• 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.

6an8 hi-level stage w. pentode CF

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anyone here try similar for fun? -this one was in my note book but never wired it

I started out with a schematic like this about 20 years ago, maybe even the exact same one, for use as the input / driver stage in a single ended amp based on the 45 tube. The initial experiment started with a 6U8 tube since I had a few hundred of them. I then separated the triode and pentode sections onto separate breadboards to make it easier to work on, and test one board at a time.

It is known that the "ideal" load for a triode tube is a constant current, so I began experimenting with current regulator diodes like the 1N5314. They proved instrumental in getting lots of gain with low distortion out of triodes like the 12AT7 and 12AX7. They just had a problem with blowing up under the voltages needed for my design. A visit to the Motorola semi sales engineering guy (I worked in a Motorola plant) got me a bunch of high voltage mosfets that I could use to make current regulators. They lived, but didn't perform like the diodes. Somewhere around 2003 IXYS invented the IXYS 10M45S, and the problem was solved. I stuck one in place of the 33K 2 W resistor and history was made.

The pentode is a classic cathode follower. The main criteria for tube choice in a cathode follower is high Gm. What device has more Gm than any tube? Yes, I stuck a mosfet in place of the pentode, and magic happened.

After about a year of tweaking the circuit wound up in an amplifier I called the TSE (Tubelab Single Ended) and now the TSE-II.

Really hate silicon? The plate load CCS could be made with a tube. Look for a pentode with nearly flat plate curves over the voltage range needed at the current where you want to run the triode. The follower could be made with a pentode, just like in the original circuit. Look for a frame grid pentode with a Gm in the 15,000+ range.
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