Does the amp take power off the 12V battery or does it get powered by the raw DC from the transformer when plugged in?
The amp works strainght from the battery. But I do not know if it still gets power from the battery when the charger is plugged into AC. The "charger" is just a wall wart transformer.
What size battery?
The battery that was originally inside the amp was smaller and it died. I put in an 8Ah battery and I believe the old one (which I no longer have) was half that physical size, so I guess it was 4Ah.
Was it OK to just put a 2x larger battery inside?
I would use the generic LM317 battery charger circuits (with output backfeed diode) that use a transistor for current-limiting. They use a resistor in the (-) to GND which would not work if the amp gets power from the transformer/bridge rectifier ground, like on your schematic.
Would that work if the amp was being used at the same time as the battery was being charged?
In fact, it always puzzled me if batteries can actually be charged at the same time as they are being used. Can they?
LM317 can do this.
The LM317 can also do this (from the LM317 datasheet):
I'm not sure how to interpret this.
Thanks...