Recto Recording Preamp Series Loop Mod

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Steve Hunter

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I took the plunge today and did the mod to change the parallel fx loop to series on my recto recording preamp. It was pretty different from the dual rec mod to do the same thing which I've seen pictures of, but fortunately those gave me enough of an idea that I was able to figure it out, thanks also to Mesa for labeling their breadboards really well. So here are some pics.

My reason for doing the mod was that having just purchased a Boss ES-8 loop switcher/midi controller/amp controller everything was going along fine until i tried using certain combinations of pedals and found I got the low frequency hum people talk about which is apparently caused by the phase shift between the dry signal being mixed with the wet signal, and others have reliably solved the same problem on Dual Rectifiers by converting the FX loop to series instead of parallel - a shockingly easy, reversible mod. This was a little tricky for me to figure out because the layout is different, so I couldn't just copy what the others did, and there are stereo returns instead of just a single one. Beginner's luck i guess, cause it worked first try and went off without a hitch.

Step 1: Clip off that potentiometer, from the "mix" knob that goes from 10% - 90%.
48414483_10103663582633028_1298644986136363008_n.jpg


Step 2: Clip off those 4 pink wires that went to the mix pot. Here you can see my testing phase, pre-solder. The Yellow and Brown wires are carrying the hot signal from the returns Left and Right back to the rest of the circuit (the next picture shows the other end of those, where you can see them labeled L and R).
48406806_10103663582707878_216602785649524736_n.jpg


49070800_10103663582677938_6794484253334700032_o.jpg


Step 3: De-solder the loose ends of the pink ones, and solder the Yellow and Brown wires into the proper places (Y=Left, B=Right), which are the two spots closest to the removed knob/pot.
48403089_10103663582832628_5219922583759093760_n.jpg


Having done this,I put my rig through its paces with the pedals that were causing problems before all on in various combinations and orders in the FX loop, and couldn't make the problem I was having before happen again, so I call that success!

Hope this helps anyone else considering this mod. I have about 5 hours of soldering experience in my life, a middle school elective course on electronics 23 years ago, and a general understanding of how to wire up a stereo. This mod really was that easy.
 
Steve! Thanks for doing this. My Recto Preamp does not have pink wires, and your pics aren’t showing up in the thread. Any way you can repost them?
 
Hi! I just tried to do it by his text manual.


1. Desolder all wires that goes to MIX pot.

2. Desolder grey wires (or in his example pink wires) from the board

3. Solder Yellow wire to the closest spot next to MIX POT on the board and Brown wire to the next spot (both right behind Left Return jack)


4. TADAAAA
 
Before performing the mod it's worth checking what tube you have in the FX loop slot and if the FX loop is a cathode follower postion. I went down the road of changing the loop to series in my Rocket 44 several years back and found installing the correct type of tube solved my noise issues. For a while I had a push pull pot so I could go between series and parallel. After putting a Chinese tube into V4 (on my amp) I didn't get the same noise issues from my delays etc when in parallel mode that I had been previosly encountering. For the record I also had problems with JJs in the cathode follower postion, not just Sovteks and EHs.

This is the old info from Mesa.

Most (but not all) Mesa/Boogie amplifiers have one or two “cathode follower”? tube stages in their preamps. Tube selection is CRITICAL in these stages.

Specifically, in a 12AX7 tube used as a “cathode follower”?, the voltage difference between that present at the cathode, as compared with the heater voltage, can be withstood or tolerated by certain types of tubes, whereas other tubes will fail. The failure of a “cathode follower“ tube will cause sound dropouts or signal loss.

For the past few years, Mesa has been using two types of 12AX7 tubes: ones originating in Russia (Sovtek EH), and ones originating in China. The Russian (Sovtek) tube is NOT reliable as a cathode follower. Of the tubes we are using today (March 2008), ONLY THE CHINESE 12AX7 IS RELIABLE AS A CATHODE FOLLOWER.

In conclusion, if you are troubleshooting for signal dropout in a Mesa/Boogie amp, suspect a cathode follower tube, and try replacing it with a Mesa 12AX7 that is marked as “CHINESE”? (silk-screened on the tube itself).

Below is a partial list of Mesa amps and cathode follower tube locations:

GUITAR AMPS BASS AMPS

Lone Star & LS Special: V3
M-Pulse: V2
Stiletto: V3 & V4
Venture: V2
Road King I: V3 & V4
Big Block 750: V4
Road King II: V3 & V5
Titan: V4
Roadster: V3 & V5
M2000: V2
Dual & Triple (2ch or 3ch): V3 & V4
Bass 400+: V2
Tremoverb: V3 & V4


Source: https://www.richardcleaver.com/2009/07/10/mesa-boogie-tube-problems/
 
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