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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

el34 instead of 6550

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I would say yes, provided that the voltages are not too high and the OPT gives a suitable load for EL34 in pentode PP, Class AB1. B+ at point A should be about 425v to 450v, the screens at point B should be at about 400v and load should be 3.4k plate-to-plate. This will yield about 50w RMS output
 
Hi there:
The o/p xfr pri impedance, really does not matter, use anything from 3.4k to 6.6 Kohm.

Set /adj the bias (Idle current) for EL-34 anywhere from -39 to -43V.

You need to add a resistor (10 ohm /2w) on each EL-34 cathode (to gnd) to be able to minitor and set the idle current, by measuring the DC voltage drop across them.
 
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Joined 2004
Pentode mode is far more "'picky" about the optimum load impedance, for maximum power and minimum distortion, than either triode or ultalinear. In pentode mode, distortion increases rapidly either side of the optimum load. If you can't get the load impedance exact, it's generally better to err on the side of lower impedance, rather than higher.

I don't know exactly what other parameters are affected by the load (Z pri as you call it). The figures I've seen for EL34 push-pull are those in Mullard data sheets.
 
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