Tuesday, April 5, 2011

LA-2A board 2

Got in another few hours after work today to work on another of the boards for the LA-2A.  This one has the circuitry for the T4B, the negative feedback loop for V1, and the output coupling cap leading to the output transformer.  Note a couple of deviations from the standard LA-2A here. First, the 10uF electrolytic coupling cap has been paralleled with a smaller Mallory film cap to help preserve good response in the high frequency range.  Electrolytic caps by themselves can a have significant, and negative, impact on tone as their impedance begins to rise at a higher frequencies.  This should take care of that.  Second, I've added some small 1K grid stoppers here (and on the other board as well).  Modern best practices demand grid stoppers for every valve! They are small enough that they will have no effect on the high frequency response, but should be sufficient to act as insurance against any weird inductive or capacitive coupling that might lead to oscillation at higher gain.

Finally, attentive readers might notice that the original 50-380pF varicap for C4 has been replaced with a fixed value silver mica cap. I'm starting with 330pF as a value that should avoid rolling off too much high end. This value will be tweaked to taste if necessary later.

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