JCM900 Effects Loop Question about signal level potentiometer in the back.....

[Gutch220]

Correct me if I'm wrong, or explain it further to me....doesn't the signal level knob for the FX-loop do TWO things? It increases the signal for the send at the same time it decreases the signal for the return, and vice-versa? So it functions as the "opposite" for the send & return?


If this is true, wouldn't the knob being turned all the way in one direction be 100% send and 0% return.............or 0% send and 100% return? (no sound)

 

[guitarbilly74]

If I remember correctly, it basically goes from a -10dB pad in the input stage for instrument level pedals (which yes, it's compensated in the return stage) to a flat 0dB that rack effects can work with, so if you're not using anything in the loop just patch it with a short pedal cable and turn it all the way up to bypass any level compensation and essentially have the preamp connected direct to the power amp.

That said, I haven't had a 900 in a while and I'm going by memory here, so I could be wrong.

But that's my (admittedly vague) recollection.

 

[Gutch220]

So, what would be the sweep of this pot?

?

 

[Kuga]

So, what would be the sweep of this pot?

?
View attachment 153367

It's a stereo pot. As your diagram show I think. Increased return signal when you decrease send signal.

 

[dtier]

It looks like the 3k3 resistors between the pots and ground set the minimum resistance so as not to completely ground out the signal at either end of the sweep.

 

[RLW59]

If this is true, wouldn't the knob being turned all the way in one direction be 100% send and 0% return.............or 0% send and 100% return? (no sound)

It never turns the send all the way down to zero.

And the return starts at 100% and goes  above 100% as you turn the knob down. With the knob all the way up, the full preamp signal goes from send to return. As you turn the knob down, the signal is cut from send and then boosted back up to normal at the return.

That's because the full preamp signal is too hot for many effects and will overdrive them into nasty sounding distortion. Lowering the signal to the effects keeps the signal from overdriving the effects, but also drops the volume. So after the effects the amp return boosts the signal back up as much as the send reduced it.

100% send (zero cut) goes to 100% return (zero boost).

50% send goes to 200% return. (Signal cut in half before pedals, then after the pedals it's doubled back up to full signal).

 

[guitarbilly74]

It never turns the send all the way down to zero.

And the return starts at 100% and goes  above 100% as you turn the knob down. With the knob all the way up, the full preamp signal goes from send to return. As you turn the knob down, the signal is cut from send and then boosted back up to normal at the return.

That's because the full preamp signal is too hot for many effects and will overdrive them into nasty sounding distortion. Lowering the signal to the effects keeps the signal from overdriving the effects, but also drops the volume. So after the effects the amp return boosts the signal back up as much as the send reduced it.

100% send (zero cut) goes to 100% return (zero boost).

50% send goes to 200% return. (Signal cut in half before pedals, then after the pedals it's doubled back up to full signal).

yes that's my understanding as well. But this is a much better explanation!!

 

[Gutch220]

I saw this, and never knew the fx-loop is for TRS stereo cables. I always just used TS mono cables with no issues.

 

[guitarbilly74]

View attachment 153371

I saw this, and never knew the fx-loop is for TRS stereo cables. I always just used TS mono cables with no issues.

yes, you don't have to use TRS cables. Some rack effects back in the 90s required balanced TRS cables so that's probably why they did it that way.

 

[RLW59]

Edit: Having to double check some stuff