Half Power Switch?

[BrianA]

Other than pulling two of the tubes and changing the output impedance, how might one go about installing a half power switch to go from 36 watts to 18 and back?

I was just about to build a Model 1962 JTM45 but I've been rethinking it some due to the volume. Might be nice to have the flexibility to go between a higher power and lower power amp. (Yes I know half power does not mean half volume )
Thanks for any imput.
Brian

[Plexi]

I think there is a pentode triode switch in the files somewhere..maybe other ways,like a switch to lift the cathodes of one tube on each side..or a switch that removes the signal to 2 tubes,one on each side..this way the voltages would still stay closer to the same,and better than removing 2 tubes from the amp.

Richie

[BrianA]

Thanks for the reply Richie.
Question: If you used a switch to lift the cathodes of two tubes, wouldn't you also have to switch the value of the bias resistor? Maybe switching off the signal to two tubes might be the way to go. I guess that way the OT would continue to see the same impedance, wouldn't it?
Brian

[Aaron]

if you had 4 tubes, wouldnt you want to have 2 bias resistors anyway?

[BrianA]

Well duhh!
I didn't see that one. I was looking at the Ac30 power section which does use one resistor.
What about the impedance question? Does lifting the groung on one tube of each side do anything to the impedance that the OT primary sees?
Brian

[zaphod_phil]

Yes, with two tubes you want double the impedance you would use with four tubes. So you would need to switch your OT taps as well.

[BrianA]

So I guess a single DPDT toggle would work. One side to lift the ground and the other side to switch from 4 to 8, or 8 to 16 on the OT.

[PhatTele]

I was wondering if anybody got this 1/2 power switch working for their 36Watt. If not, there might be some info in one of the Kevin O'Conner books where he talks about something similar for AC30s. Just curious.

[rjgtr]

I think the best option is to just lift the input signal going to one pair of tubes. That way there's no "did I get the impendence right" question and since it can be done with a simple switch, there isn't a complicated switching scheme going on. The only down side is that the unused tubes are still wearing a little, even though no signal is passed to them.