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RE: round and round we go ..........

Posted by 91derlust (A) on January 24, 2016 at 11:55:40

In Reply to: round and round we go .......... posted by vinnie2 on January 23, 2016 at 17:54:45:

Hi Vinnie,

If I have missed something in your previous threads, just point me in the right direction - no need to repeat yourself here.

As you are aware, lowering the cathode R draws more current, which in turn drops more voltage across your PS resistances, which in turn affects your plate voltage, which also affects your operating points (less voltage, more current). With that out of the way...

Is the 45 loaded by a transformer providing a 5k load?

Does your PS (potentially) supply 375V, which is then dropped to 220V using an RC node? Is this to supply the 26s and 45 from the same node? If yes, I would use an alternative strategy: 375V, then add a RC node to tap off enough voltage to run the 45 at close to textbook points (say 300V or 331V), then add another RC node for the 26s (220V)

If that is not possible, you might feel comfortable knowing that triodes that are pretty tolerant with operating points - they can be run safely and still sound fine at a variety of operating points. They don't need to be super-finely tuned to work well. Their point will change with age anyhow. That said, tubes have limitations that should not be exceeded. Again, I am sure you know this. Just sayin' as a reality check.

Finally to your question: I'd choose either the 1200 or 1500 cathode R; the 1500 R may be more ideal, but choose what sounds best to you. A 5k load should work well with either operating point. I'd take Gordon Rankin's advice not to run the 45 over 35mA... I'd be looking for a couple of mA less, say no more than ~33.1mA. You will also get a touch more power with the higher voltages, but it probably will not be noticeable.

Cheers,
91.

"Confusion of goals and perfection of means seems to characterise our age." Albert Einstein

Edits: 01/24/16

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