Do these 2:90 tubes need to be replaced?

[QuantumDot]

I'm considering buying a Mesa 2:90 power amp, but I'm not sure if I'd need to pay for a matched set of 8 x 6L6GC power tubes...

Here's an imgur album that shows the current state of these tubes:

https://imgur.com/a/U5I8j8G

As you can see, these tubes have strange brown rings or black spots on top of them. Is this caused by bad bias setings? Is this an indication that the seller has used the amp in mono and forgot to turn volume down and turn presence up?

Is only biasing the problem here and, once the bias is set correctly, they'd work as they should or are these tubes already 100% trash and there's nothing else left to do but to replace all of them?

BTW: Is there an easy solution for the 3 voice modes (halfdrive, modern etc.) if one doesn't use a triaxis preamp with the 2:90? Is there a cheap 3 switch box that I could buy that gets the job done? I'm so not ready to splurge a 100 bucks on 3 basic switches...

 

[Henz]

Greetings. Pm me. I've had a 2:90 , first version since 1994. Thats what this one appears to be. I apologize but I don't have time to answer currently.

 

[QuantumDot]

Here's a new set of pictures that I received just now:

https://imgur.com/a/qfq22A9

Still don't know what to think of the power tubes. From the side, they look prisitine, but the tube tops with the brown rings and dark spots really make me question the quality of these tubes.

 

[Henz]

Ty for more picts. I was gonna suggest that you get some gut shots too. From what I can tell, the inside looks fairly good . Clean. It has a combo of metal oxide and carbon comp resistors on the screen grids. No biggy. Carbon comp has a reputation of being just a bit more noisy. And that white plastic clip looks good coming from PT. They have a tendency to get hot and burn. Yours looks fine. Check closely at the filter caps. The big blue round components. Check for bulges. Ask who you are buying it from if its ever been maintenanced . Those metal oxides may have been installed by someone. Oh, btw, if yours has a blue light, its first version. But that low of a serial number clued me. Ok, as far as the power tubes, they look like normal wear. Usually if power tubes have darkness on its side, that would usually indicate that the tube overheated. I don't see that. For piece of mind, just purchase a new set since you really have nothing to base the tubes history unless you ask who you are purchasing it from. And then you will know all is well there. And no, they don't need biased. Where ever you purchase your tubes tell them what amp its for. In the US, the plate voltages are around 460vdc. Give or take 10%. The factory set bias is - 48vdc on the outer and - 58vdc on the inner. But again, that could vary. Voicings. You can very easily make a little switch box. Choose your box of choice. Get three switchcraft or comparable tip / ring female input jacks. Drill the holes and mount. Get three on/ off switch craft jacks . drill holes and mount. If you have basic soldering skills, simply solder lead wires from push on/off jacks to the terminals of the other three jacks. I don't believe polarity matters here. Done. I made one myself a while back but I use my Patch mate for it now. Use speaker wire to connect box to the voicing jacks on rear. Once again, you may have to cut to length and solder standard 1/4 in phone plugs. With these I would wire correctly. White lead to tip and black to ground/sleeve. That should turn voices on and off when you step on switch. I may have left things out. Ask me if you need help. If I knew how to upload picts on this sight, I would show you mine. I hope I helped. Blessings. Jim