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| | |-+  BB Build. Lil' help, so I can stop bugging Nik?
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Author Topic: BB Build. Lil' help, so I can stop bugging Nik?  (Read 1899 times)
camsna
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« on: May 05, 2014, 03:01:56 PM »

So! My dad and I are setting out to build the ultimate Bluesbreaker. We've scrimped and scratched and saved to do it the right way! Nik has been absolutely gracious with his time, advice, and question answering. I realize, though, that time spent answering my questions is time spent NOT doing important things.

My first question regards the PPIMV. I plan on installing one, but am unclear about how to the shielded cabling in order to reduce noise. Here is the diagram I'll be working off of.



Which conductors of how many shielded wires do I connect to where?! Here's what I think I'm s'posed to do:


2, 2-conductor shielded wires.

Wire 1
Center conductors from pin 1 of each half of the pot to the circuit board
Shield clipped on the circuit board side, but connected to either pin 1 of either half of the dual MV pot. Or the back of the pot.

Wire 2
Center conductors from pin 2 of each half of the pot to the grids of the power tubes
Shield clipped on the tube socket side, but connected to pin 1 of either half of the dual MV pot.

This right? Seems right to my mind.
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wyatt
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« Reply #1 on: May 08, 2014, 08:15:43 AM »

This is where the Metro Amp Forum would be a lot of help, lots of info on the Lar-mar PPIMV, but it's down right now as they try and salvage and convert their web data after a virus attack AND a bad update by their host service.

You assessment is correct, but many people have moved away from the 2-conductor coax wire and use separate, single conductor coax (RG174/U) for each run for lower noise (and thicker conductors). This means a separate piece of coax for all wire Red and the Black wires shown in your diagram.

Connect the braids connect to Pin 1 on the pot (not the back of the pot), they are Grounded by those Blue wires that return to the board.

Jeff Gehring posted a great tutorial of how to cleanly prep the coax shield...

http://home.everestkc.net/jgehring/teflon-coax.html

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camsna
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« Reply #2 on: May 09, 2014, 07:54:06 PM »

Thanks! Yeah -- I've come across a thousand links to the Metro forum during my study for this build -- its a drag that it's down!

So! Let me be sure I've got this right! You recommend a 4-wire approach in order to cut down on noise -- something in which I'm abundantly interested! Like this?

4, 1-conductor shielded wires.

Wire 1
Center conductor from pin 1 of the front half of the pot to the circuit board
Shield clipped on the circuit board side, but connected to pin 3 of either half of the dual MV pot.

Wire 2
Center conductor from pin 1 of the rear half of the pot to the circuit board
Shield clipped on the circuit board side, but connected to pin 3 of either half of the dual MV pot.

Wire 3
Center conductor from pin 2 of the front half of the pot to the grids of V5. Shield clipped on the tube socket side, but connected to pin 3 of either half of the dual MV pot.

Wire 4
Center conductor from pin 2 of the rear half of the pot to the grids of V5. Shield clipped on the tube socket side, but connected to pin 3 of either half of the dual MV pot.

That look right?

Apart from tightly twisting the heater wires, any advice for taming the potential noise on this thing? I already intend on wiring the inputs with shielded wire a la the Tube Depot JTM45+ layout. (68k resistors taken off the board and connected directly to the input jacks, shielded wire used to connect inputs to V1).

Thanks again for your help!

---
Cam
« Last Edit: May 09, 2014, 07:58:04 PM by camsna » Logged
wyatt
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« Reply #3 on: May 12, 2014, 07:56:01 AM »

That's about it.

To clarify, I haven't compared the two-conductor versus one-conductor, but that's what people have been switching too (including Ceriatone). If you have the two conductor already, I would just use it.
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