B-15 makes noises after prolonged use. What's wrong?

Edword

Edword

Jun 23, 2005
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#1
Just bought an old/new Ampeg B-15-N, and everything works great except that when I play for over a half hour it starts making faint sounds like an old radio trying to dial in a station. Same result if I switch guitar cords/instruments.

I've had amps that picked up radio stations in the past, but they started doing it as soon as I turned them on. This takes a while to start happening, and sounds like the amp itself is making the noise.

It's kind of frustrating. Any ideas?

TIA
 
#4
I'm thinking a tube issue or a capacitor is acting up. Likely thermal. Once the amp gets really warmed up it starts happening hence the half hour wait before you hear it.

Do you also get a steady faint rhythmic thumping or popping/pulsing sound in the background when it does it?
 
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#5
40Hz said:
I'm thinking a tube issue or a capacitor is acting up. Likely thermal. Once the amp gets really warmed up it starts happening hence the half hour wait before you hear it.

Do you also get a steady faint rhythmic thumping or popping/pulsing sound in the background when it does it?
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I wouldn't call it thumping or popping exactly, more like static and rising/falling frequency squealing.
 
#6
Edword said:
I wouldn't call it thumping or popping exactly, more like static and rising/falling frequency squealing.
Click to expand...

Sounds like a tube to me. But I'm not an expert so get a second opinion. If you got the popping or thumping I'd suspect a filter capacitor since you can start to hear the 60Hz pulsing in the AC line when one of those starts to go.

Maybe our resident Ampeg guru @JimmyM could chime in on this one?
 
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#7
There has to be a goood amp tech floating around detroit.

Get that amp serviced. Could be the sign of a few things, but you want to make sure everything is in good shape so you dont fry a transformer. Your repair could easily be something under $100 with parts and labor by a reasonable tech. If the amp fails in certain spectacular ways, transformers can cost up to $260+ before getting installed.
 
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#8
40Hz said:
Sounds like a tube to me. But I'm not an expert so get a second opinion. If you got the popping or thumping I'd suspect a filter capacitor since you can start to hear the 60Hz pulsing in the AC line when one of those starts to go.

Maybe our resident Ampeg guru @JimmyM could chime in on this one?
Click to expand...
I'm not an expert but bad tube or capacitor are the usual suspects.
 
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#11
It's a very nice sounding amp when it is healthy. Lots of good guesses, all need to be checked out by a tech. I'll add oxidized tube socket or tube sockets in need of re-tensioning. For additional info, follow the wiki links at the top of the page to the Ampeg Portaflex Wiki.

Doesn't sound like a big deal. The amp should be serviced.
 
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#14
Modern caps have a much longer service life than the old ones. Even 20+ year old caps seem to be fine. I mostly use snap-in Nichicon or Panasonic industrial caps these days and their performance is like night and day in comparison to the older ones.

I find that the Mallory cap cans that were used in vintage amps that run hot tend to have a short life, especially if they are stressed beyond their voltage spec at turn-on. Ampeg eventually went with a separate 600V cap for the first node because of a high failure rate. The original brown paper tube caps that Ampeg used in the B-15's and their other amps in the 60's and 70's don't stand up well long term.

I suppose it depends on the design and the cap.
 
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#15
Several good suggestions. Older amps eventually need a good overhaul if they are to be kept in service. At the least, replacement of all electrolytics, cleaning of all pots, switches and tube sockets, a voltage check (all voltages should be close to spec), and a scope trace for noise sources (bad tubes, solder joints or resistors). It's not cheap, but to keep these amps sounding good, and be safe and reliable this needs to be done. In the meantime, the OP can wiggle the tubes in their sockets (often solves noise/static problems), and substitute backup tubes (one at a time!) for those currently installed. Contact cleaner on all jacks, tube socket terminals, switches and pots should be done every few years. Beyond that, the amp should be taken to a tech for service.
 
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#17
If I had to put money on it I'd suspect a preamp tube at this point needs replacing.

You could try popping off the tube cage and lightly tapping each of the three preamp tubes with a wooden pencil or piece of dowling (NOT METAL). A thump sound is normal. If something rings sorta like a bell, then the tube has become microphonic and needs to be replaced.

But you're better off just letting a tech who knows tubes check it out. You can very easily kill yourself mucking with a tube amp if you don't really (and I mean really) know what you're doing. Besides, if it turns out a power tube needs to be replaced, it'll need to be properly biased.

If you don't know what that means or don't have the know-how, tools and test equipment to do it - you shouldn't even be thinking about any of this.
Life is too short as it is. And there are better ways to make the 8 o'clock news than playing "Edison Roulette" and electrocuting yourself.

Disclaimer: the above is presented for educational purposes only. Don't screw with tube amps unless you know what you're doing. Always have a pro service your tube amp.

Luck!
 
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#18
What ever happened to proper troubleshooting?

Hint... it's so much more effective to correctly identify the problem(s) and fix them properly than just throw parts at the problem and hope for the best.

Same thing applies to complete re-builds... ALWAYS identify any problems AND their causes BEFORE doing any parts replacement, then approach systematically. Replace and test EACH AREA OF REPAIR to be sure that you are not creating other problems.

I used to do frame up rebuilds of large format PA consoles. Systematic is the difference between a reliable repair and a crap shoot. A crap shoot on a $50k console is highly discouraged, and can be a career killer to that kind of tech.
 
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#19
agedhorse said:
How come every problem comes back with a "bad capacitor" diagnosis?

I typically find maybe 1 bad cap (at most) a year in troubleshooting. This can't be a "miraculous" discovery, right?
Click to expand...

Frying pan noise? Tube or resistor most likely. Can't say I've ever had a noisy cap at AF. Leaky coupling cap, maybe, but that sounds more like a Gatling gun. ;)
 
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#20
I had similar in a Peavey Valve King, and it turned out to be a bad solder joint at a capacitor in the effects loop. My tech found it pretty quickly. These type of problems can have elusive solutions, but if it happens reliably every time, it will be easier to find.
How old is the amp?
Good luck.
 
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