I have Mesa Mark III which have two EL 34 and two 6L6 tubes. I have questions for you:

1. Can I use EL34 tubs in all 4 sockets? I'm reading in "OPERATING INSTRUCTIONS" that "the inner tubes
must be 6L6's while the outer tubes must be EL-34's."
2. Can I remove two 6L6 inner tubes and play using only two outer EL 34 tubes in Class A mode?
You may get better answers on the Mesa board. There is a ton of technical info over there.
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You CANNOT use all EL34s. Only the two outer tubes can be EL34s. I'm not certain on the other question, but I'd be very cautious. I seem to remember from my Mark IV that was a no-no, but I'm not 100%
I'm just a kickin' and a gougin' in the mud and the blood and the beer.