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There is a thread over on Doug's forum about a battery amp. The original poster wanted to get rid of heater hum and the like but I took it in a different direction.
I gutted a tiny little cig lighter 12v to 120v 100watt inverter. It had two halves with only a transformer connecting them. I desoldered the transformer then cut away the 120v half of the PCB and was left with a current limited 12vdc in 275vct out supply. The 275vct out was in the form of a 100khz square wave- so I connected some diodes and now I have a DC powered 275 volt supply for tube amps.
I hooked the thing up to a small amp I built a while back- it's got a 5879 out front, a gain stage and CF ala marshall, a LTPI and a cathode biased pair of 6aq5's. I only used the battery to supply the B+ voltage- future versions will supply B+ and filaments.
It sounded great, better than I expected. I was rocking out for about 45 seconds when the my cheapo bridge rectifier blew up. I swiped some ultrafast diodes from a junk computer supply and built a new bridge.
The inverter was surprisingly stiff. It idled around 71 ma, rising to around 95 ma with the output section clipping heavily- voltage dropped only about a volt between rest and maximum output. I might have to reduce the 1k screen grid and 1k screen supply dropping resistors and try for more power! On the battery side of things I was drawing 1.75 amps at rest and about 2.5 at full load. Very nice!
My goal is to build a small cathode biased tube amp with 8 to 15 watts that can run using one or two valve regulated lead acid batteries. With this little inverter I should be able to make something that draws between 3.5 and 4.5 amps total and have a nice long run time of 2-3 hours using inexpensive surplus rechargable batteries. A smallish pedalboard will cover the back of the combo when not in use so I can leave the thing in my trunk for impromptu jam sessions. I could include an inexpensive 6 amp battery charger or 12v supply to use the amp "plugged in" when needed.
Here is my other thought- why haven't we been using switch mode stuff for tube amps? I'm about to build a large bass amp and it would seem that a very stiff B+ and screen supply would be a wonderful complement to a large output transformer while cutting back on the heft required to run 4 to 6 kt88's at high voltages.
A switching supply for a guitar amp needn't be advanced. I'm picturing something not too far off from a bigger version of my inverter- little in the way of voltage feedback, basic current limiting in the event of a dead short, windings for 6.3 volts, 400 volts and 650 volts. Perhaps a rewound computer transformer or two.
I've read about using dimmable electronic transformers but I thought I'd get some opinions before I try to build anything else.
jamie
I gutted a tiny little cig lighter 12v to 120v 100watt inverter. It had two halves with only a transformer connecting them. I desoldered the transformer then cut away the 120v half of the PCB and was left with a current limited 12vdc in 275vct out supply. The 275vct out was in the form of a 100khz square wave- so I connected some diodes and now I have a DC powered 275 volt supply for tube amps.
I hooked the thing up to a small amp I built a while back- it's got a 5879 out front, a gain stage and CF ala marshall, a LTPI and a cathode biased pair of 6aq5's. I only used the battery to supply the B+ voltage- future versions will supply B+ and filaments.
It sounded great, better than I expected. I was rocking out for about 45 seconds when the my cheapo bridge rectifier blew up. I swiped some ultrafast diodes from a junk computer supply and built a new bridge.
The inverter was surprisingly stiff. It idled around 71 ma, rising to around 95 ma with the output section clipping heavily- voltage dropped only about a volt between rest and maximum output. I might have to reduce the 1k screen grid and 1k screen supply dropping resistors and try for more power! On the battery side of things I was drawing 1.75 amps at rest and about 2.5 at full load. Very nice!
My goal is to build a small cathode biased tube amp with 8 to 15 watts that can run using one or two valve regulated lead acid batteries. With this little inverter I should be able to make something that draws between 3.5 and 4.5 amps total and have a nice long run time of 2-3 hours using inexpensive surplus rechargable batteries. A smallish pedalboard will cover the back of the combo when not in use so I can leave the thing in my trunk for impromptu jam sessions. I could include an inexpensive 6 amp battery charger or 12v supply to use the amp "plugged in" when needed.
Here is my other thought- why haven't we been using switch mode stuff for tube amps? I'm about to build a large bass amp and it would seem that a very stiff B+ and screen supply would be a wonderful complement to a large output transformer while cutting back on the heft required to run 4 to 6 kt88's at high voltages.
A switching supply for a guitar amp needn't be advanced. I'm picturing something not too far off from a bigger version of my inverter- little in the way of voltage feedback, basic current limiting in the event of a dead short, windings for 6.3 volts, 400 volts and 650 volts. Perhaps a rewound computer transformer or two.
I've read about using dimmable electronic transformers but I thought I'd get some opinions before I try to build anything else.
jamie
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