The roadking Ch3 and 4 uses massive amounts of preamp distortion, so if you're getting clean in a gain channel, the problem is in the preamp and most likely a preamp tube, but there is a small possibility your LDR switching from Ch1/2 to Ch3/4 is faulty...a deeper problem.
100:1 probability it is your preamp tubes, and in spite of you trying a fresh set....a couple of those are probably DOA now, too.
Bottom line is that you cannot use Russian tubes in the cathode follower positiion without them frying nearly instantly, and some Russian tubes also are so weak out of the box (like the Tungsol RI 12ax7) that it doesn't pass enough current to do anything but clean. That's my $200 worth of experience. See below.
FOR FURTHER READING:
Please list the preamp tubes you have been, and are currently using, in order of tube slot position? That will get some objective information on the table.
There's a very expensive learning process everyone seems to go through when using boogies (err....mesa's...boogies didn't have this problem), and that is their cathode follower stage (that drives the marshall style tone stack) eats Russian tubes for breakfast....0.1 seconds to failure. I blew $100 worth of brand new tubes (a super strong testing batch of Sovtek LPS no less
) in 1 hour until I ran out of them and got on the internet to research WTF was going on...
Do not use soviet built tubes in the cathode follower position. Chinese and JJ's work and most NOS. I personally think Chinese tubes sound great there, although just a hair hollow they have great dynamic character for new production. Mind you, I run low to mid gain crunch tones, so dynamic character is #1 criteria for me. If you run high gain chugga chugga and find your amp is a bit fizzy and edgey, JJ's will smooth out the fizz and add chug - and when you're at 10,000% gain, dynamic character is not really a relevant criteria.
It is NOT a coincidence that Mesa Boogie branded tubes are rebranded Chinese and JJ's.....nothing else that is new production survives in all positions of their preamps! (and maybe that "design feature" keeps you buying Mesa branded preamp tubes......hmmm.....maybe....if you believe in Machievellian business practices.) More likely it is because to make new production tubes survive Mesa would have to drop the plate voltage about 30% for that one single tube, and the extra resistor and lead trace to make that happen just doesn't make sense to them when they in fact do stock tubes that work. Further, the better part of the Mesa evolution of tone was in the 80's and chinese tubes of the era sounded terrific and Mesa dialed them in for optimum tone, and their design team just hasn't realized that tonal options and spec's of preamp tubes has changed dramatically over the past 25 years, and Russian tubes sound great but just aren't very robust at Mesa's chinese design specs.
Also, if you're using Tungsol RI tubes (also a Russian tube) in the first gain stage of your Ch3 and 4, only about 25% of them are any good out of the box, at least in my recent experience. I don't even try them in cathode follower stages, yet that tube has given me the most grief with reliability. I was replacing with a new one every time I went to a gig. And then, the last two I bought were DOA out of the box, so I just simply gave up on them. They are a weak and fragile tube that sound gorgeous *if* you can get a good one. In the case of a DOA Tungsol RI, I would get a low level flat clean sound on my gain channel.
Alas, I mostly use NOS and Chinese tubes. I do not like the JJ ECC83 for my edge of breakup crunch tones, but haven't tried the ECC803 which is said to be a major improvement.
