Blending Power Tubes

[Structo]

Anybody have any idea how the Egnater Rebel amps blend two pairs of power tubes?

They have two EL84's and two 6V6's and have a pot to blend them or choose all of one kind.

[rfgordon]

I've wondered about that myself.

What if you used a dual ganged 10k pot with opposing tapers, and connected each taper in series to the bottom of the cathode of each pair of pwer tubes? As the pot is turned in one direction, one pair is gradually biased almost off, and the other full on. I think the tapers would have to be just right to work.

Any other ideas?

[Colossal]

Yes, the tube blend circuit is interesting. The PI has two sets of coupling caps to output to each pair of tubes (6V6 and EL84). The blend circuit is mounted on its own board and uses a 500kB quad ganged and tapped pot with some other resistors and caps working in conjunction. This is sandwiched between the PI output coupling caps and the grid leak resistor pairs for each set of power tubes. The EL84 grid leaks are 220k/220k and the 6V6s are 100k/100k and both are running in fixed bias.

Hope this helps.

[ChrisM]

I know the schem is out there. I just can't find it now. There is lots of discussion of how they did the blending.

Apparently at first they basically copied what Mesa does with their blending. Of course Mesa found out, shit a brick, so Egnater had to change. Mesa has it patented iirc.

Like I said I forget what they are doing now but the design doesn't sound too great. There is a fair amount of DC on the blend pot. Very scratchy as you rotate it. From users I have heard that the control isn't very effective either. I think a simple switch to select one or the other would be better.

[Colossal]

The quad gang pot is set up like a balance circuit with opposing tapers and is undoubtedly custom made for the job.

[M Fowler]

The control is very affective and works quite well and like colossal said its a quad gang special pot. I had better put mine back together one of these days it blew a resistor on the bottom board of course.

[Structo]

Hmmm, so it doesn't sound real easy to implement.
Quad ganged pot? With taps?
That does sound like a custom piece.

Maybe I can look at a Mesa schematic to see how his did it.
Anybody know which amp uses that?

I have that little 6V6 amp I built from the Hammond organ amp that I have thought about doing this on.
It has been a great test bed for trying different things that I wouldn't dare try on my better amps.

Mark, do you like that feature of the Rebel?
I have read some conflicting reviews about that.
In a low powered amp are the power tube characteristics that different between the two to hear much of a difference in tone?

[ChrisM]

A simple way to have either 6V6s or EL84s is to hook both up like you normally would. Each tube gets its own grid stopper and screen resistor.

Then use a DPDT switch. The switch breaks the connection to ground on one pair's cathode while making the connection to ground for the other pair. Hardly any current flows through the switch so you don't have to worry about it failing or it's rating.

Its not a blend but it works and will give to distinct sounds. You can even voice each pair even more with different screen and grid resistors.

[Colossal]

Hmmm, so it doesn't sound real easy to implement.
Quad ganged pot? With taps? That does sound like a custom piece.

It's fairly straightforward (and a very cool solution) when you see it and get your brain around it, but no, it is not easy to implement as the pot is custom. The wiper on each pot has two connections complicating matters.

[M Fowler]

Tom I do like the blend feature and find that I set the blend to 50% getting some from each pair. I like the 6V6 mix the best though. The amp can get trebly and some noise but this amp is stuck into a tiny box with three boards and lots of connection.

If you spread the amp out like the TW 17 x 8 x 2 the noise problem would be reduced which isn't really a problem in the Rebel but some complain you know how that goes.

[tjauernig]

If you do it with just a switch, make sure your pt can handle the heater current of both sets of output tubes. You may also have to turn the heaters off to one set of tubes.

[tjauernig]

If you do it with just a switch, make sure your pt can handle the heater current of both sets of output tubes. You may also have to turn the heaters off to one set of tubes.