched in and out by lifting the cathode circuit out with a switch.

The thing you have to watch out for is what happens to the B+ when switching to two pwr tubes. It'll go up a bit, and it might be a bit hard on the tubes. One way around this is to have the pair running at half pwr to be biased a little colder to be able to handle the extra B+. If you use a two pole switch you can have this pairs cathode circuit change so it's a little colder at have pwr, and then switch to a hotter setup to match the other pair when all four tubes are going.

The other thing is the output imp will change when running on half pwr. This can be taken care of by plugging the speakers into the speaker jack one setting below. For example if you've got a 16 ohm vox type rig going you would plug the speakers into the 8 ohm tap.

It's a pretty simple mod to do. But you've got to do it - you'd damge the amp if you just pulled two of the tubes in a cathode bias amp. Later, PaulC

 

There is an old patent for the power control of amplifiers via the screen grid. If you lower the screen grid voltage it lowers the power. Since the amp is cathode biased the bias may well track OK too.

This is only an initial impression. I can not vouch for it since I have not built such an amp.