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Help troubleshoot my old solid state Ampeg

Discussion in 'Amps and Cabs [BG]' started by Stegman, Jan 30, 2014.

  1. Stegman


    Jan 30, 2014
    Hey all. New member here. Been lurking for the past couple of days trying to figure out what might be wrong with my amp. Thought I'd cut to the chase and just ask you guys.

    I have an Ampeg B-80N. It's an obscure 80 watt solid state combo amp from the late 70s with a 15-inch speaker. Google it and you'll see how little is out there about this amp.

    I've had it for almost 20 years. Picked it up used for $200 back in the day and it worked fine for the first 5+ years of its life.

    Anyway, here's the deal. The amp now has a lot of distortion at any volume. It growls no matter the settings and the volume. It started doing this a long while back - right around the time the kids came along and I didn't have much time to play. I put the amp away and basically forgot about it.

    I used to play a G&L bass with active electronics through it, and now I'm playing a passive Hofner Beatle Bass. I've experienced the growling with both basses.

    At first I figured the speaker must be shot, but I tested it yesterday and discovered that the distortion is also prevalent when using the headphone jack. I also followed some advice on here and pushed on the cone with my hand and heard nothing, which I understand is a good sign. There's no tears in the speaker, and it appears fine to the naked eye.

    Today I cleaned the input jacks with de-oxidizer [and the pots] hoping that might take care of the problem, but no dice.

    I removed the speaker and the head today and took a look inside. Don't really know what I'm doing, but didn't see anything that looked alarmingly wrong. I did notice some pigtail wiring for some of the back-of-the-box inputs that seem to be connected with electrical tape. Not sure if that's kosher or not.

    So, anyone have a guess as to what it might be the problem here - and, as important, how much it might cost to fix? Reading through various threads here I saw a lot about low bias causing distortion, but it seems like that might be more of an issue for tube amps.

    Thoughts? I've tried to put in as much detail as I could think of, but hit me with any questions you might have.

    Thanks in advance.
  2. I can't find a schematic, unfortunately. You say it growls at any volume, do you mean a loud hum or what?

    Without a schematic, diagnosing over the 'net is a challenge. But if it were on my bench, I'd see if the power supply caps appear to be in good shape (testing for AC ripple on the power rails) and then see if the low-voltage rail(s) are equally stable. An amp of that age is likely due for new power supply caps, but of course that may not be your problem.

    Then, once I've determined the power supply is OK, I'd inject a clean sine wave into the input, and trace the signal through the amp, going from gain stage to gain stage, to see where the signal gets distorted. Could be as simple as swapping an op-amp.

    Does this amp have a "preamp out/power amp in" set of jacks? (Could be labeled EFX out and EFX return, or something similar) Sometimes the Return jack gets corroded, it's a switching jack, but that usually means low distorted power, not what I'd call a growl. If you do have a Preamp out jack (or something similar), run a patch cord from the Pre out, into a known good amp, see if you get the same growl out of your known good amp. If so, that means the problem is in the preamp section of the Ampeg. If the Preamp out signal is clean, run a good clean preamp signal (from another amp) into the Power Amp in on your Ampeg, do you hear the problem? If so, the problem is in the power amp section of your Ampeg.

    Bottom line, it's time to take the amp to a tech.
  3. Bias is not just a tube amp thing! SS devices also get biased just above cut-off (Ampeg SVT-3's are a good example as they have been known for bias drift).
    The condition of the power supply is first to check as stated above.
  4. Stegman


    Jan 30, 2014
    Thanks for the advice, Bill. The amp has a line out jack on the back with a pre-EQ/post-EQ switch. It also has a Ext. Speaker jack labeled 8 ohm.

    I had thought of testing it through another amp, but the only amp I have is a 20 watt Peavey practice amp that can handle 4 ohms and I didn't want to fry it. That amp also doesn't have any input or output jacks other than the standard instrument input and the headphone jack on the front, so I think I'm out of luck there.

    Is there a way to run it through the Peavey without ruining the Peavey?
  5. Go from the Line Out of the Ampeg into the instrument jack of the Peavey, see if you hear the distortion.
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Since this would chain the preamps together be careful with the volume levels. Too high on the Ampeg will overload the Peavey and cause distortion. I do this when I want distortion, the first amp becomes a glorified pedal.

    You can also try plugging your instrument directly into the power amp input on the back of the Ampeg and see if you hear anything.
  7. Stegman


    Jan 30, 2014
    OK, I did as you suggested and the sound coming out the Peavey was clean - no distortion, no growl. Sounded fine.

    Then I plugged the bass into the Peavey and ran a line out from the headphone jack into the Ampeg's instrument input and heard the distortion/growl. I was actually surprised to hear anything - I didn't think the signal from the headphone jack would produce enough juice to be heard. But there it was.

    So where does that leave me?
  8. Isolates the trouble to the power amp, which can still be bias.
  9. Steve Dallman

    Steve Dallman Supporting Member

    An amp of this age, I'd start by replacing the main filter caps so I know I would be starting with a solid, stable power supply for the power amp.
  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Makes a difference if it is a huge amount of distortion, like a blown speaker (which it probably is not), or if it is just a trace, especially if it is more noticeable as the note dies out.

    A huge amount of distortion often means a bad driver or bad output transistor, or a bad connection between them. And that it isn't a good idea to spend a bunch of time listening to the distortion to figure out what it is.... that can fry components.

    Usually that kind of distortion goes mostly away at low volumes.

    A smaller amount, especially if it shows up worst as the note dies out, is generally bias.

    There are lots of other possible causes.

    I might have something on a B80N.... I have some old Ampeg service info. I'll look around. Bug me if you don't hear back.
  11. Stegman


    Jan 30, 2014
    Thanks Jerrold.

    It's a growly type fuzz that's not altogether unpleasant. I bet people buy pedals to get that type of sound.

    It is not more noticeable as the note fades out. Nor does it go away at low volumes. It's always there no matter how loud I'm playing.

    I appreciate everyone's help with this. I guess my main concern is how much it might cost to get fixed. Are we talking $75 or more like $150? If it's the latter, I probably won't bother as it's probably not worth much more than that. It's a cool, obscure little rig that has sentimental value, but I'd probably just keep it as a conversation piece - or maybe buy a head and just use the cabinet.
  12. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The problem with estimating costs is that you never know what the real issue is till it is on the bench. There are many possibilities. First you need to find a good tech that has some experience with transistor amps and can hone in on the problem in a cost effective way. You will need to pay an initial bench charge to evaluate the problem. With an older amp, there are bound to be other issues. Maybe the power supply caps need to be changed, the output bias will need to be checked. Cleaning the amp, tending to the pots and jacks are regular maintenance issues that need to be addressed from time to time.

    Costs can add up quickly and $150+ is a good possibility. On the other hand, if there is no other issues and all you need is a bias, cost will be minimal.

    You could always explain the situation in advance. Tell the tech that you don't want to sink a lot into the amp. Let them suggest a time/cost limit for a written evaluation which would include checking the bias. You can decide if you want to proceed. With a written estimate, you'll at least have that should you want to go ahead some time in the future.