NAD, but we have a problem. HELP!!!!

Got a Mesa F-50 from a TGP'er today. Arrived in good shape! Put the tubes in and fired her up and strummed a few chords. YIKES!!!!! Sounded painfully thin to me. I took out my bias probe and got 31Ma on V5 and 0 Ma on V6. switched tubes, changes tubes and got the same result. V5 get as hot as expected while V6 lights up, but barely gets warm.So, I presume I have a problem. I've contacted the seller and do not anticipate a problem, but I want the amp and would like to get it repaired.

Any ideas what the problem may be???

Thanks in advance,
Ron

Could be a lot of things. Shorted cap turning off bias on the tube. Bad output tranny (half open). Bad PI. Failed screen or grid resistor.

I'd start by looking at the bias voltages of both V5 and V6 to see if they are all the same. Check resistance of the output tranny, though the plate voltage will tell you if it's open.