6V6 & 6L6 at the same time?

Howdy. I've decided I want to try 6V6 tubes in my Mesa Heartbreaker. I always run the amp at 1/2 power which means only the outside 2 of the 4 tubes are engaged. My question is: Can I swap just those 2 tubes and leave the others in place? Or is there some danger in the mismatch?

Mesa is open MOnday through Thursdays....and they are very good at answering questions like this.
IF the voltages are acceptable and if the biasing would allow 6V6's to run there, then why not put in 4 6V6's and have the amp set up to run 6V6's?
That way, you would not have to worry that someone would switch the amp to 4 tube operation with some mismatched tubes and biasing in there.

Thank you for your response, Wally. That totally makes sense. I am just hoping to cheap things out to hear the difference by buying just the 2 tubes. The goal is to run the amp at 20 watts with the 6v6 to see if I like it. Guess I should call Mesa.

Glen, Mike Bendenelli and the other folks at MEsa have always been very helpful when I call them. IF I had more experience with that amp, I would have more thoughts on it, but I don't know what kind of plate voltage is being run or how they set up the bias. IN addition, there is the impedance matching thing. Mesa obviously has designed the amp to run a mismatch when the player opts for two power tubes versus 4, right? When you change to 6V6's, you are introducing another aspect of a mismatch there.

I can't comment specifically on this amp... It is possible to run both 6l6 and 6v6 tubes together in the same amp, though. I've done it in a twin.

However, a number of conditions have to exist, though... You'd need a plate voltage that 6v6 tubes can handle (not always the case with 6l6 amps). JJ 6v6 handle higher voltages, though, which gives a bit more room for what you can throw at them.

Next, you'd need 2 separate bias supplies for the 2 different pairs of tubes (or different cathode resistors if self-biased). Most amps have 1 pot, or in Mesa's case, none! In a fixed bias amp, you'd need to ensure the bias current is set correctly for the tube mismatch, and most amps aren't set up for this. When I ran 6v6 & 6l6 together in a twin, I had to redesign the bias supply for this purpose and add a second bias pot.

After that, there's still a problem, tonally, in that when all 4 are engaged, the 6l6 tubes, with their higher output, mainly swamp out the sound of the 6v6 tubes... though they do start breaking up earlier than the 6l6 tubes, which gets some interesting sounds when you really push the amp as one pair crunches while the other delivers big clean bottom end... Ideally, though, it would be great to have a blend/mix knob installed to decide how much of the signal goes to each tube - like what Egnater is doing with their rebel 30...

With a pair of 6l6 and a pair of 6v6, you are looking at only a very slight impendance mismatch, so I wouldn't worry about that. A quad of 6l6 looks for something around 2k primary impedance, where a pair of 6L6 and a pair of 6v6 would be looking for around 2.4K - so not a big deal. And when just running the 6v6 tubes, you'd have a bigger mismatch - 6.6k vs the likely 2k-ish OT, but the OT in a 100watt amp isn't really going to be bothered by any amount of strain from a single pair of 6v6 tubes. So nothing will go up in smoke or anything horrible... you just might not really care for the sound with that big a mismatch.

So, what I'm basically saying is that it may be possible, but it's not necessarily easy, nor will it necessary bring you particularly enjoyable results.

Why not do a pentode/triode switch? It would be a lot easier & pretty much halves the output power. Also not too bad of an impedance mismatch.

Thank you for the input, gentlemen. I'll be calling Mesa on Monday. Meanwhile, I have some googling to do as most of this is way over my head technically.

I do know from the amps manual that the bias is fixed and the amp can only take the 6v6 in "tweed" mode, which is what I use anyways. I'm just hoping I can just swap the two tubes that are used in half power mode while leaving the 6l6 s that are idle in place.

I'll post Mesa's response when I get it.

LEt us know what you find out, Glen, if you would. IF the manual says that you can use 6V6's in the tweed mode,then I am going to think that in that 'tweed' mode the voltages are reduced. I am also going to think that as long as you are the only person playing through and/or 'messing' with that amp,that it woudl be fine to leave the 6L6's in place...as long as they are in idle or tweed mode. Don't take that to the bank, though.