D3a datasheet confusion

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RhythMick
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#1 D3a datasheet confusion

Post by RhythMick »

Capture from the D3a datasheet ("as triode" section) shows Vg1 as +10V ????

Firstly I would expect g1 to be negative wrt cathode.

Secondly the curves would indicate a bias of 1.5, not 10.

Cathode bias using a 470R with 24mA works out around 11V.

What am I mis-reading ?
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Max N
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#2 Re: D3a datasheet confusion

Post by Max N »

Agree it doesn’t make much sense to me as a way to specify the op point, but that cathode R with that grid volts would work, if the Vb notation means absolute volts rather than relative to cathode? So the grid is at 10v and the cathode is around 11.5?
Max N
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#3 Re: D3a datasheet confusion

Post by Max N »

Maybe it gives more stable biasing or something?
RhythMick
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#4 Re: D3a datasheet confusion

Post by RhythMick »

Max N wrote: Fri Jun 25, 2021 10:45 am Agree it doesn’t make much sense to me as a way to specify the op point, but that cathode R with that grid volts would work, if the Vb notation means absolute volts rather than relative to cathode? So the grid is at 10v and the cathode is around 11.5?
Oh I see. Thank you that helps. So the datasheet proposes a mixture of cathode and fixed bias. I wonder why.

I know Steve (izzywizzy) of this parish uses the D3a in his phono stage and he uses diode biasing, so grid at 0v cathode at about 0.7 ish I guess (I haven't checked the datasheet for that SIC diode).
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Paul Barker
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#5 Re: D3a datasheet confusion

Post by Paul Barker »

With all valves cathode bias allows greater value grid leak resistor often by a factor of ten. Fixed bias therefore means considerably larger coupling cap.

The numbers quoted mean net bias -1.28 but hybrid of fixed and auto. Perhaps necessitated by expected RF environment it designed for, which we don’t know about in our restricted bandwidth vis a vis RF. That and the unstable nature of high transconductance valves per se.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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