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In Reply to: single ended EL84 in pentode mode posted by eedork on March 15, 2007 at 10:28:50:
Hi all,
I've not posted here before, but I do love tube audio.
So I have a question on the schematic that was posted earlier (http://www.tubeaudio.8m.com/RH84/rh84.html). In that schematic there is no cap on the screen. Is this a misprint do you think, or was it intentional? Not having any bypass on that screen would mean that you would get a gain reduction due to the negative feedback you get at the screen due to voltage swinging when the current swings up and down.(I'm the co-worker that Matt mentioned earlier)
Follow Ups:
Yet another good reason for using a separate regulated screen B+ supply. The screen grids are at ground potential as far as AC is concerned.
Eli D.
On the point of having a well regulated supply for the screen, I've considered making something like this to be a super stiff screen supply. I know that there are some people who would find the use of solid state things to be bad, but I'm open to all possibilities.By changing around R2 and R1 in this, you could make the screen voltage whatever you wanted, and it would be pretty much independent of the currents involved (so long as they weren't TOO high). Also, you could not use a BJT there, you could use a FET, though you might have to use a 10V zener depending on how much Vt+Vdsat+(regulator headroom) you needed. Just a thought.
So after rereading some of the posts, I realized that I had drawn the Maida regulator setup that was mentioned elsewhere (I didn't remember that as the name of it). Something I didn't put in it was the capacitor on the output of the lm317. I don't know if you need that for stability or if it will be OK without it.
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