[Tech] How-to: F-series serial effects loop mod

I thought this was worth extracting from another series effects loop thread as a standalone mod thread, so it's more easily searchable.

After reviewing the other information and photos available on modifying an F-series effects loop to operate in series mode rather than the stock parallel mode, i got to thinking about how it could be made switchable, so you can use it in parallel in the studio where you're less likely to be using a lot of effects, or in series for live use with EQ, delays or other time-based effects.

I came up with the following diagram to do a series/parallel switched mod, which could be done with a DPDT mini-switch or a push/pull pot.



In the original configuration, wires A, B and C are connected to lugs 1, 2 and 3 respectively on the blend pot.

In the modified configuration, when the switch is up, it functions normally, with A connected to 1, B connected to 2 and C connected to 3. With the switch down, A is connected to nothing, B remains connected to 2, and C is now also connected to 2.

All that's required is cutting wires A and C and inserting the switch as shown, then adding one new wire (shown in green) to connect the switch to the middle lug of the blend pot.

Let me know if you spot any errors in my logic... i think this should work.

Word has it that the pots used in the effects loop are 25kOhm linear-taper pots. I'm still awaiting confirmation from Mesa on this.

Any other ideas, comments or suggestions?

That's pretty killer. If I still had my F-30 I'd be all over this mod! The Stilettos have a series loop already so i'm all set...

Killer post!

So i talked to Mesa/Boogie tech support the other day, and these are in fact 25k linear-taper pots on the F-series effects loop blend control.

Does anybody know where i might be able to source a push-pull 25k linear-taper pot?

Did this mod on my Rocket 44 today. Used a 25k linear push/pull pot. It works great -- it's serial pushed in and parallel when pulled out. It's cool as you can really listen to each setting one after the other and decide what works, what has the least hum and the best sound for your needs etc. I had previously converted the loop to serial but I like the idea of being able to switch for different scenarios.

Correct me if I'm wrong...

Dialing the parallel loop to 100% is essentially making it series.

If so, would dialing the stock set-up to 100% sound "noticeably" different that running a switch with it set to series, given that I use a quality switch and know which end of the soldering iron gets hot first?

carlsoti77 said:

Correct me if I'm wrong...

Dialing the parallel loop to 100% is essentially making it series.

If so, would dialing the stock set-up to 100% sound "noticeably" different that running a switch with it set to series, given that I use a quality switch and know which end of the soldering iron gets hot first?

Click to expand...

On the Rocket 44 the parallel knob starts at 10% mix and goes to 90%. There are a number of known issues with Mesa's parallel loops -- mainly with delays creating odd phasing issues. There is a multitude of threads here about people changing these to serial.

Here's some pics from another thread here:
Before (parallel):

After (serial):