Design using variac to lower the power.

[Alan0354]

Thanks, I wonder how much power you can lower by just control the screen grid of a 20W amp?

[Steve Conner]

I think you can control it over at least a 10:1 range, but it definitely starts to sound different at the low end of the power range.

I can set my amp to the 30W range and get it as quiet as the 0.3W range by turning the screen voltage practically down to zero, but it doesn't sound that great any more.

Also it has to be cathode bias, otherwise you'd need to adjust the grid bias voltage in step with the screen voltage. The cathode bias adjusts itself automatically.

[Alan0354]

Thanks for you help.

Alan

[Ramsay]

For 50W or above, it is risky screw the power mosfet (insulated) at the chassis?
I ask this because the chassis of a tube amp gets hot a lot, after sometime of use.

[Steve Conner]

I don't like using the chassis as a heatsink. There is the issue of heat from the tubes like you pointed out. But also the chassis is often made of steel, which has terrible thermal conductivity.

On the amp build above, I wanted an invisible stealth heatsink to hide the fact that the amp was a hybrid. (If anyone found out, it would suddenly start to sound bad.) I made a new chassis out of heavy gauge aluminium and bolted the power transistors to that.

The screen grid MOSFET doesn't need any heatsinking, the TO220 tab is enough. But in a 50W amp, it might need a small heatsink.