EL34 vs 6L6
Pete,
Given a choice between EL34 and 6L6, I would have no hesitation in selecting 6L6.
At the risk of being shot down to pieces, I have to say that EL34 is not a good design from the start. This tube was favoured by many manufacturers because you could apply 800 volts on the Anode; and this means more watts per dollar in a commercial sense. The only good EL34 that I have respect for is the old GE 6CA7 manufactured in USA. If you want long (3-5 years) trouble free operation do not apply more than 400 volts on the screen and anode of current production tubes.
6L6 on the other hand is a more robust design. Philips ECG STR-318 (STR stands for Special Tube Request and please note that I am not crash hot the 318 number… I am just quoting from my ROM…..rusty old memory) 6L6GC is very good tube if you can find some somewhere. Markings are in Blue colour on the envelope. This is a true 35 watt dissipation tube. This tube can take some abuse from a beginner to tube audio.
My approach to DIY is to achieve performance at a level higher than most if not all commercially available designs. DIY enthusiast should not compromise, if possible. Price is not the object.
Since you are willing to experiment, I would recommend you to start with a 250 volt power supply and a 4200 ohm (load for two tubes in PP) primary PP transformer. You will need to apply feedback in the tetrode mode. The lower voltages will help you to try the different configurations that you propose.
You will also need a flexible output transformer. Have four 1 ohm secondaries that you can configure in many ways. These are separate windings and not taps.
You will also need two separate B+ windings on your power transformer with three taps (perhaps 220V, 160V, and 110V) on each winding. You may connect these taps to obtain various voltages. You may also consider 150VA for each B+ winding.
Personally, I do not like tube rectifiers. I use solid state devices in all of my designs.
If your goal is for high quality sound, then use choke input filter. Two stage filter is recommended.
The above are recommendations to get you started with a range of flexibility that beginners need. I presume that your experiments will continue over a period of time to warrant custom made transformers. Always be careful when dealing with high voltages. An occasional zap may be good for the heart but a few in succession can be lethal.
Mohan