Chassis ground with bus ground question

[mercsled]

I have a 50's amp with the preamp section separate. I have a ground issue since if I jumper the two pieces the noise goes down. I'm checking ground connections. This amp uses chassis ground as the tube sockets ground to the chassis but then it also has a small wire running through the amp from one ground tie of each socket and travels through the amp. Build quality is high so I assume this method worked well enough when new.

Three Questions:

1.) The bus wire in the preamp section of the three 6sn7 tubes and input jack does not connect to the buss wire of the tone control and power supply filter section of that one piece preamp. In fact the very first 6sn7 preamp tube doesn't have it's tube socket grounded to the ones right next to it. Was this a mistake in production and I should connect everything to the single bus wire?

2.) Does the lower impedance of the bus wire compared to the chassis cause the ground to travel through the bus wire rather than through the chassis and therefore not create a ground loop? I ask this since if each socket or jack is connected to the chassis and has a ground bus wire, isn't that a possible ground loop?

3.) If I am correct about the impedance of the wire, and the wire is somewhat small, would I decrease impedance in the ground bus by using a larger copper wire?

I'm not planning on completely changing the ground system of this amp as I'm sure they knew what they were doing when it was built, I'm just looking to help it out and correct the possible mistake.

 

[atmasphere]

I know it sounds unrelated but have you replaced the filter capacitors in the power supply?

If no, then all bets are off until this is done! Seriously despite what you already know about this amp, this might solve the problem. If the caps are not replaced, the power transformer is at risk of catastrophic failure.

 

[mercsled]

All capacitors have been replaced power and coupling. Almost all resistors that were slightly out were replaced. I don't except a 100% correction to my issue, it's more about info on grounding in the way this amp is set up. A small change or even piece of mind that what grounding I have is the best it can be, would be worth a little work.

 

[for_p1]

All capacitors have been replaced power and coupling. Almost all resistors that were slightly out were replaced. I don't except a 100% correction to my issue, it's more about info on grounding in the way this amp is set up. A small change or even piece of mind that what grounding I have is the best it can be, would be worth a little work.

Using chassis for grounding is a bad design. But unlike today back in 1950 some hum was acceptable. If you wan to get rid of hum, you likely will be forced to rewire everything, which is equivalent of full re-build.

 

[trobbins]

I wouldn't say chassis grounding is a bad design - it was just the most practical form of manufacture.

Within the same 'equipment' and when the powering is from one section, I'd strongly suggest that all chassis with the equipment have robust ground connections from chassis to chassis.

As some equipment used various 'connections' from parts to chassis, such as rivets or bolts or soldered wires, and some of those connections can degrade over time due to surface corrosion and due to different chassis metals, then some manufacturers ran a common wire bus that was soldered to all and sundry parts including backshells, as a way to remove noisy joints.

One way for hum to get in is for a signal from say the PT or OT to couple in to the chassis by induction and generate a small voltage signal between different points on the chassis.

Perhaps best not to leap to conclusions too early as to whether the grounding scheme is the dominant hum being heard. Certainly work to reduce any hum coming from one chassis to another, but also try and make sure that each chassis is 'quiet' for starters.