You do have a talent for coming up with solutions that are the biggest pain in the ringpiece imaginable. 
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Or just get the battery holder with flying leads and plug in the same type of battery it had before.
I would have thought that modern NiMH batteries would last weeks, so it shouldn’t be too much of an imposition tbh
I had read up on this previously and thought the system employed must be the overnight charge method . there is no apparent circuitry for smart charging inside the battery pack box - I am relatively certain that I am using the original charger which is simply a wall charger 24v x 0.15A up to now and it’s worked for at least six years.
There has to be some kind of current control, even if it’s just a resistor - you can’t just put 24V into 19.2V pack of NiMH batteries without causing some kind of harm.
Be aware that if you change the capacity (from 600mAH) you would ideally need to adjust whatever controls the current to set it at the right level.
Do you actually understand the concept of capacity?
Looked again and cannot see anything other than wires batteries and switches on it .
I wish I could be 100% certain this was the original power supply but I cannot.
This is my second BLA pre - and it came to me from the bloke that Gthang @ghtang sold it to, he may have a memory of if it is the orig supply adaptor and verify?
This is turning out to be a pain
Stop being a pussy - wire it up to the mains with yourself as part of the circuit: see what pans-out.
FOR THE GLORY OF THE JAPANESE EMPIRE!!!
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Maybe the power supply current limits and they’re using that to control the charge rate. 
Frankly it seems pretty under-designed.
You’d actually be better off buying a couple of cordless tool batteries (18V) plus a charger and having a socket on the preamp to plug them into.
Or you could build a new PSU with the charger gubbins and battery internals and switching to switch the packs between charger and pre. I doubt it would fit in the current box and unlikely to be cheap.
Maybe time to cut back on the amphetamines, m’kay?
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Not knocking your suggestions, but by the time you’ve done all of the latter, is the PSU really that much quieter than a well-implemented linear PSU?
I’ve had a Pro-ject and a Trichord phonostage that had battery PSUs, and I could discern no difference in performance between them with/without the battery PSUs…
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Pah, you have tabs of the cloth ear variety 
I can’t reliably remember, just looked back through my emails. It was august 2010 that I purchased it (10yrs ago ffs!)
I don’t recall needing a 3 pin mains adaptor.
Are you sure it was mine originally?
While partly gibberish, this is also probably true…
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I seem to remember discussing with you when I bought it and my seller and your buyer were the same person I might have dreamt it though - There was a foible yours had also that helped identify it but I cannot remember what it was perhaps it was that one battery did not read so well on the meter?
Well, if the point is that it’s a battery pre, then changing to a mains supply means it’s no longer a battery pre. They certainly have some advantages over mains supplies, but it depends on how well designed and how sensitive the actual circuitry is to power supply related factors such as noise.
I’m guessing Andy doesn’t want to change the nature of it, but even substituting different battery packs will potentially result in changes to performance.
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Maybe
Or that the switch had to be held in place, i think the correct term is a momentary swich to check the voltage?
Another suggestion: 19V laptop supply. Albeit that may be noisy unless there’s some regulation in there.
EDIT: I mean to power the pre itself, NOT as a charger.
I have thought about that and I have a number of 19v ones about
It would be better than no pre amp which is what I have now.
Another option is to replace just one side with 4 packs of AAAx4 and be careful with the charging.
I do think its a decent pre and worth some effort but OBS not worth spending too much on
I do have a soft spot for this pre amp. I remember a bake off at Gthang’s where I brought it - It went down very well
Try the laptop thing and see if it works, you never know.
Substituting batteries and messing with charging schemes can be a dangerous game. While it’s likely fine to substitute like for like in terms of packs of voltage and capacity, for the sake of safety, reliability and performance, you do need to make sure you account for any differences.