1967 Bassman problem

Discussion in 'Amp Tech Center' started by AlbertaGriff, Nov 9, 2019.

  1. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I've had this Bassman for 3 years, and it was serviced right after I bought it by my tech, who does good work. I know these amps are valued for their distorted tone, and I played this one cranked whenever I could and I enjoyed the distorted tone. In fact, it seemed I never played it clean. I was chatting with a friend who mentioned his steel player uses a similar amp and it got me wondering about my own amp - could I get a crystal clean tone?

    Turns out I couldn't. Normal channel, volume at 3, I was getting unpleasant distortion, especially at lower frequencies (like my open E string). Almost like it was getting choked. I thought it might be the cab, so I opened it up. Wiring looked a bit sloppy, and one speaker was 16 ohms.... swapped them both and wired it up nicely. My low volume, low frequence distortion is still there. I'm wondering if anyone has any hunches on what it might be? Pictures attached. I'm wondering if I have EVER heard what this amp can actually do?

    20191109_131911.jpg 20191109_131920.jpg 20191109_131928.jpg 20191109_131937.jpg 20191109_132640.jpg 20191109_132651.jpg
     
  2. tubelectron

    tubelectron Tele-Holic

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    This amp should be able to give 45W RMS clean tone, at onset of clipping, at the rated load of 4 or 8R with 400Hz sinus input. If not, indeed there's a problem somewhere.

    If you do not have the knowledge and/or the tools to troubleshoot it, I strongly advise you to rely on your tech for a complete and accurate check-up...

    But it's me, OK ? :D
     
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  3. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I have some knowledge, bit trying to see if anyone has any experience like this, has a hunch what it might be. My hunch is a cold solder joint somewhere. All the components 'look' good to me so far.
     
  4. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Bias board:
     

    Attached Files:

  5. 2 Headed Goat

    2 Headed Goat Tele-Afflicted

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    Perhaps try installing a new set of tubes. Also inquire more about your friend's steel players amp. Similar could mean a Bassman such as yours or a Bandmaster... or a Showman both of which are heaps cleaner sounding than a Bassman.
     
  6. Dacious

    Dacious Poster Extraordinaire

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    Low volume distorted clean tone - almost certainly the tube bias is way low. You're getting blocking distortion because the tubes are falling out of amplifying.

    Having a mix of speaker impedance if that's what you have won't help. One speaker is doing nearly all the work. The amp is looking for a 4 ohm load. If you have 8+16 ohm it's way off.

    Bassmans are not famous for distortion - they're famous for giving big clean middy sound that starts to break up high on the dial - Tweeds break up a bit more. Silverface Bassman - nup
     
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  7. 2 Headed Goat

    2 Headed Goat Tele-Afflicted

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    Just noticed, I see quite a few dodgy/slopping looking solder joints... perhaps take it to another tech.
     
  8. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I should probably tube swap a bit to narrow the problem.

    I replaced the speakers with two 8 ohm drivers wired in parallel for 4 ohms, amp sounded the exact same.
     
  9. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Yes I noticed a LOT of clipped off leads which I was very surprised about. This guy is normally meticulous.
     
  10. 2 Headed Goat

    2 Headed Goat Tele-Afflicted

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    Interesting. Not to sound harsh, but IMB that's really slopping soldering. I'd start there.
     
  11. Dacious

    Dacious Poster Extraordinaire

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    Maybe he didn't do all the work you see. It's doubtless been looked at by others in its history. He has changed all the little caps which is a good sign.
     
  12. 2 Headed Goat

    2 Headed Goat Tele-Afflicted

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    Good point, tho I see more cold looking soldering joints then good ones.
     
  13. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    I think he is the only tech to have opened the amp. When I tried it before buying, it had not been played in 10 or so years... and I blew up one of the original filter caps! The amp was not turned on after that, but I know my tech had a hard time after that, anded up replacing a bunch of resistors too, trying to eliminate some noise. But I don't know which ones.
     
  14. 2 Headed Goat

    2 Headed Goat Tele-Afflicted

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    Have been waiting for Wally to chime in... Upon reading the above post, I have a feeling he soon will. ;)
     
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  15. Dacious

    Dacious Poster Extraordinaire

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    Ah - if it's always sounded like this since resurrection, there's a chance your PT is toast.

    You need someone with a Megger to check it. That puts a charge down a leg and measures the resistance to check for insulation continuity

    If a cap popped you had a dead short for an instant. Often the surge of power can arc through any weak spots in the lacquer coating and allow coils to not-quite-contact. At high current draw it can arc continuously.

    One of the prime symptoms is low volume distorted output on demand.

    I would still get bias checked and maybe some connections touched up, it does drift with time. But at the same time the transformer should be tested. Just check all your earth leads, too. Jack's tight, pots tight.

    All the guys who say 'don't change caps until they're leaking goop' should read this
     
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  16. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    That would be very unfortunate.
     
  17. Dacious

    Dacious Poster Extraordinaire

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    It's only a rewind or drop in replacement. I got a new 240v power transformer made for my Superchamp by a guy who restores radios. Works like a charm. He's rewound both power and output trans for me
     
  18. Milspec

    Milspec Friend of Leo's

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    May not be the prettiest belle of the ball soldering-wise, but I doubt it was the problem. My '65 certainly plays clean when desired, so it is not normal at all. I would lean towards the bias and / tubes being the flaw.
     
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  19. Paul G.

    Paul G. Friend of Leo's

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    A Bassman should be clean at not quite, but nearly blistering volume. Distortion moves in as the volume approaches 6, and it's not "nasty" in the least.

    Do both channels distort? If so then Blown PT a possibility. Bad bias. Bad power tubes. Not as likely, but still possible OT. The soldering could be better, but I'm sure it's not the problem at all.

    Checking voltages would be your next step, checking the bias, replace tubes. Take a look for burnt components (pop! remember?). Use your nose. Do you smell anything? Chopstick joints with a signal going through, see if something changes.

    All this assumes you know how not to die when inside an amplifier. If you don't know how not to die, don't do anything and hire somebody.
     
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  20. zook

    zook Friend of Leo's

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    I Vote for the bias being off, likely too cold.
     
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