This one gave me a real hard time because of bad 50Hz hum. It came only when I connected something to the grid of the input tube, it was completely silent when the input tube was removed.
The problem with my amp is that the tone stack and the input jack are about 30cm away from the chassis. This is because I use an old fan heater housing from the 50s.
Of course, this is actually stupid, because exactly the signals that are sensitive to hum are routed through the housing. So the grounding and shielding is the most important thing. Once again, I had to learn this the hard way.
I spent hours to de-hum the amp…
The solution was to connect the central ground point where ground and chassis are connected to the ground point of the input tube.
Additionally, the input jack had to be isolated from the chassis.
The chassis is made of metal and connected to protective earth. The connection from chassis ground to earth is near the ground point of the input caps.
Here are some pictures
The input stage was removed and built new. The black wires are screened wires to the tonestack. The gray cable is from the solder iron.
The initial grounding point near to the „innerst“ input cap.
Below :
The new grounding point near to the input tube grid. Important was to isolate the input jack ground from the housing. The input signal is wired through a screened cable into the chassis. This is the only screened cable that is grounded on both sides, the screen connects the ground of the tonestack to the chassis ground point. The other two screened cables are only grounded on the chassis side. That avoids any ground loop between chassis and tonestack.
The finished amp. It has a 6“ speaker built in, and a XLR jack at the rear for an external 4ohm Speaker
And the resulting schematic after many iterations. The chassis ground point is shown in bold.
