Effects Loop in simplified AB763 Deluxe Reverb

[Ten Over]

The -20 / +4dB switch is noted on the Nanoverb spec sheet that was included as an attachment to your previous posting. Maybe it pertains to an earlier model version than yours?

The spec sheet doesn't say that the two extremes are selected by a switch. I believe what they mean is that the nominal output level is continuously adjustable from -20 dBV to +4 dBu by rotating the output knob.

[pdf64]

Ahh, ok, so it should read -20dB to +4dB, rather than -20dB or +4dB

[pullshocks]

With Gain on 5, Master Volume on 7, and the loop bypassed, strumming the open strings gave a SPL of about 80 dBZ (an unweighted SPL reading). With the loop unbypassed but nothing plugged in the loop, SPL dropped to about 77dBZ. Maybe adjusting the attenuation would deal with this.

Plugging the Alesis Nanoverb (See attachment for specs), with no attenuation on the 10K send pot, and the Nanoverb input level at noon, the input level indicator stayed in the green, did not flash red. So send level is good.

However, even with the Nanoverb output level at maximum, there is a very noticeable volume loss. SPL was only about 68-70 dBZ, compared to 77 dBZ with nothing in the loop. The specs state the Nanoverb max output level is +17.5 dBU.

The loop was designed to have nearly unity gain with nothing plugged into the loop when using a 12AX7. You are showing a 12AT7 which will not result in unity gain. You could try bypassing the cathode resistor on the return 12AT7 triode or just use a 12AX7.

The Nanoverb has a maximum output of 5.81Vrms, but that doesn't mean that 5.81Vrms is going to come out of it when the output knob is dimed. It depends on how much the input knob is rotated and what the gain of the Nanoverb is.

The signal level hitting the return triode with nothing in the loop is typically higher than the line level signal coming out of an effects device, so volume loss with a loop is going to be the typical experience.

Ooops, I did change to 12AX7 as per Merlins schematic (and yours) but forgot to make that change on my project schematic.
Thank you for filling in the blanks about the real world of effect loop use. I have never used one before. I may be able to bring the Nanoverb input level up little more without hitting the red overload indicator,which hopefully would bring up the output level.