Mixing power tube types?

[demonstratedspatulas]

Is it possible to mix power tube pairs in parallel? (Class AB push pull)

Ie, a pair of 6L6s and a pair of EL34s? Assuming separate biases per pair of course.

Assuming also a 2.2k 100 watt OT. (Would you recommend anything different here?)

Would it be possible also to throw 6550s and KT88s in the mix, assuming they are biased cold enough so that the 100 watt spec on the OT is not exceeded? (ie, a pair of EL34s and a pair of 6550s, EL34s are biased as normal, but the 6550s are biased so they don't exceed 50 watts per pair at full bore)

I really like the sound of both EL34s and 6L6s in their own way, I'm really curious if they can be mixed to give a different sound.

[xtian]

You might look at Egnater schematics. They have one or more amps that do this exactly, with a front panel pot to mix between the two pairs of power tubes.

I have not been impressed by any sonic benefit to this scheme, but that's my subjective opinion.

[bal704]

I recently built a push-pull amp with one 6V6 and one EL84. Not exactly the same thing, but it's a data point. Sounds great BTW...

[Guy77]

I have tried this with 1 6L6 and 1 EL34 in a JM style amp and it was interesting at first but then realised I preferred all 6L6 or all El34.
If I had the ability to blend the amount of 6L6 or EL34 I may have kept it in the amp. It was certainly an interesting sound and I did enjoy it for a while. Please post your progress if you go ahead with it!

Cheers

Guy

[roberto]

Mesa Simulclass sounds great.
Egnater and Brunetti pirata sound quite good as well.
You can definitely do it, and you could like it.

[demonstratedspatulas]

I recently built a push-pull amp with one 6V6 and one EL84. Not exactly the same thing, but it's a data point. Sounds great BTW...

One 6V6 pair and a one EL84 pair? Or the "push" is a single 6V6 tube and the "pull" is a single EL84 tube? Either way sounds interesting. Have any clips?

[Roe]

kt66s and el34s can work together, given separate bias circuits of course

[roberto]

The attached file is an old "cheese-paper" design I did for a guy on an italian forum who wanted to have a warm amp with the possibility to boost the volume to get some more clean range.
It has a 6V6 in SE working all the time, plus a master volume for an additional 6L6GC in PSE.

Fender Chamboost Robi 2017.jpg

[bal704]

It's one of each. Cathode Biased. I'm in the middle of a move right now, so no clips. It's an octal plexi, so it's kind of a different cat.

I recently built a push-pull amp with one 6V6 and one EL84. Not exactly the same thing, but it's a data point. Sounds great BTW...

One 6V6 pair and a one EL84 pair? Or the "push" is a single 6V6 tube and the "pull" is a single EL84 tube? Either way sounds interesting. Have any clips?

[demonstratedspatulas]

You might look at Egnater schematics. They have one or more amps that do this exactly, with a front panel pot to mix between the two pairs of power tubes.

I have not been impressed by any sonic benefit to this scheme, but that's my subjective opinion.

Thank you! I looked this over, and it looks exactly like what I'm trying to do, basically just two pairs connected in parallel.

A blend control is an interesting idea, but for now I'm happy with just a parallel connection.

Should I try something different for the OT? I'm used to using 2.2k for 4x6L6s and just have never had a chance to experiment with anything else, and not sure how the tone would be affected if I went slightly higher or lower. But that question could be its own thread.

[demonstratedspatulas]

It's one of each. Cathode Biased. I'm in the middle of a move right now, so no clips. It's an octal plexi, so it's kind of a different cat.

I recently built a push-pull amp with one 6V6 and one EL84. Not exactly the same thing, but it's a data point. Sounds great BTW...

One 6V6 pair and a one EL84 pair? Or the "push" is a single 6V6 tube and the "pull" is a single EL84 tube? Either way sounds interesting. Have any clips?

Ah gotcha! I have never built a cathode biased amp. Would love to hear some clips after the move

[gingertube]

Yes you can mix up power tubes BUT best done in pairs.
Similarly - I designed and built an amp for a friend which has a quad of 6V6, each pair has a triode/pentode mode switch and a cathode/fixed bias switch.
He routinely runs the amp with one 6V6 pair in pentode mode fixed bias and the other pair in triode mode cathode bias.
Cheers
Ian

[dorrisant]

Just sayin', install KT66s in both sockets... enjoy!

This setup seemed to be the best of both worlds, in my non-expansive testing anyway.

[JD0x0]

I would think it's doable even without separate bias supplies, if you're willing to compromise slightly and do it in pairs. Choose an OT load that will work for both tube types. Plate and screen voltages that work safely for both tubes, then, bias to the lower dissipation tube. As long as not using tubes with massively different max dissipations, you should be able to get both tube types in a safe bias range.
@ 450V, 36mA will give an EL34 65% dissipation, and a 6L6GC will be running around 54% dissipation if they can equally pull the same 36mA. 54% is still considered 'blackface spec' for 6L6GC.
EL34 application data shows 5k OT primary with two tubes @ 450V plate, and 6L6GC data shows a 5.6k load for that same plate voltage, so the primary load shouldn't be much of an issue in that case. Plus, it's a guitar amp, so you have some wiggle room since minimum THD and max output power is not usually the goal. If you're using 4x tubes, you obviously need to change your total load, though.

Separate fixed bias supplies would definitely be a safer bet and a bit more versatile, but it's more parts to deal with, too.

[demonstratedspatulas]

I would think it's doable even without separate bias supplies, if you're willing to compromise slightly and do it in pairs. Choose an OT load that will work for both tube types. Plate and screen voltages that work safely for both tubes, then, bias to the lower dissipation tube. As long as not using tubes with massively different max dissipations, you should be able to get both tube types in a safe bias range.
@ 450V, 36mA will give an EL34 65% dissipation, and a 6L6GC will be running around 54% dissipation if they can equally pull the same 36mA. 54% is still considered 'blackface spec' for 6L6GC.
EL34 application data shows 5k OT primary with two tubes @ 450V plate, and 6L6GC data shows a 5.6k load for that same plate voltage, so the primary load shouldn't be much of an issue in that case. Plus, it's a guitar amp, so you have some wiggle room since minimum THD and max output power is not usually the goal. If you're using 4x tubes, you obviously need to change your total load, though.

Separate fixed bias supplies would definitely be a safer bet and a bit more versatile, but it's more parts to deal with, too.

Thank you! This makes a lot of sense also! My end goal here is to try different tube combinations, but also different bias points for each, and perhaps try recording and see if I can A/B the differences. Wouldn't be done for a few months, but if I manage to accomplish this I will definitely post some clips