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Old 17th October 2007, 08:41 PM   #1
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Default Connecting a single plate to multiple screens?

hi all

i am planning a project which will require a single -10 line level input to feed 2 6V6 power stages (each will drive its own output xfrmr) and a VU meter.

I like to use 6SN7s for a variety of reasons. I was imagining that the single input would feed one 6SN7 section, and the plate of that section would be ties to the screens of 3 6SN7 sections - one for each 6v6 tube and one for the meter's voltage amp. I have built many many amps that use a single 6SN7 stage driving a SE output and it works great, always.

Can a 6SN7 plate feed 3 6SN7 screens tho? i was imaging that after the coupling cap I could just put a 100 ohm resistor between each screen and the screen resistor... would this work? or would i get frequency response deviation or something worse?

I have seen many many schematics for tube mixers but no info on active tube 'splitters,' which is essentially what i need here.

I would like to avoid circuits that involve any additional transformers as well... for simple $$$ reasons.

any tips???

c/
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Old 17th October 2007, 09:06 PM   #2
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First thing...by screens you mean grids...A screen is a section of a pentode and is different than a gird. triodes only contain grids.

Now to answer your question, that will depend on your circuit. Typical input capacitance with a 6SN7 is (2.2+15*4.0) pF ~ 63pF. Times three tubes is ~190pF. That being said, you will need a maximum source impedance of Ro=1/(2*pi*190e-12*fc) ~ 28Kohm at a cutoff of 30KHz.

Therefore, make sure the preceding stage has a relatively low output stage..maybe a 10-22k plate load or a cathode follower.

good luck
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Old 17th October 2007, 09:25 PM   #3
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Default reply to boris

Thanks for the advice!

ah yes of course. i meant to say GRIDS. i m spaced out today. prolly the solder fumes. ANYhow.

hmm so the 6SN7 would not work eh.

Could i directly feed the grids of the 6V6s and a voltage amp for the meter using this circuit? I have built this circuit a few times and it sounds good so far..

(see attached)
Attached Files
File Type: pdf rc20p416.pdf (41.8 KB, 20 views)
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Old 17th October 2007, 09:52 PM   #4
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That circuit will work, though I would choose a lower R5.... R4 will need to be adjusted as well then. That circuit will also work with a 6SN7, by the way, just the resistor values will need ajusted. I prefer octal tubes like the 6SN7 myself... C3 and R6 can also be eliminated by biasing the first stage just right. If you need help choosing resistor values, give a shout.
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