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Old 27th May 2011, 03:46 PM   #101
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OK, that makes it easier. You can do a common cathode stage, with a cathode follower, then feedback to the grid of the common cathode stage. Set the ratio of feedback resistor to the grid resistor (that's a series grid resistor, not the grid leak!) to 5:1. A good choice of values so as not to excessively load down the sources and, at the same time, to not have instability from stray capacitances, would be 20k and 100k. You might have to put a small cap (10-30pF) across the 100k resistor to avoid ringing. And, of course, use 1k grid stoppers to prevent oscillation.
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Old 27th May 2011, 06:33 PM   #102
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Thank you for information, SY

Also I found nice circuit in TubeCad homepage Click the image to open in full size.
I guess that adding just a resistor to reduce gain is not appropriate (see attached schematic). Any suggestions?
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Old 27th May 2011, 06:35 PM   #103
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If not in detail, in concept your schematic was exactly what I was talking about.
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Old 27th May 2011, 06:39 PM   #104
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Well I thought that common cathode stage + cathode follower needs 3 triodes. (Newbie in tubes)
Anyway, thanks a lot SY.
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Old 27th May 2011, 06:43 PM   #105
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It will need three stages if you want it to be noninverting (trivial to account for downstream). But your schematic is conceptually correct.
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Old 27th May 2011, 06:47 PM   #106
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It does not need to be non-inverting. Just to recheck values, to do some simulations and to build it .
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Old 27th May 2011, 06:51 PM   #107
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As you drew it, you have a 1k input impedance, which is asking a bit from your sources. I'd also look at boosting the open loop gain and increasing power supply rejection by bypassing the first stage cathode resistor (or using diode or battery bias). A CCS load for the first stage will take that even further. You absolutely want gridstoppers on both stages.

But you have a good base for experimenting.
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Old 27th May 2011, 06:59 PM   #108
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SY, I downloaded CCDA manual, so I'll read it all now. Also, if I dont need so much gain, I dont have to use bypass capacitor too, right?
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Old 27th May 2011, 07:09 PM   #109
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With the feedback loop, the cathode bypass won't change your open loop gain very much, but it will increase power supply rejection and most likely reduce distortion.
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Old 27th May 2011, 07:18 PM   #110
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Ok, thanks again
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