• 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.

304TL under test


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Jeffrey Jackson has some published curves that will make your mouth water.

Type 304TL is gettered by coatings on the anode, so the beginnings of a 304TL project need to include a procedure to exercise the getters at red-plate while observing grid current. (I hope that's good enough.) My build partner Iain and I are currently making a dedicated chassis with a coupla meters (for smash) to refresh-getter and test these great valves.

Type 304TL operate amazing linearly into a 5000 reflected Ohm load with the same drive voltage as a type 211 (same mu = 12, but 3 times bigger, and made to win the War).

Dude, you're crushin' it. (Bill Burr)
All good fortune,
Note that type 304TH is a high-mu version not really suitable for an audio output stage.

All good fortune,

Both 304tl and 304th have low mu, low enough for a SE amplifier. I have tried both and work excellent. The 304th is a higher mu version of 304tl gut not so high to be prohibited for SE CLASS A service. Also, their RP is between 600 and 1000 ohm according to my tests making them particularly suited for SEclassA.
Maybe we should just say that this is a judgment call. Transformers for these big guys are difficult enough to make with sufficiently low parasitic impedances that the lower the source resistance (dynamic anode resistance) the better the performance. Of course.

My argument might be that a larger class-A1 current swing and a lower dynamic anode resistance are more important than a smaller driving voltage, especially for such an extreme build. If we're not aiming for the very best possible, why go to all the extra bother of big Eimacs?

All good fortune,