el34-6l6-switchable power amp

[always-ultra]

Does anybody know of a possibility of building a power amp with the ability to switch between a pair of el34s and a pair of 6l6s?
I mean, one power stage with 2xel34+2x6l6 and one output transformer with two different primaries to match each tube pair? I think of using one tube pair at a time, not both simultaneously.

[Reeltarded]

I bet Mark Fowler has some insight into this. I think he disassembled an Egnater. They have a sweep from one set to the other, you could make that a binary sort of thing instead of variable.

[M Fowler]

Yeah colossal and I reviewed the Rebel 20 closely because it fried a resistor on the bottom board so that meant all three layers of PCBs had to come out so might as well take a good look.

It uses a four gang pot to achieve the balance/mix between in Rebel 20 case two EL8's and two 6V6's.

Another TAG member built an amp like this might need to search for that or maybe he will chime in and link that thread.

Also, a dual OT could be used.

Mark

[John_P_WI]

One easy way to do it, set it up as a 4 tube output stage and lift the cathodes of the tube pair you wish to have turned off. Don't mess with switching the high voltage transformer taps / plates.

[Zippy]

Does anybody know of a possibility of building a power amp with the ability to switch between a pair of el34s and a pair of 6l6s?
I mean, one power stage with 2xel34+2x6l6 and one output transformer with two different primaries to match each tube pair? I think of using one tube pair at a time, not both simultaneously.

Do you hope to switch on the fly? Or would you be happy configuring the sockets so you can switch tubes and rebias?

[always-ultra]

Also, a dual OT could be used.

Thanks so much! I guess thats what i need. Do you know which company produces dual-primary output transformers for guitar amps?

[always-ultra]

Do you hope to switch on the fly? Or would you be happy configuring the sockets so you can switch tubes and rebias?

I hope to be able to switch on the fly. I would like to use the 6l6s for cleans and the el34 for dirty.
Any thoughts will be appreciated

[always-ultra]

One easy way to do it, set it up as a 4 tube output stage and lift the cathodes of the tube pair you wish to have turned off. Don't mess with switching the high voltage transformer taps / plates.

Thanks for your answer! That way they would be switchable. That together with a dual primary output transformer would do the trick, i guess.

[John_P_WI]

You don't even need to worry about the dual primary OT, truly. IIRC, in the dumble files there is an example of a 1/2 power output that you could use for inspiration, with a little re-configuring for either / or set of output tubes.

[Cantplay]

Could you do cathode bias and parallel SET?

John

[Firestorm]

I hope to be able to switch on the fly. I would like to use the 6l6s for cleans and the el34 for dirty.
Any thoughts will be appreciated

Simply switching between 6L6 and EL34 won't likely do enough to get you to clean versus dirty. You'd also need to feed a hotter signal to the 34s in order to push them "over the edge."

There are lots of ways to handle the transformer side of the question: unless you are going for a specific impedance-related sound, both types of tubes will run perfectly well into anything from 4K-6K (depending on plate voltage). Or get a transformer with ultralinear taps and connect one set of tubes to the full winding and the others to the taps (a la Mesa Boogie). Since you will likely need to bias each tube pair independently, you'll need to double up the post-PI connections (four coupling caps, four grid resistors, two (or four) bias adjustment pots): this gives you the opportunity to put separate post-PI masters on each tube pair which would let you dial up the amount each tube type contributes to the output. In that scenario, you could short the master pot wipers for the EL34s to the input end of the pot via footswitch (use a relay) and go to full level on the fly.

[always-ultra]

Thank you so much for all your kind replies! That was a lot of help. This forum is the one
When i have finally built my new amp i will let you know!
Thank you again!