5E3 kit - preamp hum

[lowell]

Just had a kit amp in that was humming real bad when all controls were turned down. After scratching my head for a while and poking around I found the hum started at the grid pin 2 of V2. I then disconnected the tone control and hum went away. Couldn't understand why until I saw that the builder had grounded the first filter cap to the back of the tone pot, which is also where the tone cap for the tone control was grounded. I moved the filter cap ground to the transformer bolt and it's fixed. Thought this might help others w/ similar issue.

[Chuck H]

I've often read that it's chapter and verse to ground the filters with their respective gain stages... Hogwash...

Any hum and noise that is generated in the B+ supply line has access to the signal chain relative to the resistance in the circuit... Right??? So my practice in design has been to use more than one ground point but employ a multiple star ground type of mentality. ie: the filters for the first gain stage should be decoupled from the signal path but seperate from the main filters. And most often located in between the two. This makes sense to me and has worked out well as far as hum and buzz control. Even so, I still believe that sans the above topography, grounding filters close to the chassis ground is better than grounding the input stage filters with, for example, the input jack and cathode. These are filters after all. They are there to eliminate ripple and noise from the signal path. It makes no sense to ground them parallel to signal leads. They need a shorter path to ground to avoid sharing noise because of chassis and lead wire resistance. Considering that there is greater current and lower impedance in the filter circuit you can always expect it to inject noise if you ground it parallel to signal leads.

So IMHO you're right in your thinking.

JM2C

Chuck