• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Battery powering 12AX7

While I really like the idea of battery powered devices in audio, there can be some challenges as observed with the voltages required for a 12AX7 preamp. Admittedly, I have never made one, but instead went with a tube that could make use of much lower voltages. The 3S4 tube comes to mind, requiring only 3 volts for the heater, and is happy with 36 volts for the rest this DHT. Of course, I am describing the old 'Quickie' design by Bottle Head. 4 9volt batteries, and a couple of D cells and your on your way. The sound is more than you would expect.
 
They are always improving on batteries lately, so why not put some to use in an old audio circuit that is notorious for hum? 3.6 v lithium ion batteries are only a couple dollars a piece, so I could buy 30 of them to run in series to get to 108 v. Then I need a charger.
 
To back up to the beginning.. You have shown the amp you need. But where comes the 60V battery in the game ?
Why 60V ?Is mains unavailable or do you seek some esoteric value in running from DC ?

If it's lack of available AC mains the "classic solution" is 12V battery and some dc-dc converters, one for filament and
one for B+. Do you need more voltages ? The maybe another dc-dc does the job.
Using switching dc-dc is the way to keep battery consumption low.

Now, tell us what is the goal and what is the limits.
 
I’ve built this with a conventional power supply, but nothing I do gets the hum to a reasonable level. Therefore, I’m going to try to power it with a battery. I’m not familiar with dc-dc that you refer to. I need 18 v and 108 v for the two sides of the triode at the plates and I need a filament supply.
 
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It should be easier to track down the source of the hum, than to come up with all that.
Not to mention all the problems with using batteries, even if it does work.
Using a DC/DC converter could also introduce other sorts of noise problems.

Post all the schematics here, and some photos.
 
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Also, charging lithium ion cells requires individual charge control per cell for safety. So you'd need to charge (say) four at a time using a commonly available charger, or add charge control circuitry to your battery bank to monitor each cell while bulk charging. A bit of a pain when you have 30 cells.
 
I’ve built this with a conventional power supply, but nothing I do gets the hum to a reasonable level. Therefore, I’m going to try to power it with a battery. I’m not familiar with dc-dc that you refer to. I need 18 v and 108 v for the two sides of the triode at the plates and I need a filament supply.
Now i understand.
Well, chasing hum could be irritating.
I'd suggest you do it in discrete steps evaluating each step.
Some simple things could be done in no particular order :
  • use a battery , 6v lead acid is perfect, to power the filaments.
  • is the hum 60hz or 120 hz ( or both ?) 60 hz is from leakfields from the transformer. Moving or repositioning
the transformer might change hum level. 60 hz could leak from filament connections , will disapear if filement battery is ised. 60 hz may also come from improper routing of mains voltage.
120 hz on the other hand comes from improper filtering of B+, bad groundings. Use a oscilloscope to examine B+
see if groundings are done properly "single point ground" , ground loops etc.

Most important, do one thing at a time and measure the resulting hum.
 
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Also, charging lithium ion cells requires individual charge control per cell for safety. So you'd need to charge (say) four at a time using a commonly available charger, or add charge control circuitry to your battery bank to monitor each cell while bulk charging. A bit of a pain when you have 30 cells.
Thanks. Thats an interesting point. I'll have to look into that
 
I've spent a lot of time looking for hum in this circuit, and I've given up. My next step is going to be powering the whole thing with a battery. I tried a lantern battery for the filament supply, which helped, but I was never able to cure the hum when plugged into the mains. If you look at the bogen schematic I've built preamps using the microphone input circuit, and the hum is controllable. The line level circuit has about three times the hum with the same transformer and choke. I'll post a schematic of the conventional power supply that I use tomorrow.