Re: Single ended Class a Op stages.
Yes, Va shoots both up and down. Ideally, when you reduce current to zero, Va shoots up to Iq x Rload and stays there. In practice, because of finite inductance, it will decay exponentially back to Vb. But, you've got the idea.
What limits power is HT volts, and quiescent current - assuming you have access to an infinite array of transformers with turns ratios.
If you have a 'good' transformer and want to use that, then the turns ratio hence actual load is fixed, so you choose quiescent current to suit. Then if you have chosen your valve, you determine if it's under-run, or over-run (either on current, or anode dissipation).
If under-run, then fine, it will have a long life (unless excessively under-run - when you'd choose a less gutsy valve). If over-run, then just put two in parallel, or use a bigger bottle.
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