fx loop on 5E3 clone

[famousmockingbird]

I want to add an fx loop send and return so I can add a reverb processor. I was going to put the fx loop just before the phase inverter, I was just looking for some feedback from some members if I am doing this correct.

Here is the schematic:

[firechief]

Are you adding gain to the overall pre-amp? The cathode follower looks right, but I wonder if you even need to add a tube section after the return. What is the max output voltage and impeadance of the reverb device?

[leadbelly]

Load on the cathode follower is not so great. This is a better implementation:

[jjman]

Does the 2nd stage clip? If not, a loop would have no benefit over using the effect in front. Unless the effect unit requires a line level input.

I'm using a lexicon in a (parallel) loop but it's after my 3rd stage which clips at higher preamp settings. I drive it via a voltage divider dropped to ground and boost it's return before the PI via 1/2 a 12ax7. Then recombine with the dry signal via a mixer pot.

[Nigel Goodwin]

Originally Posted by leadbelly

Load on the cathode follower is not so great. This is a better implementation:

What's with the 100K cathode resistors? - they look far too high.

[Printer2]

You just have to look at them a different way.

[famousmockingbird]

Input
Number of Channels: 2
Format: 1/4" unbalanced
Nominal Level: -10 dBV, adjustable to +4 dBv
Maximum Level: +10 dBV

Impedence: 1MW/channel stereo, 500kW/channel mono

Output
Number of Channels: 2
Format: 1/4" unbalanced
Maximum Level: +17.5dBu
Nominal Level -20 dBV or +4dBu, front- panel adjustable

Output Impedance: 500 ohms

Here are the specs for the unit, and yes the 2nd stage will be clipping pretty hard and that is why I don't want the reverb before the gain stages.
I see from that weber schematic that the second triode in the 12au7 (the return) is another cathode follower. Which makes sense because I don't really want anything but a buffer for an fx loop. I found that schematic and thought maybe that the gain stage after the return might warm the digital processor up a little but not add too much gain.

[jjman]

Presumably the signal to the PI would normally be stronger than what might come out the effects unit. Although your unit appears to have boosting potential on it’s output, using such could increase hiss. Mine does at higher output settings. So having a gain stage after the return is probably an appropriate option. I wouldn't know how to predict the numbers for a "transparent" result so I would use a pot on the grid of return's tube stage and use a scope to set it's output at parity with the signal strength that would go to the PI in the absence of the loop. Assuming preservation of the 5E3 clipping order/behavior is a goal. Then you could replace the pot with a divider or leave it somewhere on/in the amp in case you use a different unit. Like a delay or volume pedal.