Help me save my Magnatone!

[BrokeForTone]

I just bought a Magnatone High Fidelity Custom 415. Apparently it's a bass amp, around 25 watts, high gain and low gain inputs, and has four 8" speakers. Only has volume and tone knob. I traded two Vintage 30s and $200 cash for it, so it was a very nice deal.

There's no buzzing or hum, but whenever I turn it up a third of the way or more, there's a harsh buzzing/rattling/distorting sound. It sounds fantastic until I get it that loud, then it just sounds like something's wrong. I've been playing it quietly for the time being, but the previous owner probably played it loud with the sound, because he told me it gave the overdrive tone character. it just sounds like a problem to me.

I disconnected the internal speakers and plugged it into an external speaker to make sure it wasn't a speaker/cabinet issue, but the noise was still there. Perhaps it's the tubes, but it might also be something like the filter caps, so I have begun to think after skimming over a few forums about amps like Magnatones and their maintenance. No telling when the last time these tubes have been changed.

I really don't know much about electronics or the inner workings of an amp, but I figured this would be the right place to go to see if anyone can tell me what the issue might be.

I can make a video to show how it sounds quiet and cranked, maybe that'll help yall diagnosis the problem. but if anyone can help me find out how to fix this thing, we can save another Magnatone and I'll have a lovely usable combo for jazz.

[BrokeForTone]

To clarify, because I might not have made it clear- the buzzing/distorting sound occurs when I actually hit a note. There's no other sound going on. I mean, it sounds like the speaker rattling or something. But it still made that noise through an external cabinet, and I put my ear up to each speaker and every single one of them was reproducing the noise. It's something with the preamp/power amp, apparently.

[Chuck H]

A recording of the problem would be helpful. With an amp like this it's impossible to know if it's been modified or not unless you know circuits. If the electrolytic caps haven't been replaced they're overdue, but your problem is something else. I'm going to guess that it's tubes and/or loose/dirty tube pins or sockets and/or oscillation due to lead dress resulting from a modification and/or an incorrect bias condition caused by leaky coupling caps. That's a lot to sift through for a tech. doing it as a novice will be harder. And there are other things it could be as well.

There are other issues with an amp that age that have to do with safety.

With four speakers there is likely a very specific load that amp should run into. If your going to plug into an external cabinet be sure you match that load.

With the possibility that the amp needs tubes, caps, drifted components, oxidized connections, hack modifications and needed safety upgrades I think you could best enjoy that amp after it's been to a qualified instrument amplifier tech.

JM2C

[BrokeForTone]

I'll make a video of the issue and post a link tomorrow, if I have time. I'll avoid playing the amp after video taping until the issue is sorted out.

I know for a fact that the amp has been converted to use for a 3 prong power cable, and there was some sort of capacitor that he needed to replace in order to do it. He gave me the original capacitor, along with the original little indicator light. He told me the amp shocked him before he made the change to the 3 prong, but the problem is gone. The speakers are also not original, but the previous owner received the amp with them already installed. Other than that, it is believed to be original. I wouldn't be surprised if the original capacitors and tubes are in there.

I will say, I think you're right about it being the tubes. I've had a few microphonic tubes and tubes rattling in an old combo I used to have, and this sound seems like it would be coming from a similar problem. I also layed the amp down flat and pointed a flashlight inside to get a look at the tubes and they seem very dusty and old to me. This is my first vintage amp, so I havent seen many ancient tubes haha. I don't see a lot of rust or anything on the inside, but I can't exactly see where the tubes connect.

I can take pictures into the back of the cabinet, but I don't want to open up anything myself. I want to take this to a tech with a good idea of what the problem may be. Anything past that I don't trust myself in doing, because I want a professional working on this amp.

I just hope this is something that isn't too difficult to fix. I don't mind swapping a few parts inside, as long as the heart of the Magnatone circuit is still there. I don't want to change anything that doesn't need regular changing anyway. But this isn't going to be a museum piece- I'm going to play this amp when I go to college next year.

[Chuck H]

For an amp that age of unknown service the standard would be to replace any old electrolytic caps, change to a three prong cord and remove the "death cap" (sounds like it's been done), check all the components for drift in value, check coupling caps for leakage, clean tube pins, sockets, pots and switches, replace worn tubes and check the bias conditions. once these things are done properly the amp will play like new and should be reliable. But you can't do all this yourself if you don't want to open the amp up.

Even if it needs tubes and filters you paid the nice price

[BrokeForTone]

Thank you, that's exactly the kind of information I was looking for! I know it's impossible to say because every tech charges differently, but do you know about how much this kind of maintenance could cost? I've never had to take an amp this old in for service. I can't imagine it'd be too much if there's only a few caps and tubes that need to be switched, but if there's any leakage or clean-up that needs to be done, I could imagine it'd take a bit longer and would end up costing a bit more. I just want to see if I could be potentially looking at a multiple hundred dollar fix hahaha.

[jrfrond]

My approach with something like that is to rule out the speakers forst, the tubes second. If it is neither, then it is time to start digging, and since it is a vintage tube amp, bad electrolytics and other out of spec components are likely. even if the tubes DO work, and amp that age needs maintenance.

[BrokeForTone]

Good tip! Anybody know any good techs in the Dallas area that I can trust with this job?