One may think. It seems to do it with the Mesa tubes. Amp was working just fine with the old Beijing 1990 Mesa tubes. Since I was considering getting rid of the Mark V, did not want to leave in the good tubes so I put in Mesa stock tubes. I believe I have a bad tube, V4, V5 or V6. I just did not care. It was enough excuse for me to get it out of sight. I have spent too much time on tube rolling, messing around with the tone controls, changing speakers, and such that I ever did playing the guitar through it. It is one amp I cannot stand to listen too. It still has value so it will not hit the trash just yet. If I can get it to behave properly, I will sell it. For now, it will sit out of sight as I will want to try to do something with it. What a waste of money. Wish I never bought it. Trying not to go too negative on this, some people may cherish the Mark V90. Good for them. They can have mine.
As for motor-boating. The JP2C would do that with Tung Sol, Mullard , EHX, Svetlana. Medium plate design (12AX7, CV4004). Even the long plate Mullard was worse. I have not pinpointed which tube position causes this issue. It would not be hard to just roll one of the said instigators into each position to find out. It sounds just like a boat breaking over shallow waves. Not only does it oscillate, it grows exponentially louder and louder until you have to hit the standby switch. Amp works just fine with the stock tubes. Just does not behave well with the New Sensor stuff. Matters not of the tube is used and in good condition or brand new. Trying to think if it did this on clean as well. doubt it.
As for motor-boating. The JP2C would do that with Tung Sol, Mullard , EHX, Svetlana. Medium plate design (12AX7, CV4004). Even the long plate Mullard was worse. I have not pinpointed which tube position causes this issue. It would not be hard to just roll one of the said instigators into each position to find out. It sounds just like a boat breaking over shallow waves. Not only does it oscillate, it grows exponentially louder and louder until you have to hit the standby switch. Amp works just fine with the stock tubes. Just does not behave well with the New Sensor stuff. Matters not of the tube is used and in good condition or brand new. Trying to think if it did this on clean as well. doubt it.