Note that a standard 9V transistor battery will likely not last long (perhaps a few hours for an alkaline type) in powering your amp.

I understand. I mainly did it to have a little fun and many times when I go to buy a used guitar off a person, we will meet somewhere in a public place(parking lot, etc) and this would allow me to test the guitar to make sure the electronics are functioning.

 

could I break the trace contact between the pins (scratch through it) or does the circuit need those to be bridged?

I suppose you could, but when it comes to choosing between hacking up a perfectly good PCB and mounting the battery on the outside of the case, I'd choose the latter . If you choose otherwise, this might work:

 

I suppose you could, but when it comes to choosing between hacking up a perfectly good PCB and mounting the battery on the outside of the case, I'd choose the latter . If you choose otherwise, this might work:

View attachment 80856

Thanks! I don't care about the board so I will try your suggestion.

 

Thanks! I don't care about the board so I will try your suggestion.

I built both acharging circuit and batteries inside my tiny guitar amp/ computer loudspeaker case and I can play my guitar anywhere. I made my guitar amp from a salvaged computer desktop speaker/amp that was discarded and it was made to work with a 12V AC adaptor.

I built a couple of them so me and my friend can jam in the park. I used two or three flat li-ion cells like the type in cellphones or for about 10x the power or the 18650 cells salvaged from laptops. They can be easily be float charged(meaning that the charging voltage is set to the fully charged voltage of the batteries at all times) at low currents and a single diode, anode to battery +, cathode to amp +P/S power input is all you need to switch over from battery to AC adapter.

If you have any electronics savvy, a simple LM317 regulaor or even a single transistor with a zener diode(can be made from a series-connected LEDs)_ and a current limiting resistor is all you need to set up safe charging.

All you gotta do is make sure the total voltage applied to the two series batteries does not exceed 8.4V (with two batteries) at full charge or 12.6V (3 li-ion batteries) and a LM317 is easy to set to that voltage. The first one used just a TIP41 transistor with 5 (2-cells but 7 for the 12.6 3-battery version) red LEDs in series to the base of a NPN transistor with a high valued resistor to set the open circuit output voltage to < 12.6V will do the job. The LEDs also show you the charger is charging. By using different colored LED's you can easily find a combination that sets the correct voltage. You need to mount the transistor or regulator on a small heatsink, things can get hot to the touch but not hot enough to burn you.

Its even easier with the LM317 but you can even use a 7805 regulator with a pot in the ground lead, a 1k ohm to the LM7805 output lead to the center 7805 ground lead and 5k pot to ground to set the voltage very accurately.
The most important thing is to make sure you don't apply more than 8.4 V (2-bat) or (12.6V 3-bat)under any charging circumstances and try to match the Li-ion cells from the same place or battery pack.
Limit the current to protect your AC adaptor and batteries from overload or over voltage.
if you add a series resistor, make sure you charge them at <50% of the rated C for best battery life.
The AC adapter I use is rated at 12V, shows 16V open-circuit and a powers both the amp and the charging circuit. While I am playing or not playing and plug in the adapter, the batteries are being charged, I charge them at .1A, leave the adapter plugged in all night and the batteries are fully charged in about 24 hours.
When I remove the AC adapter, the single schottky diode in series with the batteries automatically switches over to battery power.
Some techninerd told me that this idea wouldn't work, I'd end up damaging the batteries by discharging them too much..not really, when the amplifier starts to distort even a low volume I know its time to turn it off and plug in the AC adapter to recharge.

 

I suppose you could, but when it comes to choosing between hacking up a perfectly good PCB and mounting the battery on the outside of the case, I'd choose the latter . If you choose otherwise, this might work:

View attachment 80856

Did this and it seems to be working correctly. Thanks!

 

I built both acharging circuit and batteries inside my tiny guitar amp/ computer loudspeaker case and I can play my guitar anywhere. I made my guitar amp from a salvaged computer desktop speaker/amp that was discarded and it was made to work with a 12V AC adaptor.

I built a couple of them so me and my friend can jam in the park. I used two or three flat li-ion cells like the type in cellphones or for about 10x the power or the 18650 cells salvaged from laptops. They can be easily be float charged(meaning that the charging voltage is set to the fully charged voltage of the batteries at all times) at low currents and a single diode, anode to battery +, cathode to amp +P/S power input is all you need to switch over from battery to AC adapter.

If you have any electronics savvy, a simple LM317 regulaor or even a single transistor with a zener diode(can be made from a series-connected LEDs)_ and a current limiting resistor is all you need to set up safe charging.

All you gotta do is make sure the total voltage applied to the two series batteries does not exceed 8.4V (with two batteries) at full charge or 12.6V (3 li-ion batteries) and a LM317 is easy to set to that voltage. The first one used just a TIP41 transistor with 5 (2-cells but 7 for the 12.6 3-battery version) red LEDs in series to the base of a NPN transistor with a high valued resistor to set the open circuit output voltage to < 12.6V will do the job. The LEDs also show you the charger is charging. By using different colored LED's you can easily find a combination that sets the correct voltage. You need to mount the transistor or regulator on a small heatsink, things can get hot to the touch but not hot enough to burn you.

Its even easier with the LM317 but you can even use a 7805 regulator with a pot in the ground lead, a 1k ohm to the LM7805 output lead to the center 7805 ground lead and 5k pot to ground to set the voltage very accurately.
The most important thing is to make sure you don't apply more than 8.4 V (2-bat) or (12.6V 3-bat)under any charging circumstances and try to match the Li-ion cells from the same place or battery pack.
Limit the current to protect your AC adaptor and batteries from overload or over voltage.
if you add a series resistor, make sure you charge them at <50% of the rated C for best battery life.
The AC adapter I use is rated at 12V, shows 16V open-circuit and a powers both the amp and the charging circuit. While I am playing or not playing and plug in the adapter, the batteries are being charged, I charge them at .1A, leave the adapter plugged in all night and the batteries are fully charged in about 24 hours.
When I remove the AC adapter, the single schottky diode in series with the batteries automatically switches over to battery power.
Some techninerd told me that this idea wouldn't work, I'd end up damaging the batteries by discharging them too much..not really, when the amplifier starts to distort even a low volume I know its time to turn it off and plug in the AC adapter to recharge.

That sounds killer, but is probably above my skill level as of yet. I may think about trying it in the future though.