Ampeg B-12-XY: Echo Circuit

Mad Phil asked me to fix up his trusty Ampeg B-12-XY (*) bass guitar amp, having recently fired it up and discovered that the power output tube plates glowed red-hot. I’d planned to replace the electrolytic caps, but Eks, who does this sort of thing all the time, suggested that leaky interstage coupling caps can also cause that problem; the leakage wrecks the phase splitter bias and thus kills the drivers.

While poking around in the amp I found that the Echo hardware circuitry doesn’t match the schematic for either the B-12-X or B-12-XY. Mad Phil says that’s probably because he had the factory upgrade his original B-12-X to a B-12-XY for the munificent sum of $25, back in the day. It’s unlikely you’ll ever need this, but here’s what I found:

Ampeg B-12-XY - as-found Echo circuit

Ampeg B-12-XY - as-found Echo circuit

The topology resembles the -XY schematic, but with different tube sections and part values.

The Echo unit over there on the left consists of two springs with magnetic transducers on each end, evidently made by the Hammond Organ folks, who should know something about reverb. This is the bottom view, with the unit attached to the board that supports the amp chassis:

Ampeg Spring Echo Unit

Ampeg Spring Echo Unit

The input transducer, just in case you forget to label the ends before you take it apart:

Ampeg Spring Echo - input end

Ampeg Spring Echo - input end

And the output transducer:

Ampeg Spring Echo - output end

Ampeg Spring Echo - output end

Getting the thing off the speaker box posed a bit of a problem. Remove the four big screws holding the chassis to the board, tilt it carefully forward, hold it in place while you remove the six nuts-and-washers from the vibration isolators, then transport the whole disjointed affair to the workbench. Turns out you (well, I) can’t get the RCA plugs out of the Echo unit’s sockets from the top of the board, but the unit’s mounting screws are on the bottom of the board, where you can’t get to them before you remove the board. Of course, the cables leading to the aforementioned RCA plugs tether the chassis to the Echo unit with pretty nearly no slack at all.

With everything apart, I rounded the ends of the RCA plug cutouts enough to get them out from the top the next time around, with the board screwed in place atop the speaker box:

Ampeg Spring Echo unit - top view

Ampeg Spring Echo unit - top view

After putting the whole thing together with new caps, the Echo circuit didn’t work. I had cleaned the contacts and connectors, but Eks showed me how it’s really done. Apart from the rotted caps, all the other problems came from minor corrosion in switches, connectors, and tube sockets. Now I know better.

* Yes, the model numbers really end in X and XY.

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  1. #1 by peter on 1-August-2011 - 12:37

    The usual cause of the power output amplifier’s anode glowing red is indeed because of a leaky coupling capacitor (the 1nF one in your schematic, between the anode of V8 and the grid of V9). It only needs to go very slightly leaky to pull the grid up from negative to positive bias, greatly increasing plate dissipation. Usually, the grid of the output power valve is at high impedance, with a ca. 1 Mohm resistor to ground, so even a very slightly leaky coupling capacitor, allowing anode voltage of V8 onto the grid of V9, can cause the output amplifier valve to draw excessive current (and thus anode power, resulting in a nice cherry glow of the anode….)

    The problem is so common that people who restore valve radios (I’ve done a few in my time) call it THAT capacitor. Say ‘THAT capacitor was leaky’ and everyone will know what you’re talking about. :-)

    It’s a pity, because sometimes it not only destroys the output valve (which can actually handle quite some abuse) but also the output transformer. And sometimes (often?) those are *very* peculiar transformers, with sometimes extra windings for tone control, feedback, etc. Philips engineers were notorious for their creativity when it came to original new circuits (and output transformers).

    A case of a 1 cent capacitor causing many dollars of damage….

    Love the reverb spring. To Hades with bucket brigades! :-)

    • #2 by Ed on 1-August-2011 - 21:16

      call it THAT capacitor

      Eks and I came up with enough small value / high voltage caps to replace anything and everything we thought might possibly contribute to the problem: shotgunning the board worked wonders.

      those are *very* peculiar transformers

      The Echo circuit has a tiddly little output transformer, but the main audio output goes through a block of iron that’s indistinguishable from the power step-up transformer on the other end of the chassis. Individually, each of those things weigh more than any other piece of electronics I’ve handled in recent memory. It took Mad Phil’s son hauling on the other handle to help me jackass that amp back into his SUV: Archimedes would have been ashamed of us.

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