Parallel effects loop for Marshall style amps

franc · Post by **franc** » Sat May 24, 2008 11:44 am

I want to use minipots for the Send and Return levels, to save some space on the backside of my amp. But the mini log pots are only 0.125 W while a full size is 0.25W
Will this be a problem?

novosibir · Post by **novosibir** » Sat May 24, 2008 11:47 am

No problem!

Larry

franc · Post by **franc** » Sat May 24, 2008 1:40 pm

Thanks Larry!

psychodave · Post by **psychodave** » Sun May 25, 2008 11:02 pm

Larry...thanks for sending me the schematic!

How will the loop sound with PPIV master volumes?

EddyInChicago · Post by **EddyInChicago** » Tue May 27, 2008 5:56 pm

psychodave,

EXCELLENT!!!!!

Edmon

Ricky Lee · Post by **Ricky Lee** » Fri May 30, 2008 8:56 pm

I noticed since installing the loop I now have DC leaking back to the guitar from the amp (scratchy pots). Caps checked fine and changed v1 several times with no luck. Also, does anyone have this loop installed with the PPIMV as well. Seems with the both installed I get intermittent blocking distortion especially on the high notes.

novosibir · Post by **novosibir** » Thu Jun 05, 2008 7:50 am

The PPIMV and the loop can't make your guitar pots scratchy, because from these areas in the amp any possibly DC leakage can't occur on the input jack, no retroaction possible.

I assume, that you have an RF issue in your amp due to an improper lead dress - and this might cause a retroaction and your high note prob.

Larry

Ricky Lee · Post by **Ricky Lee** » Thu Jun 05, 2008 8:24 am

Thanks Larry. I found the problem. Changed a Tungsol preamp tube and high note problem went away. These modern day tubes are a crap shoot. I still have some DC causing scratchy guitar pots.

Tuco · Post by **Tuco** » Fri Jun 06, 2008 1:42 pm

Larry, I'm adding your loop to another amp, this time a Metro JMP50. I'm tidying up a few things this time and have a question.

As with my JTM45, the loop board, tube socket, and pots are going in a project box mounted on top of the chassis. The board has a star ground, which grounds with a single ground wire to the bus wire which runs across the pots in the amp chassis. Can the loop's 20u filter cap ground to this star? Or should I run a separate ground wire to the same ground point as the other filter caps? (If "yes", that's going to be a long ground wire.)

As always, many thanks for your help!

Ricky Lee · Post by **Ricky Lee** » Fri Jun 06, 2008 2:06 pm

Tuco wrote:Larry, I'm adding your loop to another amp, this time a Metro JMP50. I'm tidying up a few things this time and have a question.

As with my JTM45, the loop board, tube socket, and pots are going in a project box mounted on top of the chassis. The board has a star ground, which grounds with a single ground wire to the bus wire which runs across the pots in the amp chassis. Can the loop's 20u filter cap ground to this star? Or should I run a separate ground wire to the same ground point as the other filter caps? (If "yes", that's going to be a long ground wire.)

As always, many thanks for your help!

I'm not Larry, but I have the effects loop with star grounding which also includes the 22uf cap as well, with one wire ground leading to the chassis. The effects loop is quiet as a mouse and performs as should. However, I wouldn't recommend running the single ground wire to the BUSS but to board stand-off near v-3 or even the PI ground terminal. Also, make sure lead running to treble pot is shielded as well as the lead going to B+, or else lots of noise.

Tuco · Post by **Tuco** » Fri Jun 06, 2008 2:37 pm

Ricky Lee, many thanks for jumping in on this.

What kind of shielded wire should I be running? I didn't do this in my JTM45, and there is a small amount of noise, but at high volume, I expect some noise. If I can eliminate that, great!

novosibir · Post by **novosibir** » Fri Jun 06, 2008 2:47 pm

It's alway the best, to ground an electrolytic close to the cathodes of that tubes, it's feeding - so as Ricky already said, ground it to the star in the box, where also the loop's tube is located and grounded.

Don't forget to drill the heater's wires from amp to box as tight as possible!

As for the shielded wire I recommend antenna coax RG174/U - it's thin and flexible and only has 95pF/m capacitance.

Larry

Tuco · Post by **Tuco** » Fri Jun 06, 2008 2:57 pm

novosibir wrote:Don't forget to drill the heater's wires from amp to box as tight as possible!

Larry

Thanks, Larry! Do you mean twist the wires as tight as possible? Or make the wires as short as possible?

Jay

novosibir · Post by **novosibir** » Fri Jun 06, 2008 3:03 pm

Tuco wrote:Thanks, Larry! Do you mean twist the wires as tight as possible? Or make the wires as short as possible?

Oh yes, my German's English

Shure, twist the wires - and of course they should be as short as possible, but twisted.

Larry

Tuco · Post by **Tuco** » Fri Jun 06, 2008 4:37 pm

Larry, your English is very good, especially given the technical nature of these discussions. You'd have an easy time here, my friend. I would be tongue-tied and lost in Germany.

I just got some thin, shielded coax cable. I am assuming that I should be using the inner wire for this hookup? And not the outside stranded wire?