Is there any particular reason you can't/don't want to use a higher B+?
Compact PCB mounted transformers are commonly available and ideal for this sort of application; take a look at the hammond 229A230, it's available for around $15.
At 30v, you're going to be operating in a very non-linear region unless you increase the current to something crazy high.
You'll also be driving the grid positive at times which should generally be avoided...
For the cost of the transformer plus a few parts, you'll really free up your options for the preamp design.
Compact PCB mounted transformers are commonly available and ideal for this sort of application; take a look at the hammond 229A230, it's available for around $15.
At 30v, you're going to be operating in a very non-linear region unless you increase the current to something crazy high.
You'll also be driving the grid positive at times which should generally be avoided...
For the cost of the transformer plus a few parts, you'll really free up your options for the preamp design.
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Yes, I've been considering something similar for a tube amp build---really like the fat tone of Fender's Supersonic but would like to find a simpler, lower cost way of doing the same thing. Fewer tubes with fewer and lower B+ rails with a solid state power stage seems like one possible way of achieving that. Though from the handful of 12AX7 datasheets I've looked at so far I was thinking more like 150 or 200V.
the lowest ive ever ran a 12ax7 successfully at with not bat results was at about 80v, thats why im considering a 12au7, ive used them a few times in a 12v application. but only with one tube (in things like a booster pedal or driving a big EQ). im not sure about multiple tubes to get a high gain sound though, or what it might sound like...
It was suggested that one may want to use the low voltage space charge tubes for this type of application. They are cozy with 30VDC for plates. I haven't tried one myself yet but I do have several schematics of a low voltage 24vdc tube amp around here somewhere that use them in a push pull configuration. The 24vdc amp was even shown on YouTube. the designer built both a 24vdc and a 12vdc guitar amp, which he also demoed.
well i have been designing a preamp with as many tone options as possible, if your interested, basically, cathode caps on a rotary switch to switch between them, along with dpdt switches to change coupling caps. as well as a 100k pot with a 470pf cap to ground on the first attenuator stage (from the wiper of the gain pot to ground) basically i was looking for as much preamp tone control, without having a typical tone stack. im not sure about pots for power rails as you may need some pretty high power pots, though putting some resistors on something like a 3 way dpdt (on/off/on) on different filter stages might be a better idea.
An LM317 with a variable reference created via pot will suffer breakdown well before the thermals of supplying a signal tube are a problem. If you get into cascodes like what tomchr's doing some attention to the pass device may be needed when dropping B+ by a couple hundred volts. That's fairly extreme tunability, though.
Hadn't thought about using switches over caps (or jumpers if B+ is low enough for that to be safe). Thanks for the suggestion.
Hadn't thought about using switches over caps (or jumpers if B+ is low enough for that to be safe). Thanks for the suggestion.
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