A question for the smart people (push pull amp on one tube?)

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nacho459

Well-known member
Joined
Jul 20, 2004
Messages
339
Location
Pasadena CA
I have Music Man "65" amp with two EL34 power section. It has a low and high power switch which I assume turns one of the EL34's off. If that is true wouldn't the amp have to be single ended. I got in an argument with one of my guitar buddies last night who said, "I know guys who have pulled all but one tube out of their Marshall and it worked fine". How could you run a push pull amp on one tube? Wouldn't you only get half the sine wave?
 
I found some schematics for old Music Man amps here. The 2100-65 shows a hybrid amp w/ EL34's in the output section -- I'm guessing that's more or less your amp. It looks like the HI/LO power switch is just a tap on the secondary of the power transformer, operating the output tubes at higher or lower voltages -- not cutting one off as suggested. Very interesting design driving the cathodes with the signal instead of the grids.
 
> It has a low and high power switch which I assume turns one of the EL34's off. If that is true...

It isn't true.

> How could you run a push pull amp on one tube? Wouldn't you only get half the sine wave?

If tube amps ran perfect Class B, cut-off at idle, then it would be true.

No tube amp runs that way. Tubes have a soft cut-off. If you could push them all the way to cut-off, there would be severe distortion for small signals as they crawled their way out of cut-off. (This does happen in badly biased transistor amps.)

If the tubes ran Class A, removal of one tube would reduce the output power (and annoy the transformer with DC current) but within the new power capacity it would run single-ended producing both sides of the wave.

Most tube amps run well up into Rich Class AB, as much A as B, so they will sound fine one-legged, just a bit weak.

And possibly VERY bass-less, because a push-pull transformer does NOT like unbalanced DC in the windings.

> Very interesting design driving the cathodes with the signal instead of the grids.

It is a transistor amp with tubes doing grunt-work, NOT a proper tube amp. Below clipping the distortion is entirely transistor-like. Any tube-curvature is reduced about 500 times by transistor collector action. They take enough NFB around it so it is dead-clean up to the clip-point. Very 1980.
 
Thanks, a lot guys, I bought the amp without really examining it. I played it and thought it had a unique sound but never cracked it open. That said I'm surprised it sounds as good as it does having a solid state preamp and all.

[quote author="PRR"]It is a transistor amp with tubes doing grunt-work, NOT a proper tube amp. Below clipping the distortion is entirely transistor-like. Any tube-curvature is reduced about 500 times by transistor collector action. They take enough NFB around it so it is dead-clean up to the clip-point. Very 1980.[/quote]

This explains why the amp really sings when you dime the whole thing and really push the power tubes and transformer. It's weird this amp doesn't really clean up. Once you get past 3 on the pre volume it starts breaking up, It has a deep distortion not just a typical "fuzz on the top" distortion. It kinda reminds me of a cranked Ampeg V4, but smoother.
 
The MM amps are kind of cool. What opamps in yours the 4558s 353s or TL072s?

If you look at the MM schematics (I think you can find them at the E.Ball site) Some of the preamps have a clever soft limit circuit in the feedback of the opamp stages. I have been told the output is class B or very close to class B. Also note the screen voltages compaired to the B+ for the output section.

A friend of mine has two MM I forget the models. The one with the >100 watt rating can sound good with a Jazz guitar. The combo can sound good as well.
 

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