6N7 as drive control

[WA4WAX]

From an obscure and defunct Canadian mag.

https://worldradiohistory.com/Archive-All-Amateur/XTAL-for-the-Amateur/XTAL-1949-03-04.pdf

[Opcom]

That's a nice idea, to use a series regulator. The 6N7 is very interesting as a zero bias tube because its plate current at 300V plate to cathode, is about half the current drawn at full 'class B' output, unlike some 'class B zero bias' tubes which are nearly cut off at zero bias and have much higher plate current ratio from zero output to full output. (811, 3-500Z etc).

The data sheet seems to disagree a little though because @300V plate to cathode, it gives 35mA total for both sections at zero bias, and 70mA when driven to full output (positive grids in respect to cathode). The curves show that for DC it would need about +7V g-k at Eb=300 to pass 35mA per section.

I am having a hard time seeing how the grids can become positive enough with respect to cathode to pass 70mA, in the diagram shown in the magazine.

But if the voltage drop across the tube were 400-500V, then it would pass 70mA at zero bias (extrapolation of the curves). That is a large voltage drop and much more plate dissipation (~28W total) than it's rated (5.5W per plate).

I hope I am mistaken on this or just fail to see it. It would make more sense maybe? to use the 6N7 to control just the screen grid of the 807.

[WA4WAX]

Patrick:

You beat me to it.  I do not understand why he is doing plate and screen.  Screen alone would be sufficient.  All that aside, it is not a bad idea.