• 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.

EF86 Push-Pull "power"-amp

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So maybe this would be a good point to start



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I've done the low power thing with several different tubes; 12AU7 and 6SN7 are the classics, but for pentode sound I've found that the EF80 is the best. It can take 300 V / 10mA, and you can use it with a standard small Hammond OT. Power in the order of 1 watt, and sweet sounds. Cheap price doesn't hurt.

May I suggest the following 6BJ6 low power amplifier arrangement which I have used as a modulator/output amp in the past. The 6BJ6 Pentode's are wired as (in the vernacular of the 1930's) a "Class B Super Triode" (G2 to G1, not G2 to P) and operate unbiased without the cross-over distortion usually associated with that class. This circuit will provide from 2 to 5 Watts depending on the value of B+, the drive and the appropriate Plate Transformer impedance... the 150mA Heater's are a plus as well.

4xEF86 ...but I remember not being satisfied with the results...

That's not surprising to me. EF86 class tubes have sharp-cutoff grid structures and would not reproduce the soft-crossover characteristics of a remote or semi-remote cutoff "Super Triode" Push Pull Pentode arrangement. It's the very nature of the variable-mu grid structures of the Remote Cutoff Pentode which, when arranged in the Push Pull Super Triode Class AB2 configuration, result in the absolute highest power to weight ratio obtainable while reproducing 'sweet' audio qualities.
I was toying with the idea to build a very low output guitar amp with tons of character and distortion at reasonable playing levels. Assuming I have a custom output transformer made, what could I expect in terms of output power?

In my initial reply I suggest an unbiased low power amplifier arrangement (in the vernacular of the 1930's) "Super Triode" Pentode AB or B Push-Pull Amp" (G2 to G1, not G2 to P). which operate at zero-bias.This is by no means a new idea... as evidenced by my Grandson's string-fil line-powered 12AT7/2x12BA6 PP 4W Super Triode practice amp (actually, it's been passed-down, it's now my Great-Grandson's amp) originally conjured-up by me some 30+ years ago. However, if I were designing to your specifications today, for the most bang for the buck, I would do something like that, a pair of $3 tubes and an $18 'off-the-shelf iron, safe line power and string fil's:
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