Turning "off" One Tube in PP Output Stage

[ChrisM]

Ok I am trying to turn off one tube in a dual 6L6GC fixed bias output stage.

This is my idea...
[IMG:720:450]http://img.photobucket.com/albums/v610/ ... s/Idea.jpg[/img]
(used EL84s for illustration purposes)


It comes from the Dumble half power switch as seen here
https://tubeamparchive.com/files/124_schematic_182.pdf

Basically it shut "offs" one tube so it isnt doing much. But it is still technically on but little current flows through it. This way the primary impedance is kept the same.

Will this work, any other ideas, insight, comments?
Thanks...

[dynaman]

Push-pull with just a pair of 6L6s??

[ChrisM]

Like typically 50W output stage.
One tube's plate to the OT, the other end of the OT goes to the other tube's plate.

[dynaman]

Have you heard a P/P amp with one tube pulled? Are you simply trying to lose power?

[ChrisM]

I have not, care to explain for me?
Would my idea work?

[dynaman]

Crossover distortion, I'd imagine. Honestly, I don't know all of the different parameters that would be upset. You'd only be amplifying half of the PI's signal, but your output wouldn't be 25W. It would be significantly lower. I've ran a P/P amp with only one tube for non-playing purposes and it sounded wretched. I'm sure others with more technical experience will chime in with the particulars of what would happen.

Of course you could conceivably pull an output tube from an amp that was running in parallel SE.

[martin manning]

If you turn one tube off what you will have left is essentially a single-ended output stage biased really cold, near cut-off, resulting in lots of distortion.

[Richie]

and sounds like crap.

[ChrisM]

I am going to experience what crap sounds like first hand than. Thanks for the info...

[dartanion]

A better approach would be to leave the tube in the circuit and cutoff the input signal to a tube.

[selloutrr]

consider using a 4PDT or of the like and adding a bias adjustment to compensate for the "crap" of removing one tube.

[martin manning]

Dartanion, I think the result would be about the same. Chris, if you're trying to reduce volume you could add the so-called Lar-Mar post-PI master volume. Do you know that one?

[daydreamer]

I was just reading about the Mesa Boogie Lonestar (I think) and they have a 'patent applied for' way of doing this, so, yeah, not sure about getting specifics, they are all PCB as well.

Basically it is the 5W setting on the Lonestar changes the amp from p/p to SE (on one tube obviously).

Excuse the newbie trying to sound smart here, but the phase splitter would have to be bypassed yeah?

Sounds like a worthy cause....

[selloutrr]

they may have a circuit that flips the 180 degree out of phase in phase when it's not PP. much like combining the wet / dry signals so you don't have volume loss.
If it's mesa I'm sure it's no secret's and has probably been done before they just patten everything they do, I'm not sure how many of the pattens hold water... most are enforced with $$$ muscle.

[ChrisM]

I am not trying to reduce output level. There is a million and one other ways to do that .

It was more trying to do "class A" emulation. A friend had told me Mesa did "something similar" so I was just wondering.