Quote:
Originally Posted by NewVista
What's with Contrast pots in the cathode anyway? I don't like this as it would alter operating point of tube.
What's wrong with having a fixed operating point (in a nice linear region) and controlling the gain otherwise?
And does Brightness alter operating point also just to transfer change to kine cathodes setup?
Let's put the brightness control back up by the kine where it belongs.
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I agree that putting it in the cathode path is a bad idea. But the operating point is actually governed by adjusting both the brightness control and the contrast control in unison. I however contend that removing the mod with the capacitive bypass contrast control and putting it back as originally designed helps better maintain the correct dc component getting to the CRT.
The earlier RCA black and whites had the contrast control in the plate circuit but this involved the pot having a number of taps for frequency compensation.
I think the best solution would have been to add a diode clamp just before the CRT but this would have required another tube.