OT Ground/Common

[EFK]

In a star grounding layout, does the black or common of the OT run to the star ground, or to the chassis? Or *only* to the output jacks (which are isolated from the chassis)? What I'm unclear on is, as the black wire runs to the output jacks first, from there is it supposed to run to any star or chassis ground at all, or do I just end it at the jacks?

[raiken]

It is usually a good idea to connect the OT common to chassis ground to prevent odd noises and oscillations. Run the OT ground wire straight to the jack ground first - it is very important to do this to keep the high circulating currents in the common return wire from getting into the low-level preamp circuits. Also, be careful how you route the ground and other OT wires - keep them away from low-level preamp stages, and don't run them parallel to or bundle them with other wires if you can help it.

Secondly, if the amplifier has global negative feedback, run another wire from the ground side of the output jack back to the ground point of the stage where the global negative feedback is applied, which is usually the bottom of the "tail" resistor in an LTP, or the bottom of the presence pot.

If the amplifier has no global negative feedback, just ground it to one of the later ground points. Don't use the sensitive low-level preamp stages as ground, or you may encounter oscillations.

If you aren't using isolated output jacks, it will be grounded to the chassis at the output jack sleeve already, which usually is okay. It is still a good idea to run a wire from the output jack ground to another ground point nearby anyway, in case the jack gets loose. You should never depend on a mechanical connection of a jack nut or pot nut to maintain an electrical ground.

Randall Aiken

[hasserl]

Secondly, if the amplifier has global negative feedback, run another wire from the ground side of the output jack back to the ground point of the stage where the global negative feedback is applied, which is usually the bottom of the "tail" resistor in an LTP, or the bottom of the presence pot.

Now that's a nice tidbit I haven't heard or seen before (or didn't remember it). Nice one!

[EFK]

Thank you! I am completely rewiring and - more importantly - regrounding (in as much of a star setup as I can manage) a Selmer T'N'B 50 which is quite possibly the noisiest amp I have ever heard. The grounding scheme (has been worked on) was a mess of multiple repetitive grounds and a lot of chassis contamination. I am re-routing everything to a single point next the PT but was unclear on what to do with the OT common. The two jacks (7.5 and 15 ohm) are isolated cliff jacks and the ground from the OT runs to the jacks then to the same old chassis lug as pins 1/8 on one of the output tubes. I want to change this (giving the output tubes 1/8 their own ground to the star) and run the ground off the jacks thence on to the star point - the negative feedback ground (off the 7.5 jack) will also run to the star, although if I understand correctly you are saying it is ok to bundle those two (only) together and run a single ground line to the star.

Your site has been a wealth of information for me - thanks for taking the time to put such a large volume of information out there for everyone!

[raiken]

Now that's a nice tidbit I haven't heard or seen before (or didn't remember it). Nice one!

Yes, it helps if you think of an amplifier stage as a differential amplifier, even if it is single-ended and not a true diff amp, because it still amplifies the difference between it's input terminal and it's ground terminal, so it will be quietest when the input signal is applied directly to the two input terminals of the stage. If the ground return goes through the chassis and all over the place, it has the opportunity to pick up additional noise or unwanted feedback signals that will be amplified by the stage.

Since the phase inverter is amplifying the difference between the main input grid signal and the applied global feedback signal, it is best to reference the output return back to that stage's ground.

RA

[raiken]

Your site has been a wealth of information for me - thanks for taking the time to put such a large volume of information out there for everyone!

You're welcome!

RA

[Gaz]

Your site has been a wealth of information for me - thanks for taking the time to put such a large volume of information out there for everyone!

+1

Haha - At first I thought you were quoting Randall Aiken! But you were Randall Aiken.

I've read the article on star grounding a million times.