Mark V Issue - Need Help!!

Hi Guys, I need some trouble shooting advice.

I had a power tube short out, and a fuse blew on my MKV. I bought 4 new 6l6s, a new fuse(4A SB), installed all, hit the bias switch to 6l6. Fired it up and it sounded great for about 45 mins, then I heard a low hum, then the fuse blew again and she powered down.

Any thoughts on what I should look into? Rectifier tube?

It is #191, so it is pretty old. Do you think it is ready for a normal checkup at a tech anyway after 8 years?

Last time I bought 4 new 6L6s one was bad and did the same thing. I sent an email to Mesa Hollywood where I got 'em, they agreed it was one bad tube out of one of the pairs, and they promptly sent me a new pair. Unfortunately, new tubes can be bad out of the box.

I did a tube test with Rich at Mesa on the phone. One by one, all tubes seemed to work. Checked the rectifier tube in CH1 45W mode/tube rect and everything was good. Played for about 20 mins, and heard a volume drop, so I quickly turned it off. This time, the fuse did not burn out. I could smell some burning, but was not sure if that was the "new tube smell". No signs of burn in any socket, no visual/ finger touch arching, so...???

Getting concerned.....

Once you see smoke,,,,, you will need to get repair. If it smells really bad, like a burned clutch on a car (usually worse as it is phenolic resin burning which is not a pleasant smell) it could be one of the carbon comp screen resistors (may be on one of the center tubes). I think once the screen resistor goes, the tube will draw more current and thus burn the fuse.

I had lost one of the screen resistors and when that failed, I could not get the amp to operate any more. Sometimes the Rectifier tube can be problematic and may not always have a hard short. I had one that gave me grief until I replaced it. Note that the Rectifier tube is always active even at 90W setting but is paralleled by two silicon diodes wired in series. If the Rectifier is failing it will pull the power supply down to the 6V heater voltage. If you can't get the amp to power up again, set the amp to 90W and remove the Rectifier tube before turning on, just for diagnosis purpose only. I have also done similar things with the power tubes to determine which one is at fault. It is not common to have a component failure but it can happen.