I suspected problems with the tubes. I bought the MkIII with these tubes so I have no information about the history.
I let the tubes test on a Amplitrex AT1000. One set about 84% gain, the other set 91%. They seem OK.
I measured all the components and after that I decided to replace all components, also the can capacitor.
Slowly raise the voltage with a variac. With some music on it, again a fuse blew out.
The tubes of this amp I put aside. With the tubes of the good block I did measurements; both amps measure the same.
Now the 'bad' tubes in the good amp and again the fuse blew out. So now I'm sure one of the 6550 is bad though it measured OK on the Amplitrex.
Looking for other tubes, I found the Tungsol KT120 from NewSensor. Mr. Bob Latino wrote that the bias current has to be the same value as for the KT88.
He also wrote that it sound more 'tubier' than a KT88 (more mid bass). What is meant with 'tubier'?
Does anyone has experience with the KT120 in a MkIII at this moment? How does it sound?
I ordered the Tungsol KT120 matched quad for $48 each.