Fit Check and Presence Control

[Bretly]

So about to finish soldering this Blackvibe build and have a question on the presence mod I see on the RR site. In all his layouts, they all have a preamp bus and star ground. But this one mod the cap and resistor are always shown grounded to the back of the pot. Is there an obvious reason for that, that I am missing? Why not just send it to the pre-amp bus like everything else?

Here is where I am a. A few signal wires to run to the pots and then solder away. This is my second build and first from scratch, sourcing, machining the chassis from a blank, laying it out, etc. Hopefully it doesn’t go up in smoke, but it’s meant to be a learning experience. Pics attached. If you see something seriously off please let me know. Thanks!

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[martin manning]

I haven't studied that schematic or layout, but the presence is best grounded with the PI cathodes. It is part of the power amp, so better there than with the preamp grounds. Not that it won't work as described, plenty of commercial amps grounded the presence to the pot case.

[Bretly]

I haven't studied that schematic or layout, but the presence is best grounded with the PI cathodes. It is part of the power amp, so better there than with the preamp grounds. Not that it won't work as described, plenty of commercial amps grounded the presence to the pot case.

Well I grounded it to the preamp bus and powered up and most of everything is good. Hit the target voltage at b+ right at 465v. No oscillation or hum. The only problem is I can’t get the bias hot enough. 12w maxed out. I have basic ab763 bias board 470 2w on the board and a 10k pot with a 27k resistor to ground. Anyway I can swap the 27k resistor out to bring that up? I was going to clip an 18k in parralel to see where that gets me. Here is the board.

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[martin manning]

You could make the 470R bigger, maybe 680 or so, and see where that puts it. Probably easier to do as you say and parallel something across the 27k.