Problems to look for in a 20+ year old amp

there would be alot to look for in an old amp.
depending where you found it enters into things too.
where you found or bought it, because guitar store would probably overhauled it so it could be played in the store. ask theowner or salesperson.
otherwise there are some things to check.
has it been played recently? or has it sat unused for a long time.
if it sat unused the caps will have lost their charge. check the tubes and the connections.
look for rust and water damage either from a flood or ususally from high humidity.
where was the amp stored. high and dry or wet and humid. either would affect parts of the amp; like the wood case and metal parts.
check all solder connections.

the speaker: is the paper nice looking or torn? something you wont find visually , is the voice coil ok or frozen.?

those are a few things that come to mind.

Ive got a 1958 Gibson amp. it's original Jensen speaker works fine after all those years. I've had it cranked and it still lives.
of course it has had new tubes and a few parts repaired or replaced.

A lot of folks recommend replacing all of the electrolytic and waxed paper capacitors before you even turn it on. I've never done that, though, and I've been lucky.

I remember reading an article about old amps in a Vintage Guitar mag.
I still have the issues so I will look for any info we're missing.

Re what I said above about having not recapped old gear before turning it on and being lucky, usually I've had to put up with goshawful humming, crackling, popping and "thunderstorming" for a while before the electrolytic caps re-form, and I ALWAYS keep a finger on the filter caps to make sure they're not heating excessively. If they're badly shorted, they can and will explode, and that's not a good thing. Also I watch tube rectifiers closely to make sure their plates don't start glowing. If you're going to do this, it's a good idea to use a Variac to slowly bring up the line voltage from, say, 2/3 of normal, or to use light bulbs in series with the power supply to drop the voltage and sequentially switch them out. But as I said, I've usually gotten away with the plug in, turn on, and wait out the bad noise method, and the worst I've had to do is to go ahead and change out the caps when they wouldn't straighten out. A professional fixing one of these things for someone who depended on it would go right ahead with the recap, of course. It's just a hobby for me, so I can afford to screw around with it.

Oh, one more thing: Most old guitar amps have a two-conductor power cord with a nonpolarized plug. There's a polarity switch on the back that has two or three positions. What it's doing is switching a capacitor that has one end tied to the chassis to one or the other conductor of the power cable, sometimes with a neutral position in the middle where the cap's not connected to either. It's there for hum reduction. When it's switched to the neutral wire at the outlet, hum voltage on the chassis is shunted through the cap to ground. If it's switched to the "hot" wire, it'll usually increase the hum level. That cap is popularly called "the death cap," because if it shorts out, you have a 50% chance of having the chassis connected to full line voltage through it. (Depending randomly on which way the plug goes into the socket.) The outer conductor of your guitar cable grounds all the metal parts of your guitar to the amp chassis, so it gets full line voltage, too. If you touch something grounded while playing the guitar, you'll get seriously zapped. Often it's through the mouth as you touch a microphone.

Best thing to do with one of these amps is to remove the "death cap," and install a new three-conductor power cord with the third ground wire securely attached to the chassis. First make sure there's not already a short to the chassis, as in the power transformer. There are a number of sites with detailed instructions for making this mod. I think there may be something about it on R.G. Keen's guitar amp page: http://geofex.com/gtramps.htm If not, hunt around the links on the AX84 site: http://ax84.com/

Thats what Variacs are used for, is to reform caps. You plug the amp in it slowly bring the voltage back to proper operating voltage. Variable Voltage Transformers, although fairly common throughout industry, are not widely known or understood. Except when a few guitarist realized they could also be used to lower the voltage of tube amps to attain break up sooner.

I have tube amps that are older than the hills and work great and I also have a few that just won't come around no matter how much work I put into them. Every amp is different, many of them will last you a life time if you care for them.

Joe

Thats what Variacs are used for, is to reform caps. You plug the amp in it slowly bring the voltage back to proper operating voltage....
if you don't pull the tubes out when you do this, you will seriously shorten their life! running tubes with underheated cathodes, is not recommended. of course, if you have a tube recifier, it will have to remain in for the process. otherwise you won't be making any B+ voltage. better yet use a solid state plug in replacement for the rectifier tube during the cap forming.

of course if you're a touring pro making megabucks, who cares about tube life? run the amp a 90v all the time and get some serious tone. toss in a new set of tubes every night, just to be sure they won't go wimpy on you during your big solo.

unk

Elsewhere I've just been discussing, in reference to tube amps that haven't been turned on for a while, electrolytic capacitors' tendency to deteriorate developing high leakage current, hissing and popping as arcing occurs through the deteriorated oxide film. It happens in old caps, and it can happen to recently made caps that have sat on the shelf for a while. I just brought home some new caps from the local electronics store to replace the original screen bypass caps that I thought were making a lot of noise in the 7199 driver stages of the power amp in my 1967 Hammond H-182 organ. I just replaced them tonight, and when I turned it on I frightened myself that I'd wired them backward, because the organ was hissing and popping louder than ever and it had almost no volume. Turned it off immediately, took it apart again and checked. Nope, I'd wired them in properly just as I was sure I'd done, having checked multiple times during the process. So I turned it back on and waited a bit. The noise quieted down and the volume came up appropriately. These "new" caps had to be formed with voltage on them. Of course, I have no idea when they were actually made and how long they've been on the shelf.