DIY Mic Pre Kits, Lunchbox Mic Preamp Kits, Power Supply Kits

We'll see what our thermionic friends can do....

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A true (high plate voltage operation) tube preamp Kit with a reasonable price tag could be a huge hit.

I think you mean Tubes

12AX7A's I gather

Did you ever get anywhere with the tube microphone preamp idea? After seeing the price of most high end tube preamps, I'd be very interested in building one. I can't seem to find any schematics online though.

Which 7 pin tube are you working with in the picture? I bought a ton of 6C4 tubes which are the equivelent of half of a 12AU7. I made a 1 watt push pull amp from 2 of them but never did more than that with them.

I almost forgot. There are a lot of battery tubes from the 30s and 40s that can run on less than 100 volts if you wanted to keep the power supply fairly simple. The supply would be limited but the demand is almost nonexistant so they are fairly cheap. I bought the parts to make a hi-fi preamp from 6088 subminiture pentodes. They were once used in encoding equipment used by the military until the 1980s.

The only problem with battery tubes is their limited amplification and possible microphonics.

I almost forgot about this. I have a military R-392 mobile vacuum tube HF receiver that uses a lot of 12AU7s. It runs on 24-30 volts at about 5 amps. The tubes seem to function fairly well at lower voltages. I think they had to use more stages to make up for loss in gain from low plate voltages.

I could imagine when using low plate voltages in audio, it would take very little signal to cause the sound to distort. I think I've even heard of effects pedals that use a preamp tube with a low plate voltage to get distortion on purpose.