Passive Effects Loop Implementation

[McAbe]

Hi All! Been lurking for a while and appreciate all of the great knowledge shared here. I am in the process of building a 6v6 Plexi out of an old amp which had an FX loop in it. My question is about the preferred method to wire up a passive fx loop.

I had planned on taking the signal off of the treble wiper and sending it to the tip of the send jack and the shorting switch of the return jack. Then connecting the tip of the return jack to the master volume pot. See the attached schematic for my planned build. I see others instead send the signal to the tip and shorting switch of the send jack and then tie the 2 shorting switches together. Why run the signal through both shorting switches though? Seems like more points of mechanical failure. Anything I am missing?

[Lectroid]

Doug Hoffman has a passive loop elsewhere here, also Rob Robinette has one.

[McAbe]

Thanks. Looks like Rob does it my way: https://robrobinette.com/Generic_Tube_Amp_Mods.htm#FX_Loop. Could not find a hoffman project with a passive effects loop.

Still curious why anyone would do it the other way though.

[sluckey]

My question is about the preferred method to wire up a passive fx loop. Why run the signal through both shorting switches though? Seems like more points of mechanical failure. Anything I am missing?

Look at the linked pdf. The answer may become apparent. These jacks can be used for a send/receive FX loop, or, and this is my favorite use, preamp output and power amp input. Makes it easy to patch THIS preamp to another power amp. Or patch another preamp to THIS power amp.

I use very reliable switchcraft 12A jacks. If this really concerns you just use a short patch cable between the jacks. Then you can worry about the patch cord instead.   

Passive Loop

[pdf64]

Good stuff with the design, some nice features 
The heater fuse looks to be in the wrong place? Are you sure you even want one? If so, note that automotive type fuses are much better suited to low voltage high current than cartridge fuses.
HT switching and fusing may be better suited to the AC side of the rectifier.
I'm not sure it's a good idea to send the speaker signal down a footswitch cable. There's a chance the resulting additional capacitive load might cause a high frequency phase shift / resonance.

[McAbe]

Thanks sluckey. I think the main difference is whether you want it to disconnect the power amp when you have something inserted into the send jack or not. For fx, likely not, but for a preamp out it probably makes sense.

@pdf64 thanks for checking the rest of the schematic. I'll check the heater fuse. It was a part of the old amp, so I figured I may as well include it. Other than a complicated relay, any other way to do a footswitch on the negative feedback? The old amp has a hole in the chassis for a footswitch that I am trying to use, and I like the idea of having a boost (and less negative feedback) easily switchable for solos.

[McAbe]

Thinking about this a little more, having the send after the master volume as in sluckey's schematic makes a lot of sense because it allows you to attenuate the signal and avoid clipping whatever device you are sending your signal to. I also reworked the boost circuit to make it easier to implement given the layout of my amp.

I was not sure about moving the sag switching to the AC side of the rectifier though. I understand that switching DC voltage is hard on the switches and can cause arcing and flyback, but I have no idea how sag would work on the AC side of the rectifier. Are there schematics of amps that do that?

Latest schematic attached. Working on wiring up all the pots, jacks, and switches. Hopefully, make some good progress this weekend.

[pdf64]

Thinking about this a little more, having the send after the master volume as in sluckey's schematic makes a lot of sense because it allows you to attenuate the signal and avoid clipping whatever device you are sending your signal to.

Yes, with the passive loop I incorporated into a recent build, I’ve used that rationale too. It works fine.

I was not sure about moving the sag switching to the AC side of the rectifier though. I understand that switching DC voltage is hard on the switches and can cause arcing and flyback, but I have no idea how sag would work on the AC side of the rectifier. Are there schematics of amps that do that? …

The sag resistor would work just the same, I’ve put one on the AC side of a FWB rectifier.
What changes though is the ratio of current to voltage; for the same sag, resistor value needs to be lower, eg somewhere in the range 1/2 to 2/3 of the DC value.

[McAbe]

Same for resistors and zeners? I worked out the voltages using the Duncan PSU designer to try and get 400V B+. But I think that only models on the B+ side.

[pdf64]

Same for resistors and zeners? …

Oops, I forgot about the zeners, they won’t work the same.
I think it may be feasible to use the zeners in inverse series pairs on the AC side.
But perhaps easier to leave that bit as it is.