SLO Post Tone Stack FX Loop mod.

[VaporDemon]

I have a mod to move the SLO/Jet City effects loop after the tone stack. In this mod V4 and the Tone stack take the place of V3 making the first half of V4 the 4th gain stage. Since the 1st half V3 was a 1.8K cathode with a 1uF bypass cap & V4 has 2.2k cathode without a bypass cap should I change the first half V4 to what was there or leave it as is?

 

[glpg80]

Do you have a schematic of what you’re wanting to accomplish? It will help to describe your mod and suggest changes elsewhere if needed.

 

[Jack Luminous]

Here's a pic of my SLO post tone stack loop mod. Black lead with red end coming out of big crooked cap is loop send. Second black lead going to small resistor is loop return.

 

[VaporDemon]

Do you have a schematic of what you’re wanting to accomplish? It will help to describe your mod and suggest changes elsewhere if needed.

I don’t have a completed schematic for it. I will see if I can make one in the next couple of days.

 

[VaporDemon]

Here's a pic of my SLO post tone stack loop mod. Black lead with red end coming out of big crooked cap is loop send. Second black lead going to small resistor is loop return.

View attachment 391878

Awesome! My question is how did it work for you and also did you just swap the stack and loop or did you alter v3/v4 to accommodate the changes?

 

[Jack Luminous]

It worked great. I had a local tech do that to my SLO when I bought it about 20 years ago. I didn't look closely but I think he just moved the FX send V3A cathode follower stage after the tone stack along with V4B FX return recovery stage. He also changed the FX send attenuator resistor because the original loop was too hot. Maybe he modified the FX recovery stage gain accordingly, I'm not sure. All in all, it's just slight tweaks and the amp sounds just as it sounded before, but with a usable loop.

 

[Shask]

I did the SLO mods, but then also moved the stock loop to the regular place on my JCA100HDM. I basically copied the design from the JCA22H, because it was designed to have the loop after the EQ. I also added a master volume on the back of the amp.

If you do the loop mod, just prepare for it to be difficult. I drilled through traces on the PCB from the bottom, cut traces, added shielded cable back and forth, tie strapped capacitors together, etc.... it is definitely not for beginners.

 

[Shask]

I found a few pictures I took of my JCA100HDM loop mod

 

[VaporDemon]

I did the SLO mods, but then also moved the stock loop to the regular place on my JCA100HDM. I basically copied the design from the JCA22H, because it was designed to have the loop after the EQ. I also added a master volume on the back of the amp.

If you do the loop mod, just prepare for it to be difficult. I drilled through traces on the PCB from the bottom, cut traces, added shielded cable back and forth, tie strapped capacitors together, etc.... it is definitely not for beginners.

Awesome thanks for the info.

 

[VaporDemon]

I did the SLO mods, but then also moved the stock loop to the regular place on my JCA100HDM. I basically copied the design from the JCA22H, because it was designed to have the loop after the EQ. I also added a master volume on the back of the amp.

If you do the loop mod, just prepare for it to be difficult. I drilled through traces on the PCB from the bottom, cut traces, added shielded cable back and forth, tie strapped capacitors together, etc.... it is definitely not for beginners.

Thanks for the info and photos! I’ve actually done the mod before a few times. I also did it on my 100hdm and again now on my 50H which I modded to slo spec. Reason I posted the question is because the 4th gain stage is swapped from V3 to V4 and there should be a bypass cap on the 4th gain stage. It’s easy enough to just add the cap and see what happens. I was just looking for someone like yourself that also has the mod done and see how they approached it.

 

[SpiderWars]

It’s easy enough to just add the cap and see what happens

I would try it and listen to the saturation. Also play it for feel. There is some extra saturation going on very late in that stock preamp, if it’s not there the amp can sound grainy, less of the thick SLO saturation. Some may like it better that way but I didn’t.

 

[Shask]

Thanks for the info and photos! I’ve actually done the mod before a few times. I also did it on my 100hdm and again now on my 50H which I modded to slo spec. Reason I posted the question is because the 4th gain stage is swapped from V3 to V4 and there should be a bypass cap on the 4th gain stage. It’s easy enough to just add the cap and see what happens. I was just looking for someone like yourself that also has the mod done and see how they approached it.

I added a cathode bypass cap to the loop return stage. You can see the yellow tantalum in the pic. I did this because I had modded my Triple recto in the same way a while back as part of the bass suck loop mod. I had an extra cap, so I tried it. It made the whole amp sound bigger and puncher, so I kept it. It basically just gives a stronger signal into the poweramp.

 

[VaporDemon]

I added a cathode bypass cap to the loop return stage. You can see the yellow tantalum in the pic. I did this because I had modded my Triple recto in the same way a while back as part of the bass suck loop mod. I had an extra cap, so I tried it. It made the whole amp sound bigger and puncher, so I kept it. It basically just gives a stronger signal into the poweramp.

Yeah the stock loop recovery stage on the 50/100 JCA amps is the first half of V4 before the mod and the mod I got for it, which was something I bought early on in my amp modding journey uses the first half of V3 as the recovery stage which has a 1uF already. The mod moves V4 and the tone stack to where V3 was then goes into the 2nd half of V3 and the loop and then goes into the first half of V3 as recovery and into the Master volume relay. My other curiosity is since I modded mine to SLO spec which has the 220k plate resistor for V4 and I’m wondering if the should go back to 100k as it was originally.