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In the Forum: Analog Playback
In the Thread: 7788 tube mic pre design
Post Subject: Corrected CurvesPosted by hagtech on: 11/5/2007
Ok, once I got pin 8 connected to pin 1, the curves came out straight.  Here is the true pentode plot at 200V screen, 0.5V/step:

And here is my new data sheet plot for triode mode:

For reference, at a bias of Va = 200V, Vg = -3.0V, we get roughly Ik = 20mA, rp = 2.5k, mu = 50, gm = 20mA/V.

typical line voltage should sent 7788 deep into A2 and in clipping

Yes indeed.  I bias my amp driver at about 175V, -2.5V, 20mA.  This means an input signal above 1.7Vrms will send it into A2 (constant current operation).  However, the 2A3 needs only +/-40Vp in A1, and that translates to about 0.7Vrms at input, which would be the stated sensitivity of the amplifier.  This means I have a little headroom in the driver and it should stay quite linear under normal conditions.  I also am using an interstage tranny which is I think 1:1.5, so that gives me even more gain.  Hard to tell yet, as I have not calculated the losses due to loading (probably about -2dB). 


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