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Ampeg B-12-XY: Recapping

Just for completeness, here’s the original underside:

Ampeg B-12-XY - Underside - old caps

Ampeg B-12-XY - Underside - old caps

And with the new caps, many from Eks’ stash and a few from mine:

Ampeg B-12-XY - Underside - new caps

Ampeg B-12-XY - Underside - new caps

With all those in place, the firebottles lit up properly, the power tube plates remained dark, and it sounded great. The edge-lit engraved acrylic panel in the middle is a wonderful custom mod!

Ampeg B-12-XY Firebottles

Ampeg B-12-XY Firebottles

It’s in mint condition, with the original footswitch and a remote Echo speaker box with a pair of drivers:

Ampeg B-12-XY - ready to rock

Ampeg B-12-XY - ready to rock

It still has those original huge electrolytics, though. Eks says the best test comes after half an hour: if the cans remain cool, the leakage and ESR will be good enough.That’s the case, so we’re rolling with them. However, the amp has some residual hum that the Hum nulling pot can’t remove, plus a bit of noise, which means those ‘lytics probably hover at the bare minimum values required to keep it going.

I discovered (inadvertently, of course) that swapping the two identical 6D10 triple triode tubes killed the Vibrato oscillator. That triode would oscillate for a few seconds after the footswitch grounded the cathode, but one tube didn’t have enough gain to keep it going. More likely than not, the feedback resistors have increased in value, too. Swapping the 6D10s restored it to operating condition.

My Shop Assistant compared her tiny DSP Fender amp with this monster and concluded that DSP effects only sound good when you don’t have the original for comparison. Of course, you could lose that tiddly amp inside the Ampeg’s speaker case.

I should’a learned to play the guitar…

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  1. #1 by peter on 2011-08-03 - 12:43

    You’ve been very generous with the silver-mica capacitors, by the looks of it :-)

    Those will provide many years of reliable service.

    • #2 by Ed on 2011-08-03 - 22:02

      Eks is a heck of a nice guy…

  2. #3 by hexley ball on 2011-08-03 - 18:00

    >>if the cans remain cool, the leakage and ESR will be good enough.
    Time to break out the ESR meter you described in CC recently?

    >>the amp has some residual hum
    I figure that **all** guitar amps hum. No, it’s not because they don’t know the words — it’s the nearly 80 dB of gain following an input with a 100K impedance… It’s a pity Les Paul didn’t use balanced signals and XLR connectors on his pioneering rigs — guitar pickups would be perfect drivers for differential inputs.

    • #4 by Ed on 2011-08-03 - 22:07

      break out the ESR meter

      Those big cans were an ohm or two before reforming; I didn’t have the heart to check ’em afterward…

      I did swap out all of the lower-voltage electrolytics, regardless of ESR, and that helped a lot.

      80 dB of gain following an input with a 100K impedance

      And at 140 dB SPL, that 120 Hz hum keeps the speaker cones from settling in place…

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