two different output tubes with one negative supply?

[trentonmichael]

Been wondering about running two differnt tubes (el34 and 6l6) in a push pull config. Would it be possible to run the 6l6s via cathode bias at 10v, ground the el34 cathodes, and run a common bias signal (~ -38v) to the grids? I have a very limited understanding of cathode biased pushpull stages, but it seems like this should be doable

[Jazz P Bass]

Are you trying to come up with two separate circuits?
One for 6L6, cathode biased.
And one for EL34, negative grid bias.
Switchable, I would assume.
Sounds doable.
(Didn't Mesa or somebody do this?)
The particulars I do not have.

[jmaf]

Mesa calls it simulclass and you can either use all 4 of the same tube or mix 2 and 2. The Mark II sounds good, though IMO it's more about Mesas good quality components and build rather than the frankenstein output stage. The dovetail joints on all Mark II's I've seen seem to get loose, and the tubes too close to the massive speaker is an issue too. So they have this ultra marvelous super patented simulclass thing but screwed up on the very basics(if you need a cooling fan on a 100Watt amp, then you made it too small).

About mixing tubes, I usually ask this: if you can convince me about the why's then I can try and help you with the how's.

[J M Fahey]

About mixing tubes, I usually ask this: if you can convince me about the why's then I can try and help you with the how's.

I loved this phrase.

Applies to many other things too.

[Chuck H]

Would it be that hard to build two -V supplies??? You should be able to do two variable supplies from the same source with no interaction as long as they're each coupled with a seperate diode, right?

If running dissimilar power tubes is the goal I would think one simple circuit is a small consequense

[mooreamps]

I seriously doubt you people would have been this kind to me, if I wrote a post like this....

I've considered the concept myself ; one tube running Class A while the other tube running Class A/B... Nothing else in the amplifier would change, and when driven hard, I'd think it would sound "smoking hot"....

-g

[Chuck H]

Only one way to find out. One tube would be conducting on one half of the wave form at a lower output and both tubes would conduct during the other half with the AB1 biased tube having higher output. It could be a great overdrive experiment but I don't think it'll do much for clean tones.

[mooreamps]

Now I remember why I threw this one away... It causes a dc imbalance in the primary coil on the output transformer...
Maybe the O/T could handle it, maybe not.... At the time, I just let it go...

-g

[Steve Conner]

I assumed he meant two pairs of output tubes. Not a PP output stage with an EL34 on one side and a 6L6 on the other. Although I have tried that!

I suspect some DC imbalance will happen even with perfectly matched power tubes, if the PI or preamp clips asymmetrically, switching the two output tubes with different duty cycles. Using grossly mismatched tubes will magnify the imbalance.

This is the only explanation I can think of as to how "OT saturation" can be a factor in the tone of PP amps. In some earlier experiments I found that the guitar signal itself is incapable of saturating the OT, because it doesn't contain low enough frequencies. (That low E is 84Hz, and the PP OTs I tried would go down to 50Hz before showing obvious distress.) So that only leaves the DC imbalance from asymmetrical clipping.

[mooreamps]

No, just "two" tubes.... Although, the 2 pair of output tubes would be the hot set-up...
and even in my push-pull power amps, I just don't worry that much about "exact" dc balance...
Guess I've been reading too many of Kevin's books, I don't know..............

-g

[jmaf]

I loved this phrase.

Applies to many other things too.

It took me a while to learn that if someone walks in asking how to combine different output tubes, you can't just jump in and try to solve the transconductance and bias....you gotta take a step back and ask: why do you want to do this?

The answer will usually be: because I want to sound like Van Halen and Led Zeppelin. At the same time. And then you've solved the problem without taking out a single screw. You can't sound like Van Halen and Jimmy Page at the same time.

How do different pentodes and tetrodes respond to signals....I know they're different....so even if you regulate them perfectly for quiescent current, all are biased fine, soon as a signal comes in, the output transformer will get X current from the tetrodes and Y current from the pentodes.... I have no idea what this will do exactly, it's an interesting experiment, but not for the reasons brought forth(Metallica + Jimi, same amp, same time).

[trentonmichael]

apologies for not being more specific, it would be a push pull output stage. the schematic is just one side. the value of R is currently unknown, but i want to set this to raise the cathode (for the 6L6s only) to about 10v above ground. then use the negative supply to pull both grids (EL34 and 6L6) down to -38. would i be correct in assuming this would make the grid-cathode voltage on the EL34s -38v, and the grid-cathode voltage on the 6L6 to be -48v? or would the introduction of the negative supply throw off the the cathode bias for the 6L6?

[Chuck H]

What you propose will probably work. Though it makes for tricky biasing depending on how ideal you want to get, which really isn't a big deal WRT guitar amps. One hangup is that once you find a cathode resistor/-V ratio that seems to work for both tube types you may well need to redo the process each time you change tubes. There may also be some anamolies with bias shift due to the cathode bias on the 6L6 that doesn't occur in the EL34 making one bias or the other less than ideal at some operating point. Though this could have tonal benefits as well. You could just build an extra bias supply and be done with it.

[mooreamps]

apologies for not being more specific, it would be a push pull output stage. the schematic is just one side. the value of R is currently unknown, but i want to set this to raise the cathode (for the 6L6s only) to about 10v above ground. then use the negative supply to pull both grids (EL34 and 6L6) down to -38. would i be correct in assuming this would make the grid-cathode voltage on the EL34s -38v, and the grid-cathode voltage on the 6L6 to be -48v? or would the introduction of the negative supply throw off the the cathode bias for the 6L6?

I'm not seeing this as being drawn as a push-pull output stage.. I'm seeing this as two tubes in parallel single-ended biased class A/B..

-g

[trentonmichael]

like i said, its only one side of the push pull stage. no need to to draw the same thing twice.

[mooreamps]

I stand corrected...

-g

[J M Fahey]

On behalf of mooreamps, I think the confusion comes , not from drawing two tubes on one side of the PP only, but because of the "transformer * drawing shown.
It does not show a center tapped primary (which automatically means push-pull) nor the bottom end is labelled "C.T.".
As shown, a single unlabelled winding , usually means a single ended output transformer primary.
So the confusion is possible (at least, visually).
jm2c