input section

Here are the latest developments concerning the input section.  Also, this begins my first attempt to draw a schematic diagram for the whole thing (piece by piece).  I'm drawing by hand (obviously) in black ink on blue-grid graph paper, then scanning to PDF and processing the images with a combination of "NETPBM" open-source tools, and programs I wrote myself (originally as part of my pierced-foil art project, a different story entirely...).  Final result is a reasonably clear, and small, black & white PNG file.

A few things to note here.  I based the input jack setup on Marshall/Fender, i.e., one jack high-gain, the other with a resistor pad, but I made the pad resistors asymmetrical so that the low-gain input (2) pads down more than the original 1/2 division.  The high-gain input (1) still sees about the same 1M Ohm impedance, with about the same grid-blocking resistor (36k instead of 34k, with the values shown).  This front-end circuit seems to be an accepted "magic" set of values, used in many/most of the big-name (as well as the boutique) tube amps.  So I guess we'll stick with it -- other than making input (2) more useful by giving it the deeper padding.

In other images and text before today, I've been referring to the 6-position switch in the input section, as "HPF" (highpass filter), but now it's called "voice".  That's because it controls more stuff.  Originally, I based this control (HPF) on the Orange/Matamp "FAC" control, which selects the bypass capacitor on the output of the first triode stage.  Now, as you can see, I have changed to a dual-gang 6-pos switch, and the switch also changes the cathode circuit of the first triode.  There are three choices of cathode circuit, each with two choices of bypass capacitor: the "regular" value, and a lower capacitance (formed by two caps in series), which will give a higher cutoff frequency for the low-end.  The three cathode circuits correspond to Marshall 1987 low-gain input (positions 1-2), Fender input (3-4), and Marshall 1987/2203 high-gain input (5-6).  These are arranged in order of higher cutoff, i.e., decreasing low-end, as the knob is turned clockwise.  Normally, positions towards CCW would be used for "clean" tones, and positions towards CW would be used for "dirty", but of course the reason for the separate control is so that one can choose otherwise!  (E.g., one might wish to use a more-CCW position for a highly-distorted sound, if one were then going to dial-in a significantly non-flat response in the following active EQ module (eq1); the input stage would pass a full-range signal, which the EQ would then shape as desired.)

The second triode stage is an inverting stage with negative feedback; I've shown gain of -1, but it could be set a little higher if the loss through the passive components upstream is too high.  Most of the time, we'll want to keep the input gain set for clean headroom, and then use the switchable gain stages to produce overdrive; but we want to have enough gain to produce some reasonable overdrive with just the input knob, when desired (i.e., for the closest thing to "Tweed" sound that this amp can produce).

Hopefully, the inverting-NFB stage as above, will work well as the output stage for several of these modules.  Each module output may have to drive any of the switchable modules as input, or the inputs of the power amp if no intervening modules are enabled.  Getting all of the module gains and drive levels set right, so that the system works as intended in each of the possible configurations of module in/out (6 relays == 64 combinations total), will probably take quite a bit of experimentation, measurement, and component-swapping.