Ampeg B18N 4 Conductor Speaker Cable Hookup

[Northernpike]

I recently picked up a 1965 Ampeg B18N. The 4 conductor speaker cable had been removed. I've seen it installed several different ways on schematics.

Does anyone know the correct way to install it to the board and speaker jack?

Thanks,
Tom

[Leo_Gnardo]

The way I've seen on various B15 and B18, I s'pose B12 and others too, speaker - is pin 1. speaker + pin 4. pins 2 and 3 form a sort of "interlock" so that the amp can't pass signal to the speaker unless the 4 pin cable is plugged into a mating connector with a shorting wire jumping pins 2 and 3. On the amp side, either pin 2 or 3 goes to ground and the other to the cathode resistor of the tube that boosts the preamp signal on its way to the output drive section. With that resistor not ground referenced, no signal passes on to the power amp and presumably the amp can't be damaged, at least in that way.

It's possible to cheat the system so you can use ordinary quarter inch plugs & jacks or any other speaker connector of your choice. Simply locate the cathode resistor used for Ampeg's interlock scheme, and solder its ground end to ground, that's all. And don't forget to plug in your speaker of course.

[Enzo]

I always remembered it as enabling the B+. Looking at B12N/B15N, the link grounds the PT HV secondary CT. The cable is always the same, 2 shorted to 3, but the circuit I think depends upon the model.

[Ted]

I always remembered it as enabling the B+. Looking at B12N/B15N, the link grounds the PT HV secondary CT. The cable is always the same, 2 shorted to 3, but the circuit I think depends upon the model.

Thats how it was wired on the early ones (pre 64-ish)- it's in series with the standby switch so it genuinely disables the B+. The downside is that you can get a pretty significant belt off the back of the connector in the speaker box when in standby. The later models disconnect the ground to one side of the PI, - it just breaks the signal to the output stage and full B+ remains on everything. You can still get a slight tingle from touching the wire but its nothing like as unpleasant as on the early ones.