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If you ever played an amp which had chassis mount jacks connecting the spring reverb tank to the amp, you probably thought that annoying hum at idle was a maxim. Just an unavoidable condition of spring reverb and tube amps. It is a hum that occurs even when the tank isn't connected.

Well, I cracked the code (nerd alert!). The cause is eddy currents created by those naturally occurring in the chassis ground and the slightly dissimilar metal of the connecting jack ground being directly in contact with the chassis. I discovered and fixed it i n the AMI Dumb Luck amp I am building.

Solution: insulate the reverb recovery jack from the chassis and tap the circuit ground to the jack ground. Hum is nonexistent and the reverb is louder and pristine, even with the reverb fully saturated on '10.' My amps are dead silent, even with the volume loud.

The Repair: drill the mounting hole one size bigger, cover 2/3'ds of the jack thread with shrink wrap, place a solderable ground washer around the jack, then a 1/16" plastic washer and insert the jack into the hole. On the other side of the hole, place a plastic washer then the nut to affix the jack. Test with a DVM for and open (no continuity) with the chassis and the jack ground. Connect the circuit ground (the most centralized ground point from the circuit board) and solder to the jack's ground washer, then the lead to the recovery tube, then any bleed to ground resistor. Check for continuity from ground to jack and from tube to jack. Note that the jack ground will appear to have continuity with the chassis but trust me - it isn't the same.

The repair is pictured below, with the reverb 'send' also insulated from the chassis as an experiment but is unnecessary.

That's awesome. I love it when some analysis pays off. So many manufacturers stop at "good enough" instead of chasing more they could do.

Using the chassis as a ground conductor was a mistake/shortcut used by most amp builders. Ideally, all the circuit grounds, including all jacks and pots should go to a circuit ground bus (or star) that only connects to the chassis shield ground in a single place, right at the beginning of the power section.

Merlin Blencowe has a good article on grounding that is available in a free PDF. It explains things better than I could, but I will say, you don't need a perfect grounding scheme to get major improvements and Andrew's experience proves that.

Chocol8 wrote: ↑Mon Nov 27, 2023 10:22 am Using the chassis as a ground conductor was a mistake/shortcut used by most amp builders. Ideally, all the circuit grounds, including all jacks and pots should go to a circuit ground bus (or star) that only connects to the chassis shield ground in a single place, right at the beginning of the power section.

Merlin Blencowe has a good article on grounding that is available in a free PDF. It explains things better than I could, but I will say, you don't need a perfect grounding scheme to get major improvements and Andrew's experience proves that.

Totally agree. On many of my builds since around 1992, one of the first things I do is set up a 'signal' star ground system. Then, I connect that to the chassis with a 2ohm 5W resistor. I allow tube filament heater center taps to go to chassis ground, which further reduces the possibility of noise and hum. The rest get connected to signal ground.

It takes more time, forethought and money, but is well worth it. I demonstrated that to a friend yesterday, by putting the reverb on 10, the master volume on 10 and the clean channel on 7 of my AMI Lil Giant. With the guitar volume on zero, it was dead silent. Guitar volume on 10 and it was tough being in the same room with the amp. I am guessing well over 110dB loud.

Some reverb tanks have isolated inputs and/ or outputs. Star grounding is not good for low level signals, I would never ground my input jacks near the power section. I have built about 50 tube amps so I'm not blowing smoke.

mozz wrote: ↑Mon Nov 27, 2023 12:36 pm Some reverb tanks have isolated inputs and/ or outputs. Star grounding is not good for low level signals, I would never ground my input jacks near the power section. I have built about 50 tube amps so I'm not blowing smoke.

On more than one amp, I was noticing the hum on the recovery with or without the tank attached. So I knew it wasn't the tank jack isolation and tried a bunch of tubes, replacing cathode bypass caps, blocking caps, etc. I removed the recover jack from the chassis and connected it to signal ground and it went dead silent, unless signal was applied.

The input jacks should be grounded to the chassis, or floated with a cap and diodes. The rest of the circuit should go to the bus with the "return" flow back to the rectifier going away from the inputs. Of course electrons actually flow in the other direction, but the idea is to have the inputs away from the audio circuit current flow.

I was wrong above when I said the audio ground should connect to the chassis at the beginning. The center taps go to the chassis at the beginning, very near the power cord ground connection, but the actual audio ground reference is at the inputs using the bus method. Ideally it would connect near the safety earth connection but that is rarely practical. Likewise, using a star, the inputs do get grounded at the single ground point, but a typical layout means that requires a less than ideal long grounding wire.

Check out Merlin's chapter on grounding...
https://www.valvewizard.co.uk/Grounding.pdf