Half power sw.

[496dart]

Is there somewhere online i can look at a scem that shows how to wire in a half-power switch -for the output tubes ?

[Enzo]

Half power switch on what? Certainly it would be different on a Fender Twin versus a Fender CHamp.

[496dart]

on a 50watt -Marshall style - El34's

[Andy]

I would wire a half-power switch as a triode/pentode switch. for a push-pull output section, you'll need a dpdt switch. the switch would be connected to allow the screen grids to be switched between the screen supply and the plates of the power tubes. running a pentode in triode mode will usually yield about half the power. I'm not sure if there would be a change in tone, as I have never used an amp with such a switch.

[Ray Ivers]

FWIW, I second the pentode/triode switch idea. Here's a schematic, courtesy of Lord Valve, from Steve A.'s Blue Guitar site:



I've found triode mode to be smoother and less trebly than pentode, especially when overdriven - pentode for clean and triode for dirty can be a nice combination IMO.

Ray

[496dart]

thanks ! Ill give it a try

[Amp Kat]

Excuse me for asking but wouldn't it be a little less noisy and quite a bit less dangerous if you lifted the Cathodes instead of the Plates ? I like the Triode idea but in terms of dB it's really not that much of a difference from 2 to 4 tubes however there may be some sonic difference.

[Arthur B.]

Excuse me for asking but wouldn't it be a little less noisy and quite a bit less dangerous if you lifted the Cathodes instead of the Plates ? I like the Triode idea but in terms of dB it's really not that much of a difference from 2 to 4 tubes however there may be some sonic difference.

In a 50 watt Marshall style amp with EL34s, I would assume there would be only 2 tubes. If I'm correct, just moving the center tap of the output transformer primary from the plate suply to the screen supply would reduce the power.

[dai h.]

seems to me that would stress the choke a lot

[Matt T.]

FWIW:

I have a 100W Marshall Silver Jubilee with the pentode/triode switch and mine stays on pentode. Triode mode is less punchy, less treble, less bass, more compressed. It may be that the Jubilee is already compressed enough so triode doesn't sound as good as it might on another amp.

I also have a homebrew with a 100W Marshall PI and power section and on that one I have a switch that lifts the cathodes on two power tubes. On that amp...the switch always stays in Full Power mode. It's less of a tonal difference than triode/pentode but the amp is still punchier and ballsier in full power mode.

Both amps are MV amps with all the distortion happening pre-PI. If I were to rebuild that homebrew I would just leave any half power switch out, I like the punch/balls of 100W amps.

[Ray Ivers]

Dai,

seems to me that would stress the choke a lot

I agree - and one thing's for sure; if a Marshall- or Fender-sized choke didn't instantly saturate on PA idle cathode current alone, it surely would when the power amp started drawing signal current, and basically turn into a low-value resistor (which might give a few watts of power reduction).

I don't mean to dump on your idea, Arthur, it's just that these chokes tend to be sized only for screen and preamp-tube current, not the 6X to 10X greater output stage Ik flow. One solution would be to replace the choke with a beefier one sized for a choke-input PS, and then switch the negative plate of the input filter cap to & away from ground; you'd also have to adjust the bias voltage as well, and I'd also include a post-choke bleeder resistor to ground to ensure a minimum current flow thru the choke at all times. With 420V B+ using cap input, a choke-input should give roughly 270V, good for about 20W to 25W. At that B+, you could even go "Class A"!

Ray

[Arthur B.]

Dai,

seems to me that would stress the choke a lot

I agree - and one thing's for sure; if a Marshall- or Fender-sized choke didn't instantly saturate on PA idle cathode current alone, it surely would when the power amp started drawing signal current, and basically turn into a low-value resistor (which might give a few watts of power reduction).

I don't mean to dump on your idea, Arthur, it's just that these chokes tend to be sized only for screen and preamp-tube current, not the 6X to 10X greater output stage Ik flow.

Oh, didn't realize that detail.

One solution would be to replace the choke with a beefier one sized for a choke-input PS, and then switch the negative plate of the input filter cap to & away from ground;

How do switch to & away from something? Unless "to & away"
are not simultaneous?