Its always best to bias after a power tube change.

It's possible.

When changing across tube brands, I usually check bias (I usually check anyway).  With an amp like the Overtone, you want the power amp to be dialed in just right.  Yes, the mesa tubes will work, but if not biased correctly they may sound overly hot or cold and that can affect your judgment of them.  I assume you are just testing them, but if you were to leave them in to run overly hot for a long period of time, you'll shorten tube life and put other components in the power amp at risk for heat damage as well.

Now for some unsolicited advice.

The Mesa's are just relabeled Chinese Shuguang 6L6's.  Shuguang is sold over several other brands including, but not limited to, Golden Dragon, Tube Amp Doctor (TAD), Valve Art, Ruby Tubes, Groove Tubes.  So, the exact same tube is available as the TAD 6L6GC-STR, the Ruby Tubes 6L6GCMSTR, and the Groove Tubes 6L6CHP, TAD and Ruby dealers will probably offer the better price, and, of course, GT's will charge up to a 100% premium just for marketing BS. 

All in all, it's an okay tube, always sounded a big "small" to me, I think I prefer the JJ that Nik uses stock a more (it's got better definition), but I really prefer the SED 6L6GC over either, and would be interested in trying the new Tung Sol 6L6STR as well.  Avoid the Sovtek 6L6WXT+, a crappy tube that couldn't handle the voltages in a 40-year-old Supro Thunderbolt. 

All tubes vary greatly from brand to brand, and some even from batch to batch.  What I should have said is I always check bias, though is very possible the tubes will still fall within an acceptable range.  I assume you got the amp sans tubes.  Yes, you should be fine plugging most any 6L6GC tube in there.  Watch it closely, if the tube, especially the plates, seems to be running red (orange is fine), it's biased too hot and you want to turn the bias pot to get it cooled down some.

Haven't tried the EH's, after my experiences with the Sovtek's I never had a desire to, the Tung Sol's are made by the same company and factory and are the first tube from there I've been tempt to try in a while.


You put those in that amp and they will fail and possibly damage other parts with them.  You want a 6L6GC class tube, though some 6L6GB's will work as well (lower output) if they can handle plate voltages up to 475VDC, but a metal tube or a straight NOS 6L6G can't handle the voltages in any amp that wasn't designed for them (which pretty much means don't use them in an amp made after 1948). 


Those are the same as SED, SED used to be called Svetlana (still are in Russia), but somehow, through some loophole, New Sensor (Sovtek/EH) got the rights to the Svetlana brand in North America and the original company had to change their brand to SED, the Winged C logo is their trademark, you'll find it on original, pre-New Senor Svetlana tubes as well.

Next you are going to go to the store and buy a Digital Multimeter (DMM), you don't need an expensive Fluke like mine, they can be had for as little as US$10, though if it looks cheaply made, maybe pick a more robust model.  And we'll talk you through biasing. 

Biasing is easy on the OTS because Nik puts the bias points right in the design on the outside of the chassis, where your risk of being exposed to high voltages is minimized.   We'll assume 450-470VDC plate voltage, since that's what I keep reading that people are measuring...no need to make you dangerously poke around inside the chassis.  You put the tubes in, turn the amp on, and then insert the red probe from the DMM in the red jack beside one of the tubes and put the black probe in the black jack between the tubes (or any ground, but the jack is there..use it), then you measure DC voltage, millivolts actually...if you have to manually set the range on your DMM, make sure its' set low enough.  You want to set the bias pot, right there at power tubes so that your DMM reads around 45 mV DC, then check the other tube, it should also be around 45mV DC, if it's a little off (+/-3 mV DC), then find the setting on the pot where it splits the difference.  If it's way off (+/- 8 mV DC), then your tubes aren't matched well. 

One safety rule, just in case the amp isn't wired correctly, keep one hand in your pocket while measuring bias, until you are sure you are measuring mV and not some errant high voltage.   I'm sure they check this stuff before the amps ship, but it doesn't hurt to be safe with a new amp build.