Audiofile99, yes you can increase the voltage by adding batteries in series. With 12v batteries, 2 in series would give the 24v you are talking about. And yes, you can parallel the batteries for more run time. No danger in doing this. There is no limit to the number of batteries that you can connect in series or parallel, except for the space required for a large battery bank. Connections should be short, and kept clean and tight. For the higher voltage requirements, the 70-80v should be no problem with several series batteries. The 600vdc you refer to for some tube amps is generally stepped up inside the amp from some lower voltage that is supplied. Just supply the necessary supply voltage, and the amp will kick it up to the high voltage. But remember that whatever voltage you are dealing with, the current requirements must also be met. 1 amp at 600v is going to be 50 amps at 12 volts, or 25 amps at 24volts, or 12.5 amps at 48 volts, etc. The entire package needs to be considered for the parameters involved, including wire guages and wire run length with losses calculated. This is not to be off-putting, but just a necessary part of doing the job correctly. Glad you are interested. Please let me know if you have some successes with your experiments. If I can help you, don't hesitate to email me.