Parallel effects loop for Marshall style amps

Tuco · Post by **Tuco** » Thu Aug 23, 2007 8:01 pm

Ok, newbie alert, once again!

Recap: I have from Larry the loop schematic and a layout diagram.
I've populated a tag board (hopefully it's correct, but I'm not absolutely certain).

I've been studying the diagrams intently, hoping that I can make the right decisions, but alas here come a couple of questions.

1) For the B+ wire (Uc), where does this connect inside the JTM45? The best I can figure is either Pin 1 or Pin 6 on the PI tube. Is this correct? Does it matter which Pin?

2) The B+ runs through the 4k7 resistor. And then the diagrams show a 22u capacitor. Is this cap already part of the JTM45 circuit? Or is it a component that is part of the loop circuitry? If the latter, do I just insert the cap between the 4k7 terminus and ground?

I hope these questions make sense. I am very grateful for your help!

Jay

novosibir · Post by **novosibir** » Thu Aug 23, 2007 9:46 pm

Tuco wrote:1) For the B+ wire (Uc), where does this connect inside the JTM45? The best I can figure is either Pin 1 or Pin 6 on the PI tube. Is this correct? Does it matter which Pin?

Neither, nor! Not to the tube socket!
Uc is, where both plate R's are still connected together - or stick it to the PI's electrolytic

Tuco wrote:2) The B+ runs through the 4k7 resistor. And then the diagrams show a 22u capacitor. Is this cap already part of the JTM45 circuit?

No!

Tuco wrote:Or is it a component that is part of the loop circuitry? If the latter, do I just insert the cap between the 4k7 terminus and ground?

Yes!

Larry

Tuco · Post by **Tuco** » Sat Aug 25, 2007 4:52 pm

Back again.

Larry, thanks for your reply. It was very helpful. I should have read the entire thread first before asking, because you had already answered my questions several pages back. I am sorry about that.

I do need some further clarification about the jacks. I bought a pair of the same jacks that are used on the front of the amp for instrument inputs. However, in this thread, it is mentioned that the Return Jack should be a "switching" jack. Is this a different kind of jack from what is used as instrument input?

Thanks.

Jay

novosibir · Post by **novosibir** » Sat Aug 25, 2007 5:47 pm

No!

A input jack is a switching jack, switching the 'hot' to ground, when the plug is pulled.

Larry

Tuco · Post by **Tuco** » Sat Aug 25, 2007 6:02 pm

Cool. I bought the right thing without even realizing it.

Larry, thanks for putting up with my elementary questions.

Olaf · Post by **Olaf** » Sun Sep 02, 2007 6:01 am

Good Morning Larry,

I've tried your loop in one of my DIY amps and it works very very well, keeps the character perfectly. I just have run into one problem with it:

While the amp (4 channels, a bunch of relais, unused channels are "killed" with an opto to ground) doesn't have any switching-pop noise in bypass mode, when the loop is on, it pops heavily.

Do you have an idea where I can start to have a look ? I take it that the level before the PI due to the 2nd loop-stage is higher than in bypass mode, so the noise, which is there but non-audible in bypass mode, is amplified too much in "on" mode . . .

Thanks,

Olaf

novosibir · Post by **novosibir** » Sun Sep 02, 2007 7:09 am

Olaf, the gain in 'on' and 'bypass' mode shouldt be equal - depending on the tube's amplification factor. If not, then you may adjust the 'equity' of the loop section in both modes by varying the 68K to ground right after the mixer resistors. Try 68K, 56K, 47K or even a 39K, until you reach equity.

Concerning your switching noise:

When you've supplied your loop section with an independent supply voltage, as I've suggested, then it might be a grounding issue. All in the loop section first shouldt be grounded to one ground node - pots, jacks, mixer resistor, tube's cathodes AND the electrolytic of the voltage supply node - and then this ground node shouldt be located between the preamp ground nodes and the PI's ground node, but closer to the PI's ground.

Don't include the relay's supply ground neither to any of the preamp grounds, nor to the loop's or PI's ground!

If that all already is done correct, then my last idea (for today) wouldt be, that the loop's relay is physically located very close to any high sensitive preamp's area, so that the onset or drop of the magnetic field of the relay's coil might influence any component of the preamp's section in a bad way.

Moreover I'd recommend the use of 12V relays with a coil's resistance of at least 800 ohms and placing a discharging diode close to the relay's coil - but only a diode w/o additional electrolytic (like often seen).

Tell me more, when you've beaten the problem (and send me one of your Les Paul's

)!

Larry

Olaf · Post by **Olaf** » Sun Sep 02, 2007 8:19 am

Thanks Larry,

I think I've solved it: placed a 220k and a 22nF in series before the send pot and its almost gone. I'll try the 68k variation too.

The relay supply and grounding is ok - its not popping in bypass mode - but it could be the last relay is a bit too close, but thats a thing I cannot change in this amp anymore . . . have a look:

Thanks for your time and suggestions. Have a great sunday - and yes, one day I'll drive down to Franken and show you not my Lesters, but this amp

Olaf

Olaf · Post by **Olaf** » Mon Sep 03, 2007 10:01 am

btw: I'm using 6V Finder relays, of course always with the "flowback-stopping" diode (an UF type normally).

Its ok as it is now. Again: very cool loop. Thanks for it.

novosibir · Post by **novosibir** » Mon Sep 03, 2007 10:05 am

Olaf wrote:Again: very cool loop. Thanks for it.

Larry

jerrydyer · Post by **jerrydyer** » Tue Sep 04, 2007 4:01 pm

here is larrys loop in aciton with a Boss RV3 with reverb and a little delay.
http://youtube.com/watch?v=Bf4tAPsIKzs

stoo · Post by **stoo** » Tue Sep 04, 2007 4:48 pm

Larry! Could you send me that schematic too PLEASE!
stewartwright AT shaw dot CA
Thanks man!
Stew

novosibir · Post by **novosibir** » Tue Sep 04, 2007 5:37 pm

stoo wrote:Larry! Could you send me that schematic too PLEASE!

Already sent

Btw. my congrats - you're the 100-th board member, who has requested the loop schem! Where can I send to the champagne?

Larry

jerrydyer · Post by **jerrydyer** » Tue Sep 04, 2007 5:54 pm

stooooo gets the prize.

stoo · Post by **stoo** » Tue Sep 04, 2007 5:56 pm

stew