Over on FB a guy is selling 6418 tubes for a buck a piece. I had the thought that 10 in a row might make a good OTL amp. Opinions??!
The 6418 is a tiny little tube that was used in vacuum tube hearing aids a long time ago. Typical power output is 1 to 2 MILLIWATTS at 12% THD into a 100K ohm load. You would need thousands of them to drive an 8 ohm speaker directly. I got a sealed bulk pack box of 100 for about $20 a few years ago, so $1 each is too expensive.
I have tinkered with them and some other old hearing aid tubes for use in a vacuum tube based music synthesizer. Other than the mentioned drawbacks, they tend to be quite microphonic due to the filament that's thinner than a hair.
I have tinkered with them and some other old hearing aid tubes for use in a vacuum tube based music synthesizer. Other than the mentioned drawbacks, they tend to be quite microphonic due to the filament that's thinner than a hair.
I bought a lot of these as well for quite a bit less than $1 each. I'll sell you some of you want to experiment.
They do tend to be microphonic, as Tubelab indicated, but they sound nice when used as gain devices and are fairly linear when wired as triodes. My experience was that the microphony would die down fairly quickly after an impulse and wasn't an issue if you weren't constantly tapping on the amp. Some tubes were better than others in this regard. I used them as the voltage gain device in a headphone amp with MOSFET outputs with decent results. I still have PCBs for that project and a complete set of parts for a second one that I never built, which I could let go for a decent price if you're looking for a project.
They do tend to be microphonic, as Tubelab indicated, but they sound nice when used as gain devices and are fairly linear when wired as triodes. My experience was that the microphony would die down fairly quickly after an impulse and wasn't an issue if you weren't constantly tapping on the amp. Some tubes were better than others in this regard. I used them as the voltage gain device in a headphone amp with MOSFET outputs with decent results. I still have PCBs for that project and a complete set of parts for a second one that I never built, which I could let go for a decent price if you're looking for a project.