Parallel effects loop for Marshall style amps

novosibir · Post by **novosibir** » Fri Jun 06, 2008 5:03 pm

Thanks Tuco!

The confusion came, because the German word for twisting is verdrillen, so 'drill' instead of 'twist' jumped from my head into my fingers on the keyboard.

And yes, the inner conductor is the 'hot', the stranded outside around is the shield, what has to be grounded - but only at one end, to avoid an internal ground loop.

Larry

Tuco · Post by **Tuco** » Sun Jun 08, 2008 2:55 am

Okay, I need to ask a newb question about how to use this shielded coax wire.

The inside wire (which is quite thin and a single piece of copper) is encased in a plastic tube, which is wrapped with a thin sheet of foil, which is wrapped with stranded wire, with the whole thing encased in the vinyl jacket.

Am I correct that I should strip 1 inch of all the layers except the thin inside wire? As Larry said, this is the "hot" connector. It is the only wire connected inside the loop box. At the other end, the "hot" wire is connected, but the stranded wire is connected to the nearest ground point. Does it matter what I do with the thin sheet of foil?

Thanks for looking.

Tuco · Post by **Tuco** » Fri Jun 20, 2008 1:24 am

I need to bump to get an answer to my previous post. ^^

Plus, I'm having more trouble inside the amp. From what I can figure out on my own from Larry's schematic and from GeraniumBoy's post on page 11 of this thread, the B+ wire needs to connect inside the amp at the junction of the 82k and 100k resistors, as indicated in the attached photo.

[img]DSC_0065%20resize%20adj%202%20cmp.JPG[/img]

Is this correct?

Anyway, I hooked up the B+ wire underneath the board at this point and grounded the shield at a nearby post. And something is NOT right. When I turn on the amp and take it off standby, after a few moments the 10k and 8.2k resistors that are circled in yellow begin to discolor and the turret in between them begins to smoke. And when I turn off the amp and attempt to discharge the caps, my meter indicates that there isn't any charge to discharge.

So I'm thinking that either:

1) I have the wrong place for connecting the B+ wire
2) I have a short somewhere inside the loop box
3) I have the wrong kind of shielded coax cable.

Since I haven't yet tested this new amp, I think it might be best if I disconnect the loop for now and make sure the amp is running optimally. Once that is done, I'll tackle the loop again.

However, I am anxious to hear some opinions on what is going wrong. Please remember that I'm an electronics newb and explanations need to be simple, yet thorough. Many thanks for looking.

novosibir · Post by **novosibir** » Fri Jun 20, 2008 3:35 am

It's correct, where you take the B+ from.

Larry

Ricky Lee · Post by **Ricky Lee** » Fri Jun 20, 2008 9:55 am

Sounds like you have the shielded wire grounded at a wrong place. Try using the shielded wire to the B+ rail without grounding the shield itself. I have the same loop installed in my 12xxx without the shield grounded and it works perfectly and without noise. Also, make sure the wire mesh shield is NOT grounded to the wire leading to the B+ terminal. Since the resistors got hot you may need to change them.

Tuco · Post by **Tuco** » Fri Jun 20, 2008 2:18 pm

Ricky Lee,

Thanks for your thoughts. I will try removing the ground this morning and see what transpires.

Also, thanks for the info about changing the resistors. I had meant to ask about that.

Also, Larry, thanks for your continued help.

Jay

Riscchip · Post by **Riscchip** » Mon Jun 23, 2008 8:53 pm

First of all, thanks very much to Larry for putting this schematic together. I am very thankful to have access to it!

I just wanted to see if anyone has drawn a graphical layout for this yet? I have been going through the schematic with schematic symbol chart, but I'm not very confident I'm following it right.

If you have one, and it's okay to send...I'd be thankful to have a look.

If not...I'll head back to the schematic symbol chart for a while longer.

fivedoors · Post by **fivedoors** » Thu Jun 26, 2008 10:40 am

hi all

many thanks Larry ! loop installed and is perfect .

you are the man Larry!

cheers john

psychodave · Post by **psychodave** » Fri Jul 04, 2008 3:23 pm

Can someone tell me what wires need to be shielded? I am about to put it in and want to do it right the first time 8)

fivedoors · Post by **fivedoors** » Sat Jul 05, 2008 5:39 am

hi
i used shielded wire on all pot connections . 6 wires
4 to send/return pots and send jack
1 to treb pot
1 to mv pot

works very well

cheers john

ACDCG400 · Post by **ACDCG400** » Fri Aug 01, 2008 2:20 pm

just a question guys. is there a way you can switch the loop in and out of the circuit?

franc · Post by **franc** » Sun Aug 03, 2008 9:36 am

You can't actually switch the loop in and out of the circuit, because the loop has become part of the circuit, but you can switch the effects-signal path on or off.

What I have done is add another input jack with a simple footswitch to connect/disconnect the send signal to/from ground. This way you can disable/enable the path of the effect signal (the parallel 'dry' signal will of course always be there).

This results in a noise-free way to disable all effects in the loop.
The advantage of doing it this way (besides the noise-free operation) is that if you have, for instance, a digital delay in there, the moment you switch off the loop, the delayed signal will still be heard and not cut-off like when you hit the switch of the box itself.

I have built the loop in a couple of amps now, and the owners were always very happy with the result.

My latest build was a loop for a Silver Jubilee combo. I bridged the original loop and put Larry's loop in, with the on/off (or 'mute' as I like to call it) footswitch. Very nice result and the owner even concluded he could get rid of the boost pedal he used to have in there. His delay and the Larry loop could do the job now

.

Thanks again Larry!

ACDCG400 · Post by **ACDCG400** » Fri Aug 08, 2008 10:27 am

Thanks!

psychodave · Post by **psychodave** » Fri Aug 22, 2008 3:53 pm

Is there any issues with this loop and high volume? Does the loop still sound good with an amp cranked (100 watt Marshall)? I have yet to install it yet. One due to fear of tone suckage and my desire to find something better (as if something actually exists). 8) I really like the above poster and the idea of turning off the loop.

Olaf · Post by **Olaf** » Thu Aug 28, 2008 3:24 pm

psychodave wrote:Is there any issues with this loop and high volume? Does the loop still sound good with an amp cranked (100 watt Marshall)? I have yet to install it yet. One due to fear of tone suckage and my desire to find something better (as if something actually exists). I really like the above poster and the idea of turning off the loop.

Why not, Dave? You'd need a DPDT switch and make a true bypass.

Keep in mind that "before" the 22nF of the PI (where you normally have the middle lug of the treble in the unlooped circuitry) there's high voltage (when loop is on), so switch after the 22nF of the looped circuitry (= no DC) - which would mean that you need to add another 22nF (as input cap of the PI) for safety reasons.
You could only switch when the amp is off (heavy pops otherwise).

Let Larry confirm this - its his loop.