output. How many are needed? The question should really be: How many watts of output do you want? I have heard Klipish Horns thunder with a 1.2 watt amplifier and this amplifier would put out a little  more than 2 watts. So the math is easy enough: desired wattage divided by 2 equals the number of EL84's needed.

   The Italian audio circuit designer Andrea Ciuffoli has on his web page several simple, solid-state, no gain, power follower amplifiers: 
  Single Ended Pure Class A Power Follower
  Push Pull Pure Class A "Hot Follower 99"
  Push Pull Pure Class AB "Cool Follower 99." 
All of which are worth checking out as they show how little is required to make a good amplifier.
    Turning to vacuum tubes, what would be the simplest, quality tube amplifier possible? The answer would have to be some form of a single-ended, transformer coupled amplifier, as it would eliminate the need for a phase splitter and the problem of matching the tube's high impedance to the speaker's low impedance. Most SE tube amplifiers use an input stage and a driver stage. These stages are required only because the output tube needs a great deal of grid voltage swing to work and the feedback loop requires gain to work. But if the output tube had enough transconductance not to require anymore voltage than the average tube line stage puts out (some put as much as 60 volts peak!) and we are willing to forgo the use of feedback, then the need for the input and driver stages (and coupling capacitors, as well) disappears.
    The best choice for an output tube could have been the 8417 power pentode were not for the fact that it is no longer in production and the price for the remaining old stock has sky-rocketed. (Svetlana, here is a tube to add to your excellent product line.) The EL34 needs about 30 volts of peak drive voltage, which might be doable. The best choice today is the EL84, as it is readily available and cheap (even the buffed Russian version is a bargain). The EL84 needs between 300 to 400 volts on its plate and about 16 volts of drive voltage. The EL84 is a small power tube, but it could be used in multiples to increase the potential power

Just about the simplest amplifier possible, a Single-Ended amplifier without an input stage or a driver stage, as it is meant to be driven directly from a high gain tube line stage.

  Five would be a good number for two reasons: it would yield a good number of usable watts of output and an SE amplifier with 5 output tubes could never be confused with a push-pull design. Additionally, the Miller Effect capacitance will become more of an issue with more parallel triodes used.
   The next question then is: Which output transformer to use? Depending on the B+ voltage, a primary impedance in the range of 1500 to 2500 ohms should work well. Audio Note, Bartolucci, Electra Print, One Electron, SJS, Sowter, Tamura, Tango--all make excellent SE transformers in this range.
    Understand that low impedance output transformers require more idle current than high impedance transformers and less voltage than high impedance transformers. Therefore, while the power supply can be lower in voltage than usual, the average amount of current it must be able to source will be much higher than usual. In other words, a beefy power transformer is still required. Higher current, usually, is not a problem with solid-state diodes, but it can be a sore spot with tube rectifiers. Still, I believe it is the better compromise, as lower voltage (still

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