(That big blank panel on the bottom row, is for the "McGrath" logo. I think this will be done with masking and black paint, so that the background will be black and the letters will be bare aluminum showing through. Of course, for users of the amp, or for those who copy my design and layouts, this logo area also represents six or seven knobs-worth of extra panel space, for added circuitry.)
You can see the new four-knob layout for the reverb section (source, tone, mix L & R).
I've been planning out the circuitry to go behind those knobs, in more detail. Looks like the system will need three 12AX7 tubes, not two as I originally hoped. Two triode sections (one tube) will be needed for the reverb driver, in order to support the active tone/contour control. The driver circuit will drive a single transformer, connected to both reverb tank inputs; I believe I will provide four RCA jacks from the transformer output, two in parallel and two in series, so that more experimentation with different reverb tanks is facilitated. The reverb returns (L & R) will be amplified by non-feedback inverting gain stages (two triodes), and these stages drive the "wet" sections of the dual-gang pan ("mix") pots; the "dry" sections are fed directly from the original input signal (i.e., the output of the eq2 bypass relay), with no buffering. The wipers of the pot sections are summed together through resistors, into the virtual ground of negative-feedback triode stages (two more, total six -- i.e., three tubes).
Because of the six-position source selector switch, the bypass wiring gets a little complicated. Instead of one DPDT bypass relay like most of the subsystems, the reverb system needs two DPDT relays. Both poles of one relay are used for switching the two outputs, L and R. One pole of the second relay switches the input of the reverb driver, between the output of the source selector switch ("in"), or ground ("bypass"). The effect-send jack also feeds from the selector switch, but is not affected by the bypass relay: i.e., it is always-active. And the other pole of the second relay disconnects the "dry" signal from the mix pots when bypassed, to reduce loading on this non-buffered signal. When bypassed, instead of feeding these mix pots, the dry signal will be feeding the final master-volume pot sections: just about the same kind of load. (The source of this signal might be any one of the prior subsystem blocks, depending on which ones are bypassed or in-circuit.)
To summarize:
reverb driver input: subject to source-switch; subject to bypass.
effect send jack: subject to source-switch; not subject to bypass.
dry signal: not subject to source-switch (always from eq2); subject to bypass.
Thus, the effect send jack can be treated as a "monitor" output, which can sample the signal at various points in the chain. In addition to the obvious uses, this jack could be used to connect a tuner, a VU meter, other test equipment such as oscilloscopes or spectrum analyzers, etc.; or it could be used as a buffered line-out to drive other amps or signal chains, with the source switch providing the option for this signal to be clean or distorted.
The L & R effect returns are simply circuit-interrupting jacks, inserted at the last point before the output bypass relays (i.e., effect returns are of course subject to bypass). Thus, there is no buffering or padding, of either the effect send or the returns; the external circuitry must be able to handle and produce tube-compatible signal levels (nominally line-level, but with possible very high peak levels). I thought about putting the returns earlier in the circuit, so that they could be affected by the "mix" pots (and also they'd be tube-buffered); however, this would have made these inputs inverting. This would have also required me to separate functionality and add another pair of jacks: power amp in L & R. Instead, I can retain the dual functionality of the effect return jacks: they are also the power amp inputs when the preamp is off. (By unplugging the reverb tank outputs, one can get access to high-sensitivity inputs which are subject to the mix controls, although probably only RCA jacks will be available.)
Depending on how much extra complication I want to add, there might also be a "post-tone" effect send jack, which sends the same signal that drives the reverb transformer. Thus, the active tone/contour circuit could be used as a pre-EQ for external effects, if desired (and also, possibly my real motivation, it will be easier to monitor and instrument this tone circuit). This post-tone send jack will be subject to bypass, like the spring reverbs themselves; unlike some guitar amps, the reverbs will not be driven when the reverb subsystem is bypassed.
So... it's obvious how the overall functionality of this subsystem works, right? One can have either spring reverb, *or* external stereo effects, but not both. The bypass relay (i.e., footswitch or front-panel switch) controls whichever is active, spring reverb or external effects. The spring reverbs are operative until the external effect outputs are plugged into the effect return jacks: then the external effects take priority over the spring reverbs. If desired, one spring reverb (L or R) can stay active, by plugging into only one of the two return jacks. The effect send stays active, regardless whether spring reverb or external effects are being heard at the output. The source selector switch controls the input to the reverbs and also the effect send, but not the dry signal. Thus, the dry signal can have more distortion or a different EQ than the "wet" reverb/effects signal. The "mix" pots vary between 100% dry and 100% wet, independently for each speaker, so the spring reverb can be effectively "panned" all the way to one speaker if desired, or placed anywhere else in the stereo mix. Unfortunately, as discussed, the external effects will not be subject to the wet/dry mix knobs, for pragmatic reasons; stereo panning and wet/dry mixing will have to be accomplished by the external effects unit, itself.
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