Everything you ever wanted to know about screen grids

[EWBrown]

A long but worthy read: (I have yet to finish it)

http://www.webace.com.au/~electron/tubes/screens.htm

/ed b in NH

[Shannon Parks]

http://www.webace.com.au/~electron/tubes/

Here's the index. Several good topics. Nice find, Ed.

Shannon

[EWBrown]

http://www.aikenamps.com/UL.pdf

/ed b

[dhuebert]

After reading all about screen grids and ultra-linear operation, the only piece of info I take away from it, that seems useful, is that the screen grid should have a screen grid stop resistor. The purpose of the resistor would be to ensure that the screen grid stays negative WRT the plate. "As a rule of thumb, and noting the advice of Philips Miniwatt to instal a value of Grid #2 resistor of between 500 to 1000 ohms in each Grid #2 supply lead, we can assume that a value of 50% of the Plate to Plate primary load impedance is an approximate ideal for the Screen to Screen loading". For the Hammond 1650F this would suggest to me a value of 1900 ohms (7600/4).

Any comments-suggestions?

Don

[EWBrown]

I haven't yet finished the article, are they referring to the screen grid loading resistance or impedance? If it is impedance then they are referring to the tap position on the OPT, which is pretty standardized at 40-43% of the number of turns from B+ (CT). That would be about 16-18.4% impedance of the plate impedance (square of the turns ratio).
In a 7600 ohm P-P OPT tranny like used in the ST-35 and DIY35, the impedance of one plate to the B+ would be 1900 ohms (1/4 the total P-P impedance) and the SG taps, assuming 43%, would be about 342 ohms to B+, and 1368 ohms SG-SG.

When one adds a resistor in series with the SG tap, then the equations get a little more complex.

I've seen SG stop resistors between 100 and 1000 ohms for SE amps, "pentode" P-P amps and in some UL circuits, though this is not as common in the latter

/ed brown .

[dhuebert]

My impression from what I read is that you want to balance the DC resistance with the AC impedance. "For other values of Plate to Plate load, calculate on the basis that each Screen Grid resistor should be 25% of the transformer Plate to Plate primary load impedance "