Should I put an effects loop in a plexi build?

[loudthud]

The Kendrick 4x10 Bassman knockoff (Marshall's distant cousin) has an effects loop of sorts, there is a level control with a pull switch next to the speaker jacks. Has anyone seen a schematic for this amp?

[yotool]

The "extra 12AX7" effects loop for the plexi-style amps where the "send" is buffered w. a cascode follower and sent to a variable 250k "Send" level pot then out to the guitar or line level effects processor; and then "returned" to a common cathode (gain) stage and then to the phase inverter's input (aka the Dumbleator approach) does work to preserve the original signal gain sequence. It is a little awkward in its use however because of the way the pre-amp portion of the overdrive is originally attained in the plexi front-end. If one plays the input volumes at "low" (i.e. clean region) then the "Send" pot/effects output level needs to be raised to maintain that 500mv Peak-to-Peak signal to the stomp boxes. Similarly, if you then dime the input channel volumes (distortion-mode) then you have to back that "Send" pot way down to maintain/limit the 500mv Peak-to-Peak signal going out to the effects chain so as not to then overdrive the guitar-level stomp box front-end circuit. I have this installed and working w. the Lamar PPIMV and it isn't too bad but it's a little "fumbly" in practice. I imagine one could dual-gang the volume pots to (inversely) have the "send" signal follow their positions. The problem there is that there are 2 of those volume pots. One for the "bright" and one for the "normal" volume.

I want to try the "small audio" transformer (w. series resistor on its primary) connected right to the main plexi speaker output, and then its electrically isolated secondary winding out to the effects chain approach would avoid having to mic. the cabinet, but get the same thing going on (full amp signal chain tone [- speaker tonality])). As with a microphone, the gains on the output would need to follow the overall amps volume levels. This would also need a separate power amp and cabinet though for the "wet" signal's power drive. As mentioned by others, this would keep the plexi a plexi throughout though.

[Pedro Vecino]

The loop circuit in Marshall reissues (TL072 IC) works great. On a 1959 with post PI master is running fine.

[yotool]

The loop circuit in Marshall reissues (TL072 IC) works great. On a 1959 with post PI master is running fine.

Just wondering...the original signal when the gains are "dimed" coming out of the Plexi treble pot's wiper is as high as +- 40 Volts. This circuit can do +- 15V at best. How can that properly (over) drive the phase-inverter as when it is not in the circuit?

[NateS]

I'm with the voices saying 'this amp is not a good candidate for an effects loop', but it doesn't take but a few volts for the phase inverter to start clipping. 25vpp is still plenty of signal at the input to the phase inverter. As others have poitned out - even if you take care of scaling the signal down and back up, it's fiddly as all getout, which is sort of the antithesis of what a plexi is about.