blending output tubes

[selloutrr]

I'm working on an amp and want to run a pair of EL34's and a Pair of EL84's.
I'm willing to settle with a switch to select

EL34's
Both
EL84's

but ideal would be a bend knob so they can be mixed. live and not need to turn the standby on to select tubes.
Far left - EL34
Middle - Both mixed
Far Right - EL84

Would I be correct in thinking I can run the ground thru a dual potentiometer similar to cutting a 100W 4 tube amp to 50watts with a SPST lifting ground.
or is their a better way?

[Zippy]

Have you considered throttling the inputs to each pair of tubes?

[xtian]

Egnater does this, in his Rebel amps. But I can't find any schematics.

[M Fowler]

Egnator Rebel 30 uses a guad ganged pot 500Kb for the mix control and the blend control is a 25kb pot with a 33k 2-3watt resistor across it.

[Colossal]

Egnator Rebel 30 uses a guad ganged pot 500Kb for the mix control

The quad ganged pot is also tapped! Definitely a custom made pot.

[selloutrr]

Good to know I'm not missing something I couldn't for the life think of how to make it work with over the counter parts that would make it work with only one knob.

I guess the switch of EL34 / El84 /or both would be the way to go.

[TheGimp]

Control the screen voltage with a variable supply. single knob controls all the screens with the tube pairs controlled in opposite directions. Increase one set of screens while decreasing the other set.

[Structo]

When I have read about the Rebel amps, most owners say that there is not a real apparent difference in the tone of the two types of tubes.
The breakup range or headroom changes a bit but you don't hear a tonal difference as you would expect.

[sliberty]

I just recently played a Rebel 30 for a while in GC, and that is exactly what I experienced. Very little difference between the two types of tubes. One interesting thing - the amp was noticably louder when the blend knob was set in the middle (both tube types at 50%). Thinking about it, this is not that surprising, but I didn't expect it. At either end of the spectrum, the volume was lower than in the middle.

While I can see the marketing allure of this dual tube arrangement, after playing it, I didn't much care for the feature. It just didn't add that much to the functionality of the amp. And more than likely, since the middle setting was louder, I'd end up leaving it there all of the time. So in the end, it was still effectively a "one tone amp: (I don't mean that literally, just in terms of the output tubes).

[M Fowler]

I actually prefer to set the mix control at slightly greater to the 6V6 side.

The blending with more to the 6V6 sounded better to my ears.

[sliberty]

You must have much better ears than I do. I couldn't hear much diff.

[Merlinb]

Good to know I'm not missing something I couldn't for the life think of how to make it work with over the counter parts that would make it work with only one knob.

You could do it with a dual-gang pot, but it's not perfect.

The ends of one pot would connect to one EL34 and one EL84. Individual grid leaks would be necessary, and you'd want them to be small-ish compared with the pot. The wiper would then go to the driver stage.
The other pot would be wired the same, but to the other pair of tubes.

The reason that it's not perfect is that you won't get 100% pure 34/84 at each end of travel. The other reason is that in the centre you would have a lot of resistance in series with each grid, which might cause some treble loss.

[selloutrr]

i'm curious if thier would be a more distinct difference in tone with hard switching in comparision to blending the tubes?

I'm not one bit surprised about the volume increase with the blend in the middle. thought i'm a little surprised that wasn't compensated for so you aren't naturally inclined to go for a 50/50 blend.

i wouldn't expect a huge tonal change from el34 to el84 they are both known for a british tone. I'm sure with NOS tubes the tone is probably more striking then with current production OEMs

[M Fowler]

The Rebel is actually very trebly so Merlin's option might just be the ticket.

Mark

[Kagliostro]

I've in my drawer the project for an amp I want to build with 2 x el84 and 2 x 6v6

at the moment I haven't here a right schematic

so don't look to the cathode switch that is wrong

but only look to the doubled PPIMV (2 ganged pots for Master Volume)

Kagliostro

p.s.: I added a more readable picture