Ampeg bandaxall tone stack

[Randall]

In '62 according to the Piazza drawings, the B-15 had the tone stack between the preamp triodes and had a 4 Meg audio treble pot driving the second grid.

http://www.unofficialampeg.com/schematics/b-15-nc.gif

In '68 they put it between the second triode and the PI.

http://www.unofficialampeg.com/schematics/b-15-n68.gif

It looks the same, except now there is a 1 Meg audio pot in there. I want to build the '62/64 style circuit. I have never seen a 4 Meg audio pot. What can I do?

[Enzo]

Use a 1M or 2M pot and see what it sounds like. If you like it, fine, if it lacks something, at least you hav a working amp while you hunt for the odd pot. If you can find a 5M pot, put some resistance in parallel with it.

[J M Fahey]

The Piazza drawings, although well meant and a good resource, have some errors here and there, probably because they are based on almost unreadable photocopies of photocopies of photocopies from an already soiled and rat chewed original .
The correct value is 1M pot, and 470pF/4700pF caps, in both amps (and in others).

[Randall]

Thanks Juan, that's kinda what I was hoping. If that's the case then it certainly makes it easier. I wish I could find an original B-16NC to check for any other misprints, but from what I understand from Ampeg, they often made changes and subbed out values on the fly with no documentation. So from what both you and Enzo suggest, I will go with 1 Meg and take it from there.

Speaking of mistakes, take a look at the ext speaker jack 8/16 ohm arrangement. Am I wrong, or does that have the two speakers in series and out of phase when a second one is connected? If so, yuck!

[Enzo]

Looks that way on the originals.

[Tom Phillips]

... take a look at the ext speaker jack 8/16 ohm arrangement. Am I wrong, or does that have the two speakers in series and out of phase when a second one is connected? If so, yuck!

Well, there is no polarity marked on the speaker schematic symbol hooked to the 4 pin connector so I can't really verify how the main speaker is wired. I agree that the extension speaker jack is wired backwards from the norm but maybe the internal speaker is phased to match.

[Randall]

Take a look at the second schematic, 1968 version.

[g1]

Was thinking the same as Tom, the '68 B15N schem. could be a typo (it's not just the Piazza drawing, the Ampeg drawing shows the + on int. spkr. also). Maybe the int. spkr. was wired out of phase, or maybe the sold a specific ext. cab that was wired out of phase.
Or maybe the way the ext. jack is drawn is a typo.
But to be sure, if it was a real issue, a lot of bass players would have noticed the loss of bottom when using an external cab and I'm sure it got sorted out!

[loudthud]

Several of the older Ampeg schematics show a 4 Meg pot for the treble control. Probably impossible to find, especially in an Audio taper.

Take a close look at the way the extention speaker is wired. It's pretty ingenious the way it accomplishes connecting the extension and switches the impedance with a standard jack. The sleeve needs to be insulated from the chassis however. In many models the B+ is cut off unless the standard speaker cabinet is plugged in with the 4 pin XLR or octal plug.