I'm suprised one of the other Series owners didn't respond sooner, but here's the scoop on the Series 1/4 jack.
I don't think there's any question that it's the most complicated output jack on any bass anywhere. Basses that have active electronics and mono out often use the ring contact as a battery switch. When you plug a mono 1/4 plug in there you short together the ring and sleeve (ground) contacts. So, on one of these basses, you wire the battery + to the ring and when the contacts short together you have a power circuit and the battery is turned on.
On the original Series basses tip, ring, and sleeve are used as they normally are for something like stereo headphones. The bass pickup appears on tip, bridge pickup on ring, and ground on sleeve. To switch on power, there's an independent switch on one of the main contacts - when the plug is inserted, the contact is bent slightly and it throws another switch to turn on the power. I believe there's also another switch which is switching between the battery power and the external power as well. I haven't done this in a long time, but I believe if you have both the 5-pin and the 1/4 connections made it will be running off the batteries instead of the external power (e.g., any time something is plugged in on the 1/4, power is coming from the batteries). The wiring on the 1/4 jack looks like a forest.
This is why older Alembics only play the neck pickup when you are plugged in with a regular guitar cord. Perhaps Ron was trying to do a favor for future Series buyers - when you try it out in the store, you can tell the salesman Hey, you want $2000 for a bass with only one working pickup? and end up getting it cheap. :-)
This was OK up through the 80's. I suspect around that time, wireless units were becoming very popular and they're enough of a pain that you don't want to have to modify the transmitter plug. I built a little adapter, duplicating the innards of the power supply box and it worked fine, if only for short periods of time. I don't know that a lot of people really needed true independent pickup outputs anyway (two SVTs are at least twice as stupid as one), so by the end of the eighties, I think Alembic started wiring the 1/4 jack to have both outputs on the tip and all was happy with guitar cords and transmitters. You still get independent outputs on the 5-pin connector anyway. I'm kind of surprised that they didn't add a few more internal switches in the jack so that it would put mono out on a mono cord and switch to independent outputs with a stereo cable.
Back when all this stuff was originally designed, the only rechargeable batteries were NiCads. These are the ones that suffer from memory so you really need to charge them fully, then discharge them fully and repeat or you rapidly lose battery life. Nowadays there are NiMH and lithium secondary batteries that don't have memory problems but require a power monitoring chip in the battery pack to charge properly. I guess it could be done, but would be expensive.
Battery life is *very* short on the Series, as the onboard electronics were designed for sound rather than to save power. Your typical EMG-equipped instrument can easily get more than 1000 hours from a single alkaline 9V. With my Series, I think you're doing really well if you get 40 hrs of playing time on two batteries.
The voltage coming out of the DS5 box is higher than 18V (I can't remember the value off the top of my head, but 30V sounds familiar). When you're on external power, I think you'll probably get better headroom and dynamic range, but may not notice the difference. Strangely enough, I find that the output level is usually higher with the batteries than the external power.
