Installing a Zero Loss loop into a single ended 1x6v6 amp?

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redeyedjim
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Installing a Zero Loss loop into a single ended 1x6v6 amp?

Post by redeyedjim » Mon Feb 13, 2017 2:36 am

I'd like to add a MetroFX loop to an amp I recently built, if it's possible, and I could use a little help.

The amp is a PlexiSE. It's a 5 watt, single ended 1x6v6 design. This is the schematic:

Image

PDF version: http://www.guitaramplifierpcbs.com/Docu ... %20v11.pdf

It uses a PCB available from http://www.guitaramplifierpcbs.com/

The amp uses the PT and chassis of the Epi Valve Jr., but the circuit is entirely different. It's a surprisingly good sounding little amp, FWIW.

I've read through the build instructions several times and I'm having trouble mapping those instruction to this amp's circuit. Could someone familiar with the Loop look over the amp's circuit and tell me where to tap into it for the power and signal connections? I'd really appreciate it :help:

Thanks for any help you can provide!

Note: With the tubes in, B+ is ~340V before the first filter cap.

redeyedjim
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Re: Installing a Zero Loss loop into a single ended 1x6v6 amp?

Post by redeyedjim » Tue Feb 14, 2017 3:37 pm

Anyone here?

SDM
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Re: Installing a Zero Loss loop into a single ended 1x6v6 amp?

Post by SDM » Thu Feb 23, 2017 5:05 am

For the signal connections, same as per a 1959 type amp. "In" from treble wiper, "Out" headed to .022uf coupler (C15).

Single ended amp, so more filtering the better here, the screen node "B" on the schem should do for power source node. Use a bit higher value loop decoupling resistor than instructions state for good measure (next size higher in chart instructions for the B+ you have at node "B", guessing that'll mean you'll end up using a 10K loop decoupling resistor).

Can read note 1 at the end of loop instructions to compensate for loop current draw concerning the 4.7K resistor before node "B", but closest normal/close enough value will probably be 3.9K 2 Watt (3.3K 2 watt would be close enough too). May not be much worth bothering with this step here though as the voltage change with loop should only be 6-7 ish volts roughly estimated.

redeyedjim
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Re: Installing a Zero Loss loop into a single ended 1x6v6 amp?

Post by redeyedjim » Thu Feb 23, 2017 2:16 pm

Thank you, Steve!!!! That is as clear and straightforward as I could have hoped for. Much appreciated!

One followup question: I left this out of my OP because I did not want to make things too complicated, but this amp also has power-amp only VVR/power scaling, and B is controlled by the power scaling...but C isn't, as it feeds the preamp and thus has non-power-scaled B+. Should I use C as the power feed for the FX loop instead of B, and if I do, how does this change the value of the de-coupling resistor?

Thanks so much for your help!

SDM
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Re: Installing a Zero Loss loop into a single ended 1x6v6 amp?

Post by SDM » Fri Feb 24, 2017 3:33 am

Loop needs to keep certain and steady power to work, so yeah do not use B then.

C is fine to use as per loop instructions for choosing a loop decoupling resistor value -if the B+ is high enough. Not sure how your VVR is all set up, but your C node would be near 250 ish volts if everything was all running as per the schem above with 340V starting. If your B+ is indeed very near 250V there at C, just use a 2.2K or 3.3K loop decoupling resistor, should be fine. Will only drop several more volts to the loop board and still provide good filtering with the loop's on-board filter cap.

Again, don't know how the scaling is set up here, but assuming filtering is already decent at your C here, hopefully/likely that'll do it. Worst case, could create a new supply node for the loop specifically, but don't think that'll be needed. Good luck/have fun.

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