Phase Inverter/Presence Control Ground Question

[Matt J]

Hey folks,

I’m planning out some ODS builds in the next couple of weeks and noticed something with many of the ground schemes posted here I’d like some feedback on.

I see on many ODS layouts (like 102, 124, 183) that the 20uF capacitor for the B+3 filter node which feeds the phase inverter gets tied to the B+4 and B+5 filter nodes and is grounded at the general preamp point near the input jack, while the ground from the presence cap (usually 1uF or 2.2uF) is tied to the FX loop return jack.

In comparison, I’ve seen some mods to high gain circuits with GNFB where they separate the PI filter capacitor node and presence cap grounds from the preamp and connect them together at the output jack where the OT is normally grounded. I think Aiken Amps mentions this in some of his articles on grounding.

Has anyone tried this with their ODS builds? I normally just default to how HAD did it, but I’m curious if there’s any noise reduction benefit from this ground scheme.
Matt J.

[martin manning]

Either way works. The PI filter (B+3) negative can be grounded with the screen filter (B+2) or the preamp filter (B+4) as HAD did, but it makes much more sense to ground the B+2 and B+3 filters with the power amp grounds, which include the PI (presence cap), the power amp cathodes and the OT secondary. Those are spread across a portion of the rear of the chassis, making a total of four general ground points or areas: The preamp star, the power amp "nebula," (power amp input at the loop return, OT secondary at the speaker jacks, and power amp cathodes at the tube sockets), and the reservoir star (reservoir, HT CT, and heater winding CT).

[ijedouglas]

HAD did it 2 different ways in #0124 and #0183. #0124 has the screens and PI grounded between the power tubes (4 & 5) whilst #0183 has the screens grounded between the power tubes (4 & 5) and PI, V2 and V1 grounded at the input jack ground. I was told this does make a difference but I haven't tested it.

As far as noise floor goes, I haven't had any noise issues with either way.