MT15 power tube question


New Member
Aug 12, 2022
Looking to pick up a used MT15 locally and its a version 1 with 6L6 tubes. I know there was a switch to 5881 at some point but I'm wondering:
-Is it the same amp apart from the new tubes, or did they tweak the guts of the amp for version 2?
-If I throw 5881s in there what is the bias value I need? I know the manual says 30 but is that for 6L6 or both 6L6 and 5881?

There have been some changes to the circuit along the the way, but not for the purpose of accommodating the different power tubes I believe, you should be able to swap out the 6L6's for 5881's with a bias adjustment.

That being said, and especially given the limited tube supply right now, I wouldn't worry about swapping them, I doubt you'll hear much difference. I have one with each and can't tell.
If it sounds and feels good when you play it I would leave it alone and keep it as is until you have to change tubes.
With the power tube swap you "change" the slight final tone nuance the amp can offer, for example my original 6L6GCs are sort of more "ruff/visceral/growly" sounding, and when I put in the TAD 6L6GCs (different/taller tube with the getter on the top, Rubys have it on the side) it becomes grainier/tighter, kinda bit more "British" if that is a thing. The difference is not night and day, but enough that you can feel the difference when you play. The 5881s are said to be a bit more powerful than the 6L6s, have not tried them yet, as I am quite satisfied with what I have making my sound. But the fun part is, if you have a bit of time&money and a purpose behind the act, you can really explore the sounds with swapping preamp and power tubes ... there are many, many, many, many possibilities as Commandant Lassard would put it ;)
If we're talking about 6L6GCs, then it's the other way round. 5881s break up earlier and produce a bit less power.
Thanks for the correction, I think I read somewhere that they have a slightly higher wattage and break up later. Well, there you go, when learning from the internet and not checking facts.
Yes, the 5881 gives around 15-20 percent less output at full welly than a properly driven 6L6GC.

The 5881's alter-ego (i.e. same spec) is a 6L6WGB, just to further confuse matters.