Is an added FX loop sucking tone when not used?

[rob2001]

Hi, I've got a JCM 800 4104/2204 and before I bought it, an FX loop and a built in attenuator was added. Thanks to some help from TGP guys I removed the resistor based attenuator. Big difference there. So now it has this FX loop stuff in it. There is a send and return knob and a 12ax7 tube added. It's tied in at what appears to be the C9 cap, which is gone. I did get ideas on removing it but i'm wondering if I really need to. Is it messin with my tone being there when not in use? I can get pics if needed but i'm hoping someone would just know whats going on here. I don't need the loop and it's really some pretty ugly wiring!! Thanks in advance, Rob

[John Phillips]

If the loop circuitry is in the signal path when it isn't being used, it is affecting the tone. Whether it's 'sucking' it or not is another question... some amps with tube-driven loops actually sound better with the loop engaged. But if it's a mess, I wouldn't count on it.

FWIW, solid-state (IC) driven loops actually affect the tone far less than tube ones, either for good or bad. As long as they're not being driven into distortion themselves, they can be extremely transparent.

[rob2001]

Thanks John, pretty sure i'm gonna take it out. It didn't sound too bad when using it but it started making wierd noises and crackling so I stopped using it awhile ago. So if I got the right info I can just remove the wires at the board and replace the c-9 cap, which if I got the correct info, will be a .022uf 400V cap. Metro has the Sozo's. It's jumpered to V1 and V2 so i'd remove those as well. Does that seem correct?

[ChickenLover]

It should be fairly easy to bypass the loop to see how the amp sounds without it. You probably would only have to disconnect where the preamp goes into the Send circuit...then reconnect the preamp out to the PI input (and simultaneously disconnect the Return circuit from the PI input). So now the preamp just goes straight into the PI...taking into account the Master Volume (I'm not sure how this loop is configured).

If I had to guess...the loop might lop off a tiny bit of high end which might make the amp sound a little fizzier wothout it (or it might sound a bit more open/airy).

[Structo]

Maybe the jacks on the loop need to be cleaned?

[epluribus]

If the loop circuitry is in the signal path when it isn't being used, it is affecting the tone. Whether it's 'sucking' it or not is another question... some amps with tube-driven loops actually sound better with the loop engaged.

Hey John.

Since we're talking about modding and fixing loops, got a link to a schematic showing a loop done right? Or, say, maybe a tube-powered circuit and another built SS?

I'd love to know more about how to do this properly.

Thanks as always, JP. :BEER

--Ray

But if it's a mess, I wouldn't count on it.

FWIW, solid-state (IC) driven loops actually affect the tone far less than tube ones, either for good or bad. As long as they're not being driven into distortion themselves, they can be extremely transparent.

[stvnscott]

I tend to gravitate toward series passive loops. It does the job with most gear and is totally transparent when nothing is plugged in.

[John Phillips]

Since we're talking about modding and fixing loops, got a link to a schematic showing a loop done right? Or, say, maybe a tube-powered circuit and another built SS?For a tube loop done right, look at a Fender Rivera-series Concert. It isn't 'transparent', but it sounds good, and has useful level matching which can also be sued to change the gain structure of the amp if you want.

For a solid-state loop done right, look at (no kidding!) a Fender Hotrod. Jump a cable from the Preamp out to the Power Amp in and see if you can hear a difference :).

The only thing I don't like about the Hotrod's loop is the use of the FX return/Power Amp In jack to switch the signal path, which necessitates the use of one of those crappy brittle plastic jacks, because a standard Switchcraft or Cliff can't do that separate from the audio path. (The Concert used a now out-of-production Switchcraft with an isolated switch.)

A better solution is the new Marshall loop they put in the reissue 1987 and JCM800 - hard-bypassed using a manual switch, not the jack.

[epluribus]

For a tube loop done right, look at a Fender Rivera-series Concert. It isn't 'transparent', but it sounds good, and has useful level matching which can also be sued to change the gain structure of the amp if you want.

For a solid-state loop done right, look at (no kidding!) a Fender Hotrod. Jump a cable from the Preamp out to the Power Amp in and see if you can hear a difference :).

The only thing I don't like about the Hotrod's loop is the use of the FX return/Power Amp In jack to switch the signal path, which necessitates the use of one of those crappy brittle plastic jacks, because a standard Switchcraft or Cliff can't do that separate from the audio path. (The Concert used a now out-of-production Switchcraft with an isolated switch.)

A better solution is the new Marshall loop they put in the reissue 1987 and JCM800 - hard-bypassed using a manual switch, not the jack.

Thanks John. This is a bridge I'm going to have to cross soon as I figure out all the cool stuff I can do with the extra 12AX7 in my guinea pig amp. It's a huge help to see how a loop has been successfully done, then sort of reverse-engineer it to understand why it works. (Even better when you wire up your own abominations of the original and discover why it doesn't work. :))

BTW, as I was diggin' around the net on this subject, I came across this:

http://www.schematicheaven.com/dumbleamps/loop.pdf

Hey, if it says Dumbl*...

--Ray

BTW, on the SS version...this strikes me as one of those places SS may actually be an advantage. If what you want is a highly linear and transparent component, I don't see any reason why SS couldn't be a terrific solution. And on that note, I ran across this...

http://www.ironsounds.com/

...but it's not as much fun as learning to build your own...

[epluribus]

...and has useful level matching which can also be sued to change the gain structure of the amp if you want.


...boy, gotta have a lawyer for everything these days...:rolleyes:

[rob2001]

Ok then, so I cleaned the jacks and cleaned the send and return pots. The cracking is gone and everything sounds good. I can't say it's a bad design as far as a loop goes but I'm not much of a tech. It sounds OK when used but It seems tricky to find the right levels on the pots depending on what i'm running in it. Since i'm running fairly low gain my time based stuff sounds fine out front. I guess i'm just thinking of squeezing all I can out of it.
For now I decided to just leave it. If it's not destroying my tone I surely can't complain, the amp sounds fantastic as it sits, it just sounds much better when the loop isn't being used.

I noticed what John mentions about changing gain structure with the levels. I get very different tones by running the loop levels low and the master way up vs. loop levels high and master low. I'm not sure how that fits into the equation but there is a difference. I'd think the loop is acting as a second master in a sense, but it's routing the signal differently.

Since I opened it up I can see someone did some work on this amp. Besides the loop and attenuator there are a few other suspicious things that make me wonder what was done. At some point i'm gonna send it out for restoration to stock. Then again, it sounds killer so maybe I sould just leave well enough alone! Thanks for the help, Rob

[Ted Witcher]

Thanks John. This is a bridge I'm going to have to cross soon as I figure out all the cool stuff I can do with the extra 12AX7 in my guinea pig amp. It's a huge help to see how a loop has been successfully done, then sort of reverse-engineer it to understand why it works. (Even better when you wire up your own abominations of the original and discover why it doesn't work. :))

BTW, as I was diggin' around the net on this subject, I came across this:

http://www.schematicheaven.com/dumbleamps/loop.pdf

Hey, if it says Dumbl*...

--Ray

BTW, on the SS version...this strikes me as one of those places SS may actually be an advantage. If what you want is a highly linear and transparent component, I don't see any reason why SS couldn't be a terrific solution. And on that note, I ran across this...

http://www.ironsounds.com/

...but it's not as much fun as learning to build your own...

Do you know of anyone with experience of the Iron Sounds loop?