Load line and biasing issues

[Breadbored]

I'm sorry if this has come up a lot in the past, I've looked around the ole interwebs but I haven't really found a good answer to my problem. I'm planning a scratch build, an Orange OR120 preamp with a 2xEL34 cathode biased power amp and I figured that if I use trannies for a 50W JCM800/Plexi it would work fine. One less preamp tube than the JCM and the same amount and type of power tubes so of course the power handling and secondary voltages would work with the tubes I plan on using. So far so good right? But when I started drawing up the load lines for the output stage it was clear that the 3200ohm primary impedance would be WAY too low (345-0-345V HT winding). I looked around for different power and output transformers but all the numbers seem to match up.

Looking at the load lines the tubes would operate well above Wa for both the class A and B lines, but obviously this combination works somehow. How would I go about finding the value of the cathode resistor without going through a bunch of molten tubes before I get it right? Of course I could look up other amps with similar HT/primary impedance, but I'd rather know what I'm doing and land pretty close to a correct value.

[pdf64]

… 2xEL34 cathode biased power amp and I figured that if I use trannies for a 50W JCM800/Plexi it would work fine. …

Cathode biased is different to fixed. Needs to be biased much hotter, close ish to class A 360° conduction, so the HT needs to be lower.
See 380V HT 3k4 cathode biased operating conditions on p4 of https://frank.pocnet.net/sheets/010/e/EL34.pdf

[tubeswell]

What ^pdf64 said^


Also, remember that screen voltage and bias voltage have an interactive effect on tube operating point. You probably want the screen voltage to be somewhat lower* than the plate voltage if you're going to bias the output tubes at Pmax - to stave off a sudden rise in screen current when you hit those Class B power chords. Others may have a more finessed assessment. EL34s are finickity tubes when run hot.

[Breadbored]

Thanks for your input! There's really no reason that this amp couldn't be fixed bias instead, but as the noob I am I don't really understand why that combo of transformers would work with fixed bias either. Wouldn't the tubes still run above Wa with fixed bias, or am I missing something here?