# halving power



## ezcomes (Jul 28, 2008)

i was thinking about this...

since pulling a pair of power tubes effective 1/2's the output...when building an amp, say 100W with four output tubes...could a switch be wired into the power section to cut the signal to the two tubes, rather than take them out...

i was looking at a kit online...comes in 100W...would this be effective?


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## Rugburn (Jan 14, 2009)

ezcomes said:


>


I think if you just follow this schematic *very carefully *everything ought to be just fine.

Shawn.


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## Wild Bill (May 3, 2006)

ezcomes said:


> i was thinking about this...
> 
> since pulling a pair of power tubes effective 1/2's the output...when building an amp, say 100W with four output tubes...could a switch be wired into the power section to cut the signal to the two tubes, rather than take them out...
> 
> i was looking at a kit online...comes in 100W...would this be effective?


I wouldn't do it! The unused tubes would still be in parallel with the working tubes. So they would possibley interact with the other tubes as the plate voltage would swing with the output power driving into the output transformer. I'm not sure what would be the effect of essentially having idling tubes appearing as a dead load on working tubes but I wouldn't want to take the chance before I had EVERYTHING figured out and measured!

'Course, if it was somebody else's amp and they were smaller than me...!

It would also be a very inefficient method. The unused tubes would be wasting a LOT of power just being lit up and sitting there!

The two usual methods of achieving half power are to wire in a switch to disable the screen voltage and tying each screen to its tube's plate or the old Garnet method of putting a big common cathode resistor to raise the bias level to all the output tubes.

Both methods will change the tone a bit in low power mode, especially the usual screen voltage method. The reason is that disabling the screen voltage makes the output tubes into triode instead of pentode amplifiers. Less power and much cleaner! The cathode resistor method cuts power by raising the total bias voltage, running the tubes on the "thin" and lower power side of the bias adjustment.


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## ezcomes (Jul 28, 2008)

interesting...cool...will definately have to think on this


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## keeperofthegood (Apr 30, 2008)

Ive seen it for SE amps, but is this what you mean for PP? Shouldn't that switch both tubes too?


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## Wild Bill (May 3, 2006)

keeperofthegood said:


> Ive seen it for SE amps, but is this what you mean for PP? Shouldn't that switch both tubes too?


In a PP amp you would use a DPDT switch to accommodate both tubes.

Check out a schematic for a JCM900 or newer Marshall with a HI/LO power switch. That's how they do it.

WB


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