Hi all,
I was playing with a 12J8 tube before and found that it did okay. I was using those little 1K:8 audio transformers that Radio Shack used to sell, which are capable of a few hundred mW into a speaker. Not ground-breaking, but decent for headphones. It's altogether possible that a larger transformer could provide better fidelity and soforth.
https://frank.pocnet.net/sheets/127/1/12J8.pdfAnywho, I am proposing a headphone amplifier that will use these as output tubes. I would like to implement some kind of gain stage prior to the tetrode so I can effect some negative feedback. That might make the tiny transformers more willing to perform well. I do remember that the sound was kind of rich in the highs and I could mellow it out by placing a 100nF capacitor from the plate to ground. I might look for a 12U7 (which I'm convinced is a 12AU7 proven for low-voltage) or I might use one of my clear-top RCAs.
Either way, I will provide more information when there is more information. I'd like to make this one exceptionally hum and noise free, something I haven't achieved with other tube amps I've made. The last headphone amplifier I built using tubes was a 12AU7/IRF510 model. IIRC, the gain of a 12AU7 is something like 5 at 12 volts if the next stage is a high-impedance load like a MOSFET or even an LM386.
I'd consider getting better transformers, but I also have no interest in getting Edcor transformers that will cost at least $50-60 for two models (when shipping is factored in) and will take months. Maybe some feedback from the experts here could be helpful.
Ed