half power switch?

[chuckb]

Hey guys, i'm looking to put an 1/2 power/triode mode switch (,15/7 watts) on my Matchless Lightning clone. Can someone direct me on how to go about doing this ? I have never done this before ,so any help would be greatly appreciated.

Chuck.

[hasserl]

Triode operation connects the screen grids to the plates. You need to wire up a 2 position On-On spdt toggle switch so that the center terminal is connected to the screen grids and one outer terminal is connected to the standard screen grid supply, the other outer terminal is connected to the plate. With the switch in one position the screen grids are connected to their normal supply (pentode), in the other position they are connected to the plate (triode).

[Jazz P Bass]

Would that not take a DPDT switch?
That way each tube is isolated.
http://www.google.com/products/catal...d=0CHUQ8wIwAA#
Switch - Wikipedia, the free encyclopedia

[Chuck H]

It's a cakewalk. Lot's of diagrams on line for doing it. The tone is cool, but not really a half power version of the same sound as when the tubes are operated as pentodes. Triode mode sounds a little more middy, less dynamic and there's a definite top end roll off. It is significantly quieter. But since roughly 7 to 10 watts is still pretty loud ( a Champ is actually about 3.5 watts. Not 5 watts as is posted all over) I find it a more useful mode just for tonal variation than volume reduction.

A more useful volume reduction might be to squash the screen voltage by tapping a totem filter assembly.

[chuckb]

hey chuck, could you elaborate on squashing the screen volages dy tapping a totem filter assembly? How would I go about dooing that?

chuck.

[Chuck H]

I'll need to do some MSPainting. I'll get back.

[Chuck H]

I modified a Lightning schem and the "totem" arrangement I mentioned is in the red circle. The switch gives you three power options. Then I thought about it again and decided to include a diagram of a simple attenuator that I use sometimes. The attenuator is infinitely variable and sounds quite good. There is some tonal detriment at really high attenuation levels but not bad and it'll get you down to bedroom practice volumes. The last one I installed in a combo was done on an aluminum plate that I screwed to back panels.

[chuckb]

Hey chuck, thanks for the info. The caps in the totem arrangement are they 6.8uf or 68uf @450v and the switch is a spdt? The attenuator is interesting ,I'd like to give that a go also.

Chuck.

[Chuck H]

Hey chuck, thanks for the info. The caps in the totem arrangement are they 6.8uf or 68uf @450v and the switch is a spdt? The attenuator is interesting ,I'd like to give that a go also.

Chuck.

The caps are 68uf and only need to be 200V. The switch is a SP3T like a lot of impedance switches. Many switches don't like high voltage so be sure to check that spec. I've never had to source a switch for this purpose since I've never done this circuit (but it's a common circuit for power reduction in many commercial amps). I hope the cost of the switch isn't too prohibitive.

The attenuator is a great thing to have. My personal unit uses a reactive load in place of the load resistor and this does sound a little better, but is much bigger and more expensive. For modrate attenuation the reistive version works just great and is cheap and easy to make. If you want to be able to use it with amps bigger than 25 watts you'll need to up the ratings on the rheostat and resistors.

[chuckb]

Hey chuck,thanks for all the needed info,very much appreciated. I have to pick up some 68uf caps as I don't have any of that value here at the moment. I'll let you know how it works out and I'm sure to have more questions concerning this before I'm done.

Chuck.

[Bruce / Mission Amps]

Find a schematic for a Gibson, Goldtone GA15RV, the remodeled version of the old Trace Elliot Velocette... copy that pentode triode switch circuit.
It works well and the power output in faux triode mode is about 40% or so of the pentode mode.
I've used this circuit a number of times on <22watt amps... nice but darker and duller so be prepared for that comment.

http://www.gibson.com/Files/schematics/GA15RV.pdf