Output tube blending

[MarshallPlexi]

Question: Anyone know how to blend two different push pull pairs in one amp using one output transformer?

As best as I can think of, the best way to do this would be with a 4 ganged pot and take the grid voltage from the phase inverter and blend it between the two output stages. It would take 4 pots to keep the each signal out of phase.

It seems like this setup would ground the unused grid of each output tube section with the control was fully rotated to select the other set of tubes.

Basically would it look like this but with another pot just like this stacked on top of this pot?



If any of you guys have experience with this, please let me know.

Thanks!

[tboy]

Considering the possibility of actually finding a 4-gang pot, I'd recommend driving each pair with it's own phase inverter.

[MarshallPlexi]

I think you're right. This is actually the best of all worlds since you could then tweak each PI for the tone you are looking for. Very cool idea!

[defaced]

I run mixed pairs by giving each tube it's own coupling cap after the PI and each pair has a separate bias supply.

[MarshallPlexi]

Okay bear with me on this. So we have two phase inverters. One for each power section; say in this instance, 4x EL84's (27W Class A - Cathode Biased) and 2x EL34's (50W Class A/B fixed bias).

In order for this to work, the OT would have to have a center tap B+ that is common to all tubes and have taps for each set of tubes. Something like:

6.6k---3.4k---0---3.4k---6.6k

It seems like this would work great using one OT.

To take the idea to it's fullest, you could have 2 OT's with each phase inverter feeding it's respective power section and ultimately it's own OT. This would be cool since each power section would be separate from each other and tweaking of each filtering supply could be done to tailor the sound of each section. I would imagine that the OT's secondaries could then be run in parallel much like the JTM100. This would mean that each OT would have to have a secondary of 8-16-32 in order to achieve 4-8-16ohm operation once the OT's secondaries are paralleled. This would give a total output power of 77W when both sections are used. The blend control would essentially be able to select between 27W at it's lowest to 38W in the middle and 50W at the highest.

Does this make sense? I'm just shooting off the top of my head so if I'm missing something please chime in.

Thanks for the good idea!

[defaced]

Doesn't Mesa have a patent on a setup that mixes Class A and Class AB? Simulcalss? Check out one of their circuits.

Here's a schem of what I talked about above:

[MarshallPlexi]

Any thoughts on the blend pot for the preamp that will go to both phase inverters?