Sluckey's power switching scheme.

[labb]

I built a Marshall 18 watt super lite that is really loud. Have been asked if I can reduce the out put some. Tried to explain that that what the Gain and volume controls are for but the player wants more...Once again Steve to the rescue..Stumble on his "Marshall 18 Watt Power Dampening Switch". Simple mod so I decided to give it a try. Works.. I did use a pot instead of resistors and a switch. I know that this is simply changing the bias of the PI but what does it actually do to the circuit...Kinda wondering if it affects anything in a bad way..Thanks.

[sluckey]

The higher resistance biases the PI colder, ie puts a choke hold on the tube. This probably affects the balance of the PI so the two outputs may no longer be equal. I don't remember ever looking with a scope to confirm that. I played with it about a week. Never did permanently install it in any of my 18W amps. There was a pretty long discussion on this several years ago. Several people were trying it out. Search the forum for "Power Dampening". Maybe the thread has not been purged.

[labb]

Found a few references but nothing in depth. The amp gets pretty thin if I dial in the whole 50K, I will let the players have a shot at it Wednesday's rehers and see what they think...Seems to be ok untill I start to get to full turn down.

[sluckey]

Thin... That's kinda what I thought too. Never tried it in any later builds either. Volume control works for me.