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In Reply to: RE: A lot depends on where the screen tap is placed. posted by Ralph on October 11, 2024 at 12:16:05
A beam power tube or a power pentode is not very linear connected as a pentode, or UL (no matter where the taps are) or as a triode.They are all very non-linear compared to a direct heated triode (45, 2a3, 300b, 211 or 845).
Here is a 6l6 triode connected. It is more linear than when used in UL (no matter where you put the taps for the UL) but still no where near as linear as a DHT.
and here's a 300b direct heated triode.
For those who might not understand what I'm saying, pick you operating point, plot your load line and then see how much the plate voltage changes for a given decrease of the bias voltage (less negative on the grid caused the current through the tube to increase so the plate voltage is reduced) and then see how much the plate voltage changes for that same amount of increase of the grid bias (more negative grid bias caused the current through the tube to decrease so the plate voltage increases). In case you don't know, the music signal simply modifies the grid bias and the result is a change in the current flowing through the tube. The first half of a input wave form is positive causing the current through the tube to increase and the second half of the input wave form is negative causing the current flowing through the tube to decrease.
With a linear tube, with a proper operating point and a proper load line, the plate voltage change be symmetrical (the plate voltage will increase and decrease equally for each half of the wave form). If it doesn't, then distortion is the result. It all has to do with the spacing between grid voltage line following the load line. When they are equally spaced then the output wave form is symmetrical and the HD is low.
Below you can how one half of the output wave form will be much bigger than the other half. What you are looking at is harmonic distortion caused by the grid lines not being equally spaced along the load line. Now pick an operating point of the 300b plate curves above and plot a load line and see what a difference evenly spaced grid lines make.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 10/11/24Follow Ups:
The 6L6 is a particularly good example of this. It needs a judicious amount of feedback to produce reasonable linearity, even in UL and triode modes. The same is true of the EL34. There are exceptions though. The 6V6 offers very good linearity in triode mode, and is useful in SE with no feedback at all (assuming damping factor is not an issue).
It was an early one, and if you wish a wee bit lower output Z, apply a bit of plate-to-grid( in pentode mode no less) sufficient to knock its mu down to 3.5 and *THEN let us compare it to a 300B.
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.
Tell us more! Do you have a schematic with component values?
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