MT15 5881 Can I swap in 6L6GC's?


New Member
Nov 17, 2022
I have my buddy's MT15 that I have been checking out and love it. I am considering ordering one. However, the new ones come with the 5881's. I am wondering if i for some reason do not care for the 5881's can I swap in 6L6GC's in it or is there more to it than a simple tube swap?

Thanks in advance and sorry if its been asked 100 times. I searched but didnt see that anwered clearly.
Yes, you’ll just need to bias with a multimeter. Very easy.

There have been a few small changes/revisions to the MT15 circuit, but I don’t believe any that change the ability of the power section to run 6L6 or 5881. Mostly just tweaks to reduce noise, etc.
Good rule is to bias anytime you replace power tubes. General rule is you can use a 6L6 in place of a 5881 but not always the other way round. A true 5881 {like the Tung-Sol used in the HDRX-20 dissipates a max of about 23watts while a 6L6 dissipates at least a max of 25watts. A 6L6GC dissipates about 30watts. There are exceptions to both types. Also, amps are usually biased from 50% to 70% of the max dissipation. So, if your RCA NOS "blackplate" 6L6GC's are biased to 70% {pretty hot}, and you throw a set of 23watt 5881's in, they may red-plate or not last very long without a re-bias. There are some mil-spec 5881's that can handle higher plate voltages very well like the 5881/6L6WGC, the 5881WXT+, the 6p3s-e, and some NOS 5881's. It depends a lot on the amp design and particular voltages/power supply/ tranny used.