Ampe SB-12 breaking up in a bad way

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by samdjr74, Jul 14, 2006.

  1. samdjr74

    samdjr74 Supporting Member

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    Hi All,

    I bought an Ampeg SB12 from another forum member a wile back and I love it. But recently it's been breaking up/crackeling after extened play periods. It only seems to do this when I'm running my tonebone and reverb pedal through it. The amp head was checked over by the person I ought it from and by my local tech, both gave it a clean bill of health but I never brought the cabinet in for a check up. So I was wondering, do you think my tonebone is overdriving the speaker or do I just have a bad patch cable? When I run the ampp with no distortion I have no problems and when I run my tone bone and reverb through my other amp they work fine too.

    Thanks,
    Sam
     
  2. donnyjaguar

    donnyjaguar Member

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    So you're saying that after playing through your TB for a while it starts to break up. Now if you go direct at a similar volume does the problem go away?
     
  3. samdjr74

    samdjr74 Supporting Member

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    You mean if I bypass the TB alltogether, yes it does go away. But if the TB is used on my other amp I don't see the same problem.
     
  4. AL30

    AL30 Member

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    Man do I LOVE those amps.

    If it's only doing it with your pedal then there's a few possibilities (and probably some I'm going to miss).

    1) Bad cord
    2) Bad pedal/jack
    3) Bad tubes

    Also, has that amp had a cap job? When I picked up my SB-12 it "sort of" worked. I gave it a cap job and fixed some shady wiring - problem solved.

    Is that amp connected through the side latches? These are a common source of problems in those amps. If the latches or top become loose the amp will vibrate and loose the connection causing intermittent operation. Check the latches and the top. Latches can be purchased at http://www.fliptops.net/ and if the top is loose some standard weahterstripping should help.

    How's the speaker?

    AL
     
  5. samdjr74

    samdjr74 Supporting Member

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    It might be a bad cable, tubes check out fine, caps are good and it's been modified for a 1/4 jack to the speaker instead of the latch connection.

    The speaker is a newer Fender but I don't know how new. I'm thinking it's the speaker myself but shouldn't I hear the problem all the time?

    And to better desribe the problem, it's like running too much power through car speakers kind of sound.
     
  6. Wakarusa

    Wakarusa Member

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    Just curious, but did it come from El Paso, TX?
     
  7. samdjr74

    samdjr74 Supporting Member

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    No, it came from Philly
     
  8. Wakarusa

    Wakarusa Member

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    Ah, then t'isn't the one I worked on. Which output tubes is it set up for?

    I'll cast my vote with AL30 but add that since it seems to happen after you've been playing a while that heat may well be a factor. Could be a tired component that, when it gets good and warm, starts to underperform. Have you tried it through a different speaker cabinet?
     
  9. samdjr74

    samdjr74 Supporting Member

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    I'm not sure Todd. I got it from a guy named Bruce. The tubes are as follows 1 5AR4, 2 12AX7 and 2 7868. I haven't tried it through a different speaker but I could try tonight if I get a chance.
     
  10. Wakarusa

    Wakarusa Member

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    Good luck finding replacement 7868 :) I think EH makes one, but I couldn't find anyone who stocks it. I'm sure I'll hear screams of vintage preservation, but you might consider replacing the 9pin output sockets with octals and re-wiring for either 7591A (electrically identical to a 7868, just an octal base) or for 6L6.
     
  11. AL30

    AL30 Member

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    The tube store carries 7868's. http://www.thetubestore.com/index.html

    At least they say they do. I haven't ordered from them yet.

    My SB-12 is acting up too. It's intermittent. Can't find it yet. I recently took it to the shop (it's beyond my limited ability and patience to find the problem). Maybe it's the same problem? If I get it back soon I'll let you know. I have a suspicion mine is thermal related.

    AL
     
  12. Wakarusa

    Wakarusa Member

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    Just for fun, here's an SB12 re-worked with new caps and octal sockets. Had to add the spacers under the cage to allow for the taller 6L6 outputs. It's also got a "tail" for the speaker jack. The cabinet connections are unreliable and we didn't want to drill a hole.


    [​IMG]
    [​IMG]
     
  13. samdjr74

    samdjr74 Supporting Member

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    Yeah, I know those 7868's are hard to find. I wouldn't mind modifing it for a 6L6 but is it a hard job to do?
     
  14. Wakarusa

    Wakarusa Member

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    Not too bad. The ceramics I used were drop-ins for the 9 pin sockets. I might have had to drill mounting screw holes, but frankly don't recall. The original sockets used rivets, but they're either aluminum or cheap pot metal and drilled out easily. Then you just connect up the wires to the right pins. You also need to change the voltage to the bias circuit -- that's the pair of resistors mounted on the rectifier socket in the picture.

    One other thing to keep in mind is that electrolytics, regardless of declared working voltage, get a bit of a memory for the voltage they actually see. If they start seeing a different voltage they can fail. If you swap for 6L6, the bias voltage is different enough that at a minumum the bias circuit caps should be replaced.

    Finally, as noted above, the cage for SB12s that came with 7868 is shorter than the cage on later SB12s that shipped with 6L6. You'll need to either use the shortest 6L6 you can find (the Tung Sol 5881 may actually fit. I seem to recall that the TAD shorties were very close) or jack the cage up on spacers and VERY carefully move the "Ampeg" glass logo down a hole or two. If you want to keep the tone the same as it is now by wiring for 7591A I think the coin-base tubes will fit, but others will require the cage spacers.
     
  15. AL30

    AL30 Member

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    Whoa !! Nice packing job. Spragues?!? I wouldn't of thought they would fit on the board.

    What did you use for the 600V cap?

    Did you notice any tonal differences between the 6L6 and 7868? I think the 7868's are rated at 19 Watts and the 6L6 at 30 Watts? Just wondering what, if anything, the swap did?

    AL
     
  16. Wakarusa

    Wakarusa Member

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    The Spragues don't fit on the board. Note the addition of terminal strips (bolted to the board through the old eyelet hole) to hold the larger caps. The 600V cap is replaced by a pair of 350V caps in series held down with a chassis mount and tie wrap (I hate RTV. Yech.) and a fair amount of heat shrink to insulate everything. It would have been nice to put balance resistors in the rig, but there really wasn't enough room and I figured this setup has enough in the tolerances to live without them.

    While we're poking around another nice change is to correct the wiring of the AC mains to put the fuse in the hot leg before the switch. That's why there's a Westinghouse splice in the one PT lead.

    The 6L6 biased up nicely and sounded just great. They're definitely different than what you get with 7868/7591A (particularly the compressed tone), but there's just so much more variety in available 6L6. At last check the only guys making 7868 are EH(Sovtek) and for 7591A it's EH or JJ.

    One thing I forgot to do while I had the amp was to measure the primary impedance on the OT. The generic value for 7868/7591A is 6.6K plate to plate, so in some respects EL34 would be a better choice, but I don't know if the teeny PT heater winding would take the added draw. KT66 would also sound awesome in these (hi OT primary Z and cathode biased is a killer setup for KT66) but it'd look ridiculous with the cage sitting several inches off of the chassis :) And, finally, when Ampeg switched to 6L6 I think they kept the transformers as before, so they at least thought the change would work :)
     
  17. fullerplast

    fullerplast Senior Member

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    Part of the great tone of the SB-12 is due to those tubes. I'd suggest just finding a source and buying a few spares. EH makes new production and NOS isn't terribly hard to find either. I made a new wood base plate for mine and just use it as a small portable head. It fits perfectly into a small B&D canvas tool bag! Great amps, and way louder than they look. Unique Baxandall tone stack is cool too.

    Here are some 7868 sources to check out:

    source1

    source2

    source3
     
  18. samdjr74

    samdjr74 Supporting Member

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    If I really needed them I'd probably go for the EH's but my tubes checked out ok. Do you think I should have them tested again?
     
  19. AL30

    AL30 Member

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    Tubes are just the easiest thing to check. Pull one out, put one in. Worst case scenario is your tubes are OK and now you've got a set of extras - which you're going to need eventually anyway.

    I think the transformers are listed the same on both schematics.

    I have to agree with that 100%. I've got a bunch of 6L6 amps and the tone is very different. The SB-12 is a really unique sounding amp. What a sleeper.


    AL
     
  20. Wakarusa

    Wakarusa Member

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    So I had to go back and look at my job notes on this one...
    We went with 6L6 because the owner didn't trust the new production 7868 and didn't want to pay the price for NOS. Ignore all of those "good luck" comments. I was drinking at the time.:jo

    FWIW, I've never heard the EH7868 so have no idea how well they do. I've tried the JJ 7591A in my old Reverberocket; they're nice, but I prefer the Sylvanias :)
     

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