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floobydust wrote:

The final ratings on later ST-style glass 45s was 10 watts on the plate and 300 volts cathode to plate max. Recommended bias current is between 34-36ma max. Short answer... I prefer no more than 34ma on the 45 cathode. It's a coated cathode type on a thin filament wire and pushing them will run them down quicker. If you can manage 35ma and match the set I'd say you're good to go. The only problem is your available supply voltage. As you're "backing into" a set of fixed parameters (plate voltage supply is already defined), you might need to alter your driver operating point to get the 45 in this range and maintain optimal performance. Of course, you know better than I about the driver stage and where it's optimal operating point is. I would do some (bias) trimming on the driver stage first and see if you can reduce the 45 idle current to no more than 35ma while keeping the driver stage operating properly. If not, you might end up trimming the cathode resistor to get there.

Floobydust,

I hear what you are saying but I do not know if I agree with you. It kind of odd – I ask advice and I disagree with the advice but it is not that disagrees but rather it does not conform to my view of “common since”. Let me to lay down my view, perhaps my view is inaccurate and you will be able to point out where I am wrong. 

The source of my disagreement is 35mA as some time of absolute number that has own meaning. Yes, the typical operation of 45 reportedly 250V and 35mA that makes 8.7W. Furthermore I presume that it is high possible that you guys who are experienced with this tube found that at this operation point the 45 does the best.  Highly possible and I would not debate it. However, let remind to each other that a proper operation point is a subject of voltage, current, load and the most important from my point of view – a symmetrical clipping under the given voltage, current and load. Only with context of the symmetrical clipping the tube give up max power and experience no voltage and no current starvation under any signal amplitude (until it clips).

Now, let see what happens in my case. Firs my channel runs no LF and it is DSET. In classic SET configuration power is derivative of LF as inductance dies first. In case of SET employed as DSET and has not LF the consideration of power as current vs. voltage are not there. My MF amp would hardly clip by power of output stage but rather by overdriving the stage very deep into A2 and by clipping the driver stage. If so the symmetrical clipping of the output stage might not be a subject and therefore is might not be the only right operation point. In my case I run the tube at 208V and at 35mA and 40mA or at 7.5W and 8.3W, what is near the same plate dissipation as 250V/35mA (8.7W). If I do not have objective to get max power from the tube then why 208V/40mA might be worse than 250V/35mA? You suggests that “it's a coated cathode type on a thin filament wire and pushing them will run them down” but current does not burn cathode, I presume temperature does as a derivative of power. It you have a fuse that goes off at 1A and 120V then it will not go off at 1A and 12V. This is my argument, I do not know if I right but it makes “common sense” to me.

There is another subject. The common practice to use 45 is to load it against 6KOhm. I am loading it against 9.5K and will be soon against 11K. This is 2 times idler load and I am sure that power-perfect operation point will be moved from 250V/35mA. So, do I need to worry about 40mA in my case or I might use whatever stick to the wall as long it is within 8W?

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