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FX Loop in SLO-100

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6/22/2004 7:00 AM

FX Loop in SLO-100

HI guys,

taking as closer look at the SLO schematic, I wondered why on earth Mike Soldanoe used two full 12AX7 for the loop. He could have easily done the Send stage with a cathode follower and the Return with a single triode.

And is the 1uF decoupling cap on the Send cathode follower correct on the schematic? Why such a huge cap? A 0.1 uF would be fairly enough?!?

Did anybody already alter the Loop or come up with a modded version?

Another thing that has been discussed here is the fact, that with delays there will be a jump in signal level if switching from OD to Clean, since the Clean MV will probably be set higher, thus the last delay signal will be much louder (as written in another thread on ampage).

Why did MS not move the Clean MV in front of the Loop? A good place could be in front or after the 330k after the V2B (??).

Any ideas guys?

Greetings,

Dominik

6/22/2004 11:54 AM

I think that because of the way the preamp is "switched" that a MV placed before or after the 330K would also load down the Lead channel. I built an amp that has a similar clean channel but with a Bogner-ish Red channel for the Lead channel and I let both channels sum into the gain stage before the tone stack and then into the tone stack. Then the there is a 1M-A "Lead Master" after the tone stack (this pot is switched IN/OUT of circuit when switching channels). The tone stack (or Lead Master if in Lead mode) feeds a "tapped" cathode follower Send stage (with Send Level control) and then there is a typical Return stage with the Return Level ultimately being your Master Volume. It uses 5 preamp tubes total (including PI) and 3 LDRs. I've run my delay and a volume boost in the loop and it works great EXCEPT that there is a VERY quick surge in volume when switching from Lead to Clean. I've sidestepped that somewhat by making the levels of the two preamps closer in gain but it's still there. The schematic for preamps and LDRs is here:

http://www.lilypix.com/photos/showpic.php?aid=2247&pid=31087&uuid=283

I didn't write the schematic of the Send or Return stage. My Send uses a .47uF (not .047uF) coupler after the Send cathode follower. I think you need a big cap there to get full bandwidth. I'm not sure why MS did it that way, seems like there is a better way. I thought I had found a better way but I'm not quite there yet.

6/22/2004 5:56 PM
Matt	OK, I converted my schemo to JPEG and some funky things happened but it still looks OK and is readable. The PI and power amp is pure Marshall aside from a 3-way switch that changes the NFB a little. http://www.lilypix.com/photos/showpic.php?aid=2247&pid=31363&uuid=283

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