Tube amp loses volumn

[i5studio]

Hi,My mid 60's Alamo amp loses volumn and gets very distorted after about 30-45 minutes of playing time.Any ideas of where to check would be apperciated.
Thanks

[pdf64]

Have you tried alternative 'known good / tested' tubes?
Are you competent - safe! - to test inside a live amp?
Pete.

[dodona]

replace the tubes, and if it doesn't help then the capacitors.

[DLNB]

I had the same issue with an old Lifco amp. I replaced all of the capacitors except the large metal cans and it fixed the problem.

[Chuck H]

I had the same issue with an old Lifco amp. I replaced all of the capacitors except the large metal cans and it fixed the problem.

The large metal cans are the most important ones to replace in these older amps. Even if your initial problem was solved you are on borrowed time with old power supply filters (the "large metal cans").

Old paper in oil caps DO seem to be prone to leakage. The problems this can cause are a serious detriment to tone, but that's all. Filter caps failing can do major damage to transformers and/or power tubes. The power tubes and transformers are the expensive parts of the amp. Don't be a cheepskate in this area. It will cost you in bad, ripple ridden hummy tone and possibly expensive damage to power tubes or transformers. Just sayin'

Most older amps have "large metal cans" that are no longer available. Some retro fitting is usually required. If this keeps you from doing proper repair and maintenance to the circuit your just being lazy and neglecting to recognize reality. Replace the filter caps and any other elecrolytic capacitors in the amp. If the film caps test leaky, replace those too.

[DLNB]

When looking into replacing the caps I read somewhere online that if the amp is not noisy, no hum or anything that I should leave those cans alone. The amp is super quiet. Almost freaky how quiet the amp is when cranked up so I just left them. BUT If they should be replaced then I'll definately do that.

[nashvillebill]

There is some terrible advice floating around on the Internet. People advising NOT to change those caps...that's absolutely ludicrous. Most of these folks don't have a clue and they are merely repeating other people's bad advice.

Electrolytic capacitors made in the 1950's or even the 1960's won't last 45 or 50 years. If you are familiar with Hewlett-Packard, they made some of the worlds best test equipment in their time. I have one of their signal generators from the early 1960's and their owners manual proudly boasts: "this unit was made with the highest quality electrolytic capacitors which should last from 5 to 10 years before requiring replacement". Hmmm.... 5 to 10 years, I could see trying to stretch to 15 or maybe even 20, but over 40 years?

Would you say, "hey I know the owners manual says to change my car's oil after 5000 miles, but hey the car still runs, so there's no need to change the oil?" Of course not.

Electrolytic caps have a known failure mechanism. It's well documented that their seal can and does detiorate over time, and the cap will then dry out. This may occur relatively slowly and it may not be immediately evident--the amp will still "work" and you may think it "sounds just fine". However, the caps are losing their capacitance, and their internal resistance (ESR) is increasing. The bottom line is that the amp is not capable of performing as well as it should. At some point the caps may even fail violently, and can be quite messy.

For a DIYer, changing the caps usually is not a big deal and really not terribly expensive. It's just a bit of preventive maintenance that should be done; change those caps now, and get a couple more decades of trouble-free life out of the amp. For the DIY'er it also gives them a chance to get up close and personal with their equipment and make sure everything else is in order.

Change those caps!

[DLNB]

I'm going to start doing a little research and see what I need to do to change those cans. I remember from last time I looked into it that I need two caps per can, and possibly a variac.
Anyway I do enjoy working on the amp so this will be a nice little project for me.