important note re reverb and effect-loop phase

I haven't started to draw the "official" schematic for the reverb section yet, but I observe that in my previous writings here, as well as in many of the diagrams in my notebook, there is a serious problem: the "dry" signal is out-of-phase with the "wet", at the mix knob.  I had been planning to take the "dry" signal right from the input of the whole circuit section, i.e., non-inverted.  But at the position in the circuit of the "mix" knobs, the "wet" signal is inverted, having passed through one inverting gain stage.  After the mix pots, the signal is inverted again, so the overall reverb/effects path is non-inverting.  The fix is that the "dry" signal must be taken after the first inverting stage.

With my new conception that effect sends and returns need to be line-level and not higher, the effect returns will now share the same input circuits as the reverb tanks, so this phasing problem and its solution apply equally to the external stereo effects loop, as to the internal spring reverbs.

(Note that the "dry" signal for the mix pots, is not the same as either the "bypass" signal which appears at the outputs when the relays are in bypass mode, or the effect-send signal.  Both of these latter are non-inverting signals, as discussed in an earlier post on the reverb section.  Thus, unlike in the case of the effects loops associated with the active EQs, the send signal here will remain always-active.  Also, this effects loop is in-phase.)

Just wanted to get this important correction out there (mainly to myself, so I don't forget).  Schematic coming soon...

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Ummm... more about this... dry signal taken from existing first stage will not work, because this stage is after the source-switch: the dry signal must not be affected by the source-switch, it must always come from the eq2 output position, i.e., the "natural" input signal for this stage.  It is the contrast between the distorted dry signal, and a reverb signal which is fed from an earlier, cleaner signal location, which the source switch is intended to enable.  If both dry and wet are fed from the earlier point chosen by the source switch, then there's no point to the later stages, they would be feeding nothing.

So... I don't like this, but the initial conclusion would seem to be that *yet another* 12AX7 is needed for the reverb section: a total of four.  Yikes.  I feel a great desire to get this tube count back down again, I was already unhappy with three.

One way to reduce the tube count, would be not to buffer the outputs of the mix pots.  This circuit only has one target (load) to worry about, i.e., the inputs of the power amps.  It's either driving the power amps, or it's bypassed.  Other circuit sections have to be able to handle a number of possible output loads, depending on which bypass relays are energized, and also depending on the position of the reverb source selector (which puts some extra load on whichever output it is monitoring).

My original idea for the power amps had the master-volume pots right at the inputs, then followed by tube stages.  But this will only work well if all the other preceding circuit sections have good, low-impedance tube output drive: if the output of the reverb section is "passive", coming from the mix pots, there will be poor performance directly driving the volume pots, and there may be a noticeable change (mainly, in sound level), switching the reverb between "in" and "out".  The power amps could perhaps be reconfigured to have a tube stage ahead of the volume pots; but if this requires actually adding a tube, then nothing is saved: it only makes sense if the reconfiguration can be done within the current tube-count (I'll look into it).

However, maybe the best way is for me to just, reluctantly, get used to the idea of four tubes for the reverb section alone.  The final stages in the reverb circuit, are closed-loop inverting stages, which give better performance for the resistive mixing networks, because the resulting virtual grounds at the tube inputs, ensure significant separation between the dry and wet signals.  Without the virtual ground, the resistive networks might have enough "leakage" that 100% dry or 100% wet settings are not attainable.

Well, that's my current stream-of-consciousness about all this!  As you can see, it is a work in progress, and significant issues remain in play, yet to be solved to my true satisfaction...