I have an old Carvin XV-212 (originally bought in 1989), which I have not used much at all for 5+ years. The pots/sliders on it have the typical corrosion that is taken care of by cycling them, so that is no issue. I am still getting drops in gain when switching channels, so not sure of "proper maintenance" for the amp. A friend said that the cones/edge material on amps typically do not go bad (I have had foam edges go on speakers over the years, so was #1 concern), but the tubes do (3 large, 4 small in this). Two of the switches are plunger type knobs, which may be the "issue," but I have cycled these several times with no change in performance, so a tube may be dying in it.
 
What should I check for to bring this back online, and how to know if a tube is going bad?
 
Thanks in advance for any insight.
 
Edit: I bought this in 1989, not 1992!! LOL

Bring it to a qualified tech. You will be glad. Those are some really nice/decent amps.

Carvin's site actually just confused me even more with this - the tubes on my amp have always had a red glow to them!
 
I just played around with it a bit more... never have had the power switch set above 25%, but continued cycling knobs and let the amp get warm and stabilize, and seems to be perfectly fine. I am still incredibly confused by Carvin's own picture, since the tube "cores" have always been red, and their site implies that a "good tube" has no glow to it??
 

There is a small heater element at the heart of a tube. This gets hot, heating the cathode and releasing electrons into a cloud around it.  Then there are 1 or more wire mesh layers to control the electrons. All of this is inside more metal jackets forming the anode.  This is the part you see on the outside and it should NOT glow red, or any other colour.
 
Depending on the type of tube you may or may not be able to see inside as far as the heater.  Mostly you can. This is the only bit that should be glowing like a tiny lamp.

That is a great picture. :-)
 

 
That picture is showing a pair of tubes "red plating" from a bad bias adjustment. While it may the case that those two tubes are bad, it is more correct to say that the bias circuit on that side of the amp is in need of maintenance or a simple adjustment.
 
The red plating is caused by the tubes being biased too hot which is to say that they are drawing more current then they are capable of handling.
 
The glow you see is not the normal glow one sees in a tube that is adjusted for it's useful bias range.
 
The glow shown in that picture is illustrating what a tube looks like just before the metal plates inside it melt and cause a lot of bottled lightning... or worse.
 
It's probably a good idea to find a tech who can help.
 
The switches and pots can be problematic but many times it's better just to cycle them as you describe rather than squirting cleaner in them... a lot of times the cleaner just makes it worse a few months down the road.
 
It's possible that one of your tubes is worn out. For example; phase splitter driver tubes have a hard life and in my experience are usually the first tubes to actually wear out under normal operation. If a tube is the problem you may just need one, or maybe a couple, so it pays to figure out which one is the problem.
 
There are many small capacitors in the amp that wear out slowly but surely. They may still work, but not so good, for years. The symptom may be temporarily fixed with a fresh tube but if a worn cap is the actual cause it's best to find and replace the bad cap.
 
 
In the example shown in Carvin's photo, which may or may not have any thing to do with the issue your amp has, I most often find that a capacitor in a bias circuit has gone bad, and or a potentiometer in the bias adjust, or bias balance adjust, is so dirty that it is acting "bad" and the fix is to repair the circuit. Usually, once that is done the old tubes are still OK. If you red plate the power tubes too long... well, you can replace the tubes too... but if you don't fix the bias adjustment problem your new tubes will go bad very quickly.
 
 
 
 
That's the quick version.

Thank you for the responses! So the "red" in that picture is the metal plates internal to the tube.. this has never glowed red, but the cathode is visible both above and below the plate. I took a few pictures and understand theirs better now, since with a flash enabled the glow is not visible in the pictures well at all (plus theirs is low resolution).
 
The first two are taken with the amp on for 5 minutes with "auto flash" enabled:
 

 
 
 
Once that flash went off, the cathode glow was "gone" so I took another with the flash off:
 
 
 
The red glow from the cathode is very visible (both above and below the internal plate), but the surrounding plate has no glow to it.
 
As I said, I let this warm up after my OP and cycled all the controls quite a bit (for about an hour)... a few still have static on them so they are not "clean enough" yet, but everything seems to function fine. I have rarely run this amp above the 25% power setting and even then the jump in volume from 0-2 is a lot (always has been actually). The room is large that I ran it in (25'x13'x10'), but has hard surfaces, so I didn't spend much time above 2 on the gain.
 
Comparing this "100W" amp versus a "300W mixer," the amp can overpower the mixer with ease - is this simply due to the power concentration being in a tighter frequency range combined with the Fletcher-Munson curve effect? The "threshold of pain" (for me) can be reached with very low power settings on this thing in an enclosed space quick.

Those "look" OK. If I were you I would get a known good 12AX7 and begin to swap the small tubes out one at a time and see if you get any change.
Remember that if you open the chassis, DON'T TOUCH ANYTHING... even if it is unplugged and has BEEN UNPLUGGED for a while there are things in there that can kill you. A capacitor can even rebuild a charge after having been discharged. However, if you're comfortable just looking, you might take a look is changing the tubes does not fix it. Look for anything that looks burnt on one end or the other. Look for anything that may have some loose. Put one hand in your back pocket and wiggle anything suspect using a wooden chopstick. Never use both hands in an amp. Taking a picture of the guts or several pictures while being CAREFUL and posting them may get someone to tell you if anything looks wrong or maybe suggest where to check for a loose solder joint using the CHOPSTICK. There are videos on discharging caps, it isn't that hard to do, if you're comfortable giving it a try. If not, get someone else to do it. Just never do it while tired, drinking, or incapacitated in any way. Never forget that it can kill you, ony use one hand, wear rubber soled shoes and you'll probably be fine.
I learned to build simple tube amps from a kit and then from parts and if I can do it, anyone can... just never be careless and use a lot of respect.
I really don't think the problem you are describing could be the power tubes, the big ones... my first idea is that one of the small tubes, probably all 12AX7s has a bad half. Sometimes a half of a tube will be used for extra gain. That would be the easiest thing to try because all you have to worry about is hot tubes, let them cool before you pull them... one at a time. You can also turn it on and up.. and then lightly tap the tubes, all of them, with the chopstick or anything that will tap but not break the glass. See if you get a ringing sound or any odd sound when you tap them. Sometimes you can swap out a 12AX7 to another place in the amp. The first ones, closest to the end are usually the ones that are most critical to your sound. I think left to right looking into the back of the amp. They may be marked V1, V2, etc. V1 and V2 are where you want your most stable tubes and possibly a really good NOS tube, according to the tone you want. Tubes and speakers and the easiest things in an amp to experiment with and can make a huge difference in your tone. Speakers can change things more than you can imagine.
Groove Tubes aren't bad but I'd get some JJ tubes, at least a couple of them for that amp.
I don't know but I bet it will run either el34s or 6L6 tubes... with a re-bias. Does it have a switch for running either EL34 or 6L6 ? A lot of carvins do. You culd probably run KT66 tubes too, if there is room. Have some fun trying things once you get the problem sorted out... I bet it is a 12AX7. You may try just swapping the one on the far right for the one on the far left. If that doesn't fix it. Put the far left tube back where it was and put the tube that you took from the far right end into the second from the left socket. If you don't have extra tubes, I would try that first.
If it isn't a tube it is probably a capacitor or a loose solder joint... but it could be anything.
So again... check the small tubes first. Tap the tubes to see if any make a noise. If you are comfortable with it, open the chassis and look for anything that looks burned. Wiggle wires, lightly with a chopstick with one hand behind your back TOUCH NOTHING IN THE GUTS... it can KILL YOU even if it is OFF and UNPLUGGED. Take some pictures.
Look on Youtube for some tube amp basics videos. There are some for the basics and for SAFETY. Gerald Weber has some for basic safety that are decent enough and are easy to understand.
Really, it is fun once you get started... and a simple Tweed Champ style amp is easy to build and very satisfying. I started with a kit from STF, you'd quickly learn a lot about amps.
Karyn and ampfixer.. and Mike are all good help on here.
 
Good luck.
 
J

Thanks J, I think my biggest "sin" with this amp is that I have left it sit idle more often than it should be. When I first began playing guitar I took a more logical approach to it based on seeing others (fail) and swore "no effects till you prove you can play... but a good amp is needed." I went with tube because I had read that digital tube emulation was lacking (MOSFET was just coming online then) and after delving deeper "Carvin" kept popping up. I installed pop-out casters in it right away, since I would wheel it on concrete floors through college and play it late at night in 6-story stairwells (the reverb of a concrete stairwell is AWESOME). Overall... I would say this amp has had a few hundred hours of play time, and mostly at unstressed wattage, so the "degradation" would be from "just sitting idle." It seems most is corrosion of exposed switches and pots, so I will take that route at first and get a habit of switching it on more. It is ironic that I will default to jacking into a DI box and playing through SONAR rather than power this amp up
 
Thank you for the warnings on the cabinet. I am a former submariner, and at all times on a boat, there are 2 systems within 6' that can kill you instantly. I once had an electrician come to me white as a ghost because he discharged the 500V capacitor used to flash the AC field on a Ship's Service Motor-Generator by backing into it during preventative maintenance! After making sure he was okay, I told him that he was one lucky bastard.
 
I will have to research on tube replacement/biasing, but for now will hold off on that since it seems a lot of technique/know-how (which I do not have yet at all), and may also be totally unnecessary. Just a couple hours of using and wiping contacts is bringing it back to what I remember (which is incredibly beefy and loud, LOL)... maybe part of the reason why I haven't used it so much... this guy is meant to be "on a stage."

There is a small heater element at the heart of a tube. This gets hot, heating the cathode and releasing electrons into a cloud around it.  Then there are 1 or more wire mesh layers to control the electrons. All of this is inside more metal jackets forming the anode.  This is the part you see on the outside and it should NOT glow red, or any other colour.
 
WHAT A WOMAN!!!

If you decide to clean those scratchy controls, be sure to use a proper cleaner with a residual lubricant. If you spray a strait cleaner/deoxidizer into the faders they can seize solid.