From what I understand its ok to run just 2 tubes in class A. I had 1 tube go bad so I put 2 tubes on the ends . The 1 that went bad was on the left end. This should work till I get my new tubes. Which will be in about a week or so. And they are the same numbered tubes too.
Another question would be did the wattage drop using this method..
z28lt16, I apologize for possibly misunderstanding what your current tube situation is...from what I understand here, one of the end (far right or left) output tubes blew. It's still in there and you are still using the amp.
With that being the case, you want to run the Mark IV in Simul-class in tweed mode. Running the amp in Class A, you are essentially running the outside tubes at Class A voltages...thus disengaging the inside power tubes. This amp design falls into the Push-Pull category of amps. If you are using just the outside two, and one of them is blown or bad, you
will stress the amp to a point of component failure. This is a costly mistake to allow to happen...that is unless you like your local Mesa tech an awful lot :shock: Seriously, you can damage the amp running the amp this way. If you
must use the amp until new tubes arrive, do so in Simul-class. This will relieve some of the amp's stress...just not all.
Regarding the lower output, that's because you don't have the complete circuit of the "push-pull" happening...it's just doing one side of that in that section of the amp.
Running in "Tweed" mode will also drop certain voltages within the amp. This will also provide some relief to an improperly operating amp.