A friend of mine recently bought a Mark V combo a few months back. It came stock with 4 6L6 power tubes. He decided to switch out those tubes for EL34's, so he went out and bought two matched pairs. And to no avail, hes not getting any sound. So he put the 6L6's back in and the amp works fine. The Mark V comes with a self-biasing switch on the back of the amp specifically for swapping 6L6's to EL34's and vice versa. Change the tubes, flick the switch to EL34, and boom, your there, right? Are there any other steps in this process? Does he need 4 matched EL34's? What is it that hes doing wrong? He always comes to me for these answers, but Im stuck on this one.
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Not really sure what is up.

I'm also not an amp tech nor super familiar with the Mark V. Off the top of my head the only thing I can think of is that the 4 tubes are not closely matched enough but my common sense tells me it should still work (but maybe not sound that good).

What does the manual say? Has he tried running on only 1 pair (2 tubes)? Mesa likes you to buy their tubes because they come to you already matched up at a certain bias level for their amps and are ready to go. I would find out exactly what that bias range is and check the tubes for exactly what they are. If they are not right, ask to exchange them for the correct values. BTW - tubes don't have a bias, amps do. But tubes are checked to see how 'hot' they are and then grouped or 'matched up' accordingly.
Last edited by 311ZOSOVHJH at Mar 30, 2011,
If there's no sound at all, the only thing I can think of is that he didn't put the tubes all the way in. Doesn't make a whole lot of common sense though.
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Did he get the tubes in Mesa's bias range? They have to be a matched pair but they also have to be in Mesa's bias range. And is he installing them correctly. In other words is he putting them in a YXXY configuration.
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i am pretty sure it is heavily advised to buy a quartet instead of two pairs, but if the two pairs are rated similar or the same it wouldn't make a difference.

however i doubt that that would cause no sound to come out of the amp.