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RE: question.......

" When the tube is not run as hard (as the standard 60 ma.), it can stand far more input swing without distorting."

Dennis please understand I'm not making a judgement on what you claim to hear but the above just isn't true.

If you lower the plate current, without increasing the plate to cathode voltage and increasing the load impedance to rotate the load line more towards the horizontal, the tube will run into distortion faster. Just the opposite of what you say.

This is not my opinion. This is just the matter of fact.

Just plot load lines each way and see for yourself.

When you lower the plate voltage, leaving the slope angle of the load line the same (in other words, leaving the load impedance the same), the load line to the right of the idle point runs into the non-linear area sooner. The non-linear area is where the grid lines are bunched closer and closer together causing harmonic distortion.

The plate voltage rate of change is less going to the right from the idle point vs. the rate of change going to the left.

This lack of symmetric behavior is the cause of harmonic distortion.

To achieve low HD we want to pick an operating condition (idle current, idle Vpk and load impedance) where the plate voltage rate of change is the same, left vs. right along the load line.

Paul Joppa's formula gives good results. For your operating point you should be using a 3.9k output transformer.

OPT Z = (VPk/current)-(2.38*plate resistance)

If you're really not interested in power (and you have indicated that) you could use an ever higher load impedance. But a lower load impedance will give you more total power but more HD at all power levels.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

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