How to select screen R's?

[daz]

Can someone enlighten me on how to select screen resistors on my marshall style build? Right now i have the typical 1k per tube, but there is also a 1k common. I find that without the common the voltage is very close to the plates, which it shouldn't be, correct? Anyways, i'm wondering whether there are reasons that dictate what values to use or if you should just use whatever sounds best. I find more resistance seems to take the hard edge off the feel and it gets softer as you up the resistance. anyways, if someone can enlighten me in LAYMAN'S TERMS i'd appriciate it.

[Rob Mercure]

Daz,
First off lemme recommend that you consult the Ampage archives as very specific values of screen resistor have been discussed for very specific amps over the years so you might stumble across exactly what you're looking for - around 10 years or so of knowledge stored there.

In any case the primary reason for the resistor is to prevent the screen from attracting too many electrons that are destined for the plate - the higher the resistor the lower the electrostatic charge which attracts the electrons and the higher resistance the current path for those electrons. Since the screen is much less massive than the plate if it get's hot it melts and sags closer to the cathode - or more in line with the electron stream if it's focused -until it goes into a meltdown cycle and shorts. Fairly brief overheating can permanently distort the screen grid on some tubes (some EL34s come to mind).

Now where this gets a little funny is your experience - a lot of folks feel that too much screen resistance makes the amp harder feeling. But this seems to be subjective and here's where I suggest consulting the archives.

For myself I generally like around 1 K for 6L6 and El34s but have been known to substitute slightly higher or lower values when parts stocks are low without neqative consequences. The actual value seems to be more critical for folks who crank up their output stage constantly.


Rob

[daz]

I remember someone else here saying that MORE screen R gave a softer feel and less was hard. So i'm a bit confused. My reason for asking was because of feel. at the moment my main goal is to get as elastic a feel as i can. (yup, i'm using a sag R in the B+)