vacuum-tube
What is the minimum voltage I need to provide my tube to be able to produce some sound?
I am trying to debug my first tube amp, but I can only power it with a 11,8V supply instead of the recommended 13,3V.(PCL86)
Could it be that the triode and pentode doesn't open because of this?
Or in an other way: What filament voltage do I need relative to the recommended one to be able to use a tube?
A modest increase in R2 should suffice to re-bias the tube stage for 12V at the anode, P1 should be OK. Either recalculate using the load line method, or increase R2 until Va=12V, which will probably be around 200 ohms.
The output stage will still "work" as-is, but 125mA is quite a high current, so dissipating twice the power here will probably require heatsinking both FET and regulator for reliable operation.
Naturally you can't run the heater off 24V! So you still need to arrange a 12.6V or 6.3V supply for that.
One alternative would be to run both the heater and existing output stage off a regulated 12.6V supply (avoiding the increased power dissipation, though I would suggest heatsinking anyway), and a relatively low current 24V anode (B+) supply maybe using a voltage doubler then a regulator.
However this would mean you could no longer DC couple from V1 anode to Q1 grid cough sorry, gate. I would suggest AC coupling to Q1g, and providing the DC bias with a 1 Megohm "grid leak" resistor to a 6V supply (just a voltage divider with decoupling).
simulate this circuit – Schematic created using CircuitLab
I agree with @Dave about the modelling. You only need to worry about the resistance at operating temperature. The lower resistance at room temperature might increase the warm-up time, but unless you're worried about that, it should have no significant effect.
An option would be to create an ~8V DC supply and use a boost converter to create a regulated 300mA DC supply. A simple circuit using the LM2577 (or one of the ubiquitous modules) should work fine. You can probably make the circuit smaller and cheaper, and easier on the transformer.
If you do decide to use the capacitor voltage multiplier, make sure that the ripple current rating (RMS ripple current) of the capacitors is observed. Cheap electrolytics may not last long in this service, particularly if they see high temperatures from your tube ('thermionic valve', sorry), on top of the self-heating from the ripple current.
Best Answer
The RCA receiving tube manual RC-23 is quite vague about the amount of permissible variations, however it does warn that insufficient filament voltage can lead to degraded performance and reduced tube life. Of course excessive filament voltage can also shorten tube life.
Since tubes were normally operated from unregulated mains transformers, I would think your -11% would be acceptable, but perhaps not if that nominal voltage also has a large tolerance and/or you're working the tube close to the maximum ratings.