Passive FX Loop Pre or Post Volume in 5F2A

[cusebassman]

Hey all - experimenting with a passive effects loop in my stereo 5F2A build, following the Rob Robinette mod -

https://robrobinette.com/Generic_Tube_Amp_Mods.htm#FX_Loop

It works post-volume pot, but with no master volume, the volume pot is the sole volume control, and at lower volumes, sends much less signal to any connected effects. What are the pros/cons of placing the effects send/return before the volume pot versus after?

It would be interesting to add a master volume to each channel to allow the volume pot to act as an effects send level, but I'd like to avoid that for the moment if possible...

[Papa Jim]

I'm kinda interested in your stereo schematic. Could you possibly post it too. May be helpful with your question for those not familiar with it.

[sluckey]

I'm kinda interested in your stereo schematic. Could you possibly post it too. May be helpful with your question for those not familiar with it.

Click on his name. Then click on show posts. There are only 18. You'll find his stereo threads.

[cusebassman]

I have a wiring diagram (attached) but no official schematic - this amp was adapted from a post over on tdpri:

http://www.tdpri.com/threads/stereo-5f2a-build.476120/

Added TMB tone stack, figured out the grounding scheme (recent thread here, as sluckey mentioned), and it sounds fantastic - like a stereo bedroom-volume 5F6A Bassman (minus the push-pull, of course).

[pdf64]

Putting an fx loop in such a preamp seems pointless to me; what's the drawback of putting fx before the amp input?
For the loop to be passive seems positively detrimental to performance.
Aiken explains the issues about halfway down his Q&A page http://www.aikenamps.com/index.php/technical-q-a

[cusebassman]

Interesting read. I was looking at the effects loop mod as a way to place effects like modulation, chorus, tremolo, and reverb after the first preamp section and EQ, to allow for the use of boost/overdrive pedals that drive the first 12ax7 stage to distortion before sending the signal to those effects.

Since completing the stereo amp, I've been running my signal chain as follows:

Clean boost --(mono)--> distortion --(mono)--> rotary pedal --(L,R)--> stereo delay --(L,R)--> stereo chorus --(L,R)--> stereo reverb --(L,R)--> Amp channels 1 & 2, respectively.

The rotary pedal acts as the point in the chain where the single guitar signal gets split into a stereo signal, and after that the left and right signals are sent through the rest of the chain separately, and then into each side of the stereo amp.

To be fair, right now I am generating most of the distorted sound in the pedals, rather than the amp itself. But I wanted to make the amp versatile enough to allow a player to place certain effects past the preamp stage if they wanted to drive the preamp section and avoid distorting those added effects.

[pdf64]

So each channel of the amp is now more akin to a BF Champ https://el34world.com/charts/Schematics/files/Fender/Fender_champ_aa764_schem.pdf than a tweed Princeton?
For your intended use, (ie input stage may be overdriven and fx loop used at low channel volume control settings), then putting a passive series fx loop after the volume control would probably work out best.

[cusebassman]

Huh, look at that. I've never studied the BF schematics, but yes, it's much closer to that BF Champ circuit, just with a slightly more complex TMB stack. I do wonder if an overdriven first preamp stage would be too strong a signal (pre-Volume) for pedals like the following:

Boss RT-20 Rotary
TC Electronic Nova Delay
Strymon Flint

Not that I intend to run the amp full-out, but turning the volume all the way up on each channel would have the same effect as running the effects loop pre-volume. There's also the potential preference for having the loop post-volume so that the volume alters the response from the pedals...

[pdf64]

If you were overdriving the input stage, then the fx fed directly from it would get >30dB overdriven, unless they have the facility to attenuate their input signal. Hence the benefit of a properly engineered loop.

[cusebassman]

Thanks for all the help, that clears things up. I'll give the post-volume pot a try, and see if I like the sound over just running everything into the front of the amp. Incidentally, what do you (or others) consider the better way of adding an effects loop to retain the ability to hit the front end hard but not fry any effects in the loop? I know there are buffered effects loop circuits (and pre-fab modules) available, but I wonder how much people around here have used those (and if so, where in their circuits). I'm also curious if those solve for the problem of having a wide potential dynamic range coming from the preamp section of the amp.