Finally Modded the Loop of my old 2-Channel Triple Recto....

[Shask]

I finally did something I have been meaning to do for a LONG time. I modded the Loop of my old 2-Channel Triple Recto. I know the Parallel to Series mod is popular, but I actually decided to leave it Parallel because I plan to use some rack units with it. I am talking about the "Loop Tone Suck Mod". When you use the loop, you lose all your bass. It is like you have to set it about 4-5 notches higher to have the same amount of bass, so when you turn on the loop it sounds all neutered. There is no bass, so it is tighter and brighter, like a different amp.

There is a badly chosen cap value on the return stage of the loop. The Cathode bypass capacitor needs to be much larger. When doing the math I found the stock value starts cutting bass at about 300-350hz. Far too high. Now it goes down to like 50hz or something. There is a 2.2uF tantalum cap that needs to be changed to 10uF.

After changing this, the amp sounds almost identical when turning on the loop. Maybe a slight less bit of presence, but that is easy to change by blowing on the knob. I actually think it sounds better with the loop on, and I don't think I would ever say that, lol. It is easier to balance the volume knobs, and my EQ pedal sounds more natural.

Anyways, just rambling, lol. Figured I would mention it since so many people hate the 2-Channel parallel loops. They all suffer from this same problem. While I had it apart I took some pics, so here are a few if you like looking at that sort of thing....

 

[KailM]

Nice work! Reminds me of my old 6505+ 112 when I modded its effects loop — neutered and fizzy with the loop prior to the mod; a tone monster afterward.

 

[cardinal]

I use to have a Triple Rec just like that and really wish I hadn't sold it.

 

[wakjob]

What's the resistor value that the cap is bootstrapping?

From my experience, 2.2uF will pass most all guitar frequencies, maybe changing that resistor value would be easier.

 

[narad]

Nice! Really want a chrome chassis triple at some point.

 

[Shask]

What's the resistor value that the cap is bootstrapping?

From my experience, 2.2uF will pass most all guitar frequencies, maybe changing that resistor value would be easier.

The return stage has Cathode values of 2.2k and 2.2uF. If you use an online calculator to plot a frequency graph, the gain starts dropping around 600hz and is about 2.1 db lower at 82hz (low E on guitar). If you go down to about 60hz, which is close to a B on a 7 string, the gain is down over 3db. That is about half the volume on the bass end of what it was before the loop. I found I had to move the bass knob about 4 notches (11 to 3 oclock) to make up the difference.

With this mod (which is the value Mesa recommends), the gain starts dropping around 200hz, and is only .14db down at 82hz, and maybe .3db by 60hz. Huge difference in keeping those lows the Recto is known for.

If you change the resistor you would change the gain of that stage, and I didn't want to mess with the gain.... only the frequency cutoff. Both parts are next to each other, so neither would really be easier. The new cap is larger, so it is probably better to change it since it does not sit as flat as the old one. The worst part of this mod is that these parts are right under several of the tie-strapped wires. It was kind of like playing that board game Operation trying to solder the part without hitting any of the wires with the side of the soldering iron.

 

[wakjob]

Ah, I always assumed changing either the R or C in an r/c filter would get the same cutoff.
http://www.muzique.com/schem/filter.htm

EDIT: I see, it's not OPamp driven.

 

[Shask]

Ah, I always assumed changing either the R or C in an r/c filter would get the same cutoff.
http://www.muzique.com/schem/filter.htm

EDIT: I see, it's not OPamp driven.

That is how the filters work in between the tube gain stages to set the frequencies that get distorted to make a Marshall a Marshall, a Mesa a Mesa, etc....

The tube gain stages, you set the gain by selecting the plate and cathode resistors, and then increase gain by frequency with the cathode capacitor. You set the low end cutoff. You can also parallel a capacitor across the plate resistor to set the high end cutoff. The low end and high end cutoff is determined for each tube gain stage, and then the frequencies are emphasized by filters in between the gain stages. Tube amp design in 3 sentences, lol.

 

[wakjob]

Yeah, I know how voltage dividers work.
I just thought the FX loop was opamp driven.
My bad.