Grounding- AB763 AB165 AA864 etc.

[J Rindt]

I looked at the grounding info in the "Library" and it all makes sense. But I am trying to utilize some existing connections.  With a typical Fender Bassman head, is it better to separate the grounding of the pots from the grounding of the preamp cathodes?  Is it kosher to run the preamp cats to ONE ground lug on the chassis, and then run the pot grounds to a few different grounds along the brass "bar"?
Does it matter if the phase inverter shares a cat ground with V1 V2 and V3?
Thank You

[plexi50]

Shared cathode grounding is fine and the way it's been forever. Grounding for the preamp and pots is about the same. You have that brass plate on most fender amps. That needs to be spot grounded. The preamp cathode grounds are in most cases 1/4" away from the brass plate. Keep all you power supply grounds as near the PT as you can and away from the preamp ground area. I know there is a hole in the chassis where the first PS filter ground  solders right under the normal channels volume pot. Most times it's not an issue

I have had no issues i can think of with all the cathode grounds including the PI cat to the same ground. But on the Bassman you will see that the PI cat some times does have it's own dedicated ground to chassis and other times they will all be tied from V1 all the way up to the PI

Different model years had some differences but all work and sound great for the most part

[J Rindt]

10-4
Thanks Again

[HotBluePlates]

Simple rule of grounding: all the grounds of a given circuit should physically form the smallest loop possible.

The "physical" part and the practical execution is where the simple rule may not be so simple.

Take a single tube stage. It has a grid reference resistor running from the grid to ground (either a resistor, or a pot). It also has a cathode resistor running to ground, and at the far end of the plate load resistor there is a filter cap running to ground.

The circle from the grid, through the grid resistor, ground, up through the cathode resistor and to the tube cathode is called the "grid circuit" or "input circuit" (for a grounded-cathode or common-cathode stage). The circle from the plate, through the plate load resistor, through the filter cap, ground, up through the cathode resistor and into the tube cathode is the "plate circuit" or "output circuit" (again, for a common-cathode stage). Each of these "loops" or circuits shoudl ideally be as direct and physically small as they can be. Keeping the loop direct lessens the change of having a low-level signal ground contaminated by large hum currents from a big stage as exists at the power transformer secondary, rectifier and first filter cap. Keeping the loop physically small lessens the chance of the wiring picking up hum, RF or other noise electrostatically like an antenna.

So your best plan is to have the ground lug of a volume control connect to the cathodes of the tubes it is between with short wiring. Fender already does this (sort of) by running a wire from the cathode resistor ground eyelet to the brass plate under the controls. Not perfect, but it usually does the job.

If you don't have real hum problems in an amp, or have them but still have original filter caps, I wouldn't mess with the Fender method. There's nothing sacred about the way they did it, but it has worked well enough in a large number of amps, and there's no sense in accidentally created a grounding problem by trying to fix a perceived grounding problem (if you're not having noise that cannot be fixed any other way).

[J Rindt]

HBP -
Thank You...... for explaining the electron path of travel. I have a better understanding of it now. The importance is not lost on me.
Thanks Again

[Fresh_Start]

There is a very helpful diagram hiding in Doug's Library:
http://www.el34world.com/charts/currentflow.htm

Here's the grounding page (just because this comes up so often):
http://www.el34world.com/charts/grounds.htm

Here's Randall Aiken's grounding essay:
http://www.aikenamps.com/

Aiken recommends "star grounding" while Hoffman's approach appears very different.  However, Hoffman's grounding methodology closely resembles the "galactic grounding" Kevin O'Connor describes in The Ultimate Tone.  Basically, it's a collection of small stars designed to meet the criterion of the "smallest physical loop possible" HBP describes above.  Works for me.

You can spend months studying different theories about grounding, or you can copy a "known-good" layout.  I learned the hard way that crossbreeds don't tend to work out well.

2CW

Chip