Parallel effects loop for Marshall style amps

paulster · Post by **paulster** » Fri Jul 27, 2007 7:49 am

Larry

What are your thoughts on putting a tip-ring-sleeve jack in the amp as a passive insert point and then putting the loop in an external self-powered enclosure? A Larrylator, so to speak, as MacG named it.

As well as minimising the intrusion into existing Marshall-style amps it could make it an easy bolt-on to other types of amps without loops as well.

Paul

novosibir · Post by **novosibir** » Fri Jul 27, 2007 8:32 am

paulster wrote:What are your thoughts on putting a tip-ring-sleeve jack in the amp as a passive insert point and then putting the loop in an external self-powered enclosure? A Larrylator, so to speak, as MacG named it.

Shure! You may disconnect in the amp between the Treble's wiper and the PI's input cap - or in MV amps between the Master's wiper and the PI's input cap and run both spots to a stereo jack.

Then you need additional B+, ground and heater supply from inside the amp, which you can fiddle trough a transformer's grommet to the external box. Much more save would be, to run inside the amp this 4 cables to a 4-pole XLR, mounted to the back of the amp, where you plug in the 'supply cable' from the external box.

Then you may have an external 'Larryator', which you may connect to the amp only with a stereo phone jack and a 4-pole XLR - and w/o any additional power supply

The stereo jack on the back of the amp although has to be wired, that tip and sleeve are connected, when nothing is plugged in, otherwise the amp w/o the Larryator plugged in would be internal disconnected.

Larry

paulster · Post by **paulster** » Fri Jul 27, 2007 9:31 am

novosibir wrote:Shure! You may disconnect in the amp between the Treble's wiper and the PI's input cap - or in MV amps between the Master's wiper and the PI's input cap and run both spots to a stereo jack.

That's exactly what I had in mind. I was thinking about powering it separately rather than bringing B+ outside of the amp a) for safety, and b) so that it could be used with other amps more easily. It would be a simple mod on other amps without drilling holes to replace a speaker jack with a TRS shorting jack and then interrupt the input to the PI with it.

Next problem: finding a tiny transformer with B+ and filament windings.

bickster · Post by **bickster** » Sun Jul 29, 2007 6:11 pm

Mr. Larry can I please have a copy of this loop. I have a plexi build based on the Mark Huss 6V6 and the chassis is punched for 4 novels. Try rbickfordatzoominternet.net please.

Bob

novosibir · Post by **novosibir** » Mon Jul 30, 2007 6:05 pm

bickster wrote:Mr. Larry can I please have a copy of this loop.

Yes Sir! It already should have been dropped into your mailbox

Larry

paulster · Post by **paulster** » Wed Aug 15, 2007 4:35 am

paulster wrote:Next problem: finding a tiny transformer with B+ and filament windings.

Larry

What voltage are we aiming for after the 4K7 voltage dropper, i.e. across the 22uF cap? I'm trying to plan my B+ supply as it'll be standalone.

Paul

novosibir · Post by **novosibir** » Wed Aug 15, 2007 4:51 am

That depends on your previous voltage at the PI node and the tube you're using in the loop section. The voltage drop across the 4.7K is about 10...15V

Larry

paulster · Post by **paulster** » Wed Aug 15, 2007 5:01 am

This one won't be powered off Uc - it's going to be external with its own power supply (and a 12AT7 tube). That's why I'm after the target voltage since I won't need the 4K7.

novosibir · Post by **novosibir** » Wed Aug 15, 2007 6:20 am

I'd recommend a supply voltage of at least 310V up to 350V, what's valid for 12AX7 as well as for 12AT7, but with a 12AT7 I'd recommend 470/22K for the CF cathode and 680/68K for the recovery stage.

Larry

MacGaden · Post by **MacGaden** » Wed Aug 15, 2007 7:38 am

novosibir · Post by **novosibir** » Wed Aug 15, 2007 8:18 am

The 270V/.08A wold be ok - rectified about 360VDC - due to the ripple there must be a first 22

MacGaden · Post by **MacGaden** » Wed Aug 15, 2007 8:22 am

[quote="novosibir"]The 270V/.08A wold be ok - rectified about 360VDC - due to the ripple there must be a first 22

paulster · Post by **paulster** » Wed Aug 15, 2007 8:25 am

[quote="novosibir"]The 270V/.08A wold be ok - rectified about 360VDC - due to the ripple there must be a first 22

novosibir · Post by **novosibir** » Wed Aug 15, 2007 8:28 am

185VAC is much too low! That's ~235VDC after the rectifier and 205VDC after the cascade thing to clean off the ripple.

Larry

MacGaden · Post by **MacGaden** » Wed Aug 15, 2007 9:57 am

novosibir wrote:185VAC is much too low! That's ~235VDC after the rectifier and 205VDC after the cascade thing to clean off the ripple.

Larry

Hmm. Larry: Do you know of anyone who makes a PT that would work for this ?