Mixing tubes - 6V6 & EL-84

[Curbdog]

Any reason why I can't modify the output section of my 2 - 6V6 amplifer project (an Express) to add another pair of tubes, as cathode-biased EL-84s? What if any concerns are there in having both a fixed-bias push-pull pair and a cathode biased push-pull pair? Thanks.

[benoit]

The only thing I can think of off the top of my head is you should make sure the 6.3v winding on your PT can supply the required extra current.

Ben

[Phil_S]

Are you intending to run both pairs at the same time? I can't fathom how you are going to tap the PI to feed the cathode biased pair, as the fixed bias feed rides that grid input. I'm not real technical, but it seems to me you've got to split the signal first to two separate phase inverters. That would be a show stopper for me. I suppose you could use one tube and build a pair of cathodyne splitters, but I don't know.

If you intend to run them one pair at a time, you'll need some switching to make it work, shut off the bias feed, ground the cathode resistor, put the desired pair into the circuit.

To me, this sounds like too much work for too little reward.

[Phil_S]

Are you intending to run both pairs at the same time? I can't fathom how you are going to tap the PI to feed the cathode biased pair, as the fixed bias feed rides that grid input. I'm not real technical, but it seems to me you've got to split the signal first to two separate phase inverters. That would be a show stopper for me. I suppose you could use one tube and build a pair of cathodyne splitters, but I don't know.

If you intend to run them one pair at a time, you'll need some switching to make it work, shut off the bias feed, ground the cathode resistor, put the desired pair into the circuit.

To me, this sounds like too much work for too little reward.

[j-po]

Just put two pairs of coupling capacitors on the PI and run each pair to its own bias supply. Might sound nice with the EL84 clipping a lot while the 6V6 less.

[Andy Le Blanc]

Two pairs of couple caps means two pairs of grid resistors, which will change the gain of the inverter.
Going back and forth could be done with a cathode switch.

[Zippy]

http://www.schematicheaven.com/boogieam ... eangel.pdf

[Structo]

The new Egnator Rebel 20 does just that.

It has a sweep control that you can blend the two power tubes at any point or all 6V6 or all EL84.

[stoo]

Just put two pairs of coupling capacitors on the PI and run each pair to its own bias supply. Might sound nice with the EL84 clipping a lot while the 6V6 less.

+1
Tons of amps have 4 o/p tubes.
Stew

[Ron Worley]

I would love to know how Bruce mixes the 2 tube sets on the Rebel 20....

It would be cool to see what that would do with an Express topology.

Ron