PORTRAIT OF A TUBE EL34 is a tube capable to deliver maximum 100 Watt from a push pull pair, at 800 Volts, but only some very good tube brands like TFK actually can do 800V. Besides, from what I know, actually only TFK can be loaded at 800V, and even for those it is not recommended, as it hardly ever done with historical equipment. As a rule of thumb, a Push-Pull stage will give maximum 2x the plate power of the tubes as output power (that means both tubes together). So if both tubes are 25 Watt types, you get maximum 100 Watt output power. And yes, EL34 can do that. But.. 800V over an octal socket is no good idea. It gives leakage, inside the glass part of the tube base. Glass may develop small leakage current when hot. If this happens, the heat from the leakage itself will warm up the spot further, and the tube can self destroy. Besides, 800V for a Barium Cathode is very high, and only possible when you have EXTREMELY tight control over the vacuum. And EL34 can not have that tight control, no matter how many getters, or whatever. The plates get simply too hot, and the vacuum at 800V can not be maintained like this for a long time. So at 800V you need to change tubes more often. So EL34 is used at lower voltage normally. Not because designers know (ehem...) and respect the reasons above, but simply because lower voltage is more comfortable. HOWEVER, output power goes down from that a lot, and end up somewhere around 50 Watt for 400...500 Volts plate voltage. So the efficiency of the tubes go down, you develop heat, and not output power.
The solution is called EL50 / 4654
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This tube uses a top cap for the plate connection, allowing higher plate voltage. Still... 800V is a lot. There is a downgraded version, that will work at 600V maximum, it is the 4654. From this 4654 some wonderful big coke bottle tubes can be found. Sockets I habe hundreds of NOS still. China sockets are around, and Yamamoto has a low cost ceramic socket, even gold plated. Now comes the goodie... The 4654 is only 18 Watt plate dissipation per tube, but now look at 400Volts the push pull stage will 48 Watt at 2,5% distortion. So that is 2.5% at full power, no feedback. Note the tubes run at 18 Watt dissipation (each) for this, which is less than EL34. And still 48 Watt. EL34 at 400 Volts will give 45 Watt at 6% distortion.
So, if you are non-commercial, and a DIY of the kind who wants to go an extra mile to get better results, and you don't like China Cracker amplifiers....my recommendation is to take any good EL34 circuit, and adapt it for EL50. The main reason to use it, I would say is the significantly lower distortion Conclusion: EL50 is NO EL34. EL50 (or 4654) is the better choice when you operate at medium voltage, it uses less power from the power supply, generates less heat in the tubes, and will give still more power at (much!) lower distortion. The disadvantage is the plate cap, but for that you get a far better tube.
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