Earthing and lead dress

[Raja_Kentut]

I fully agree to the above.
In this case we also have HF oscillating. Now next to the resistance of wires we have to think about their inductance. (and also their capacitances)
The inductances can build a resonance with capacitances in the circuit. The tubes themselves have internal capacitances.
Together with the amplification of the tubes this makes up an oscillator.
There are ways to work against this HF :
Use „gridstoppers“ that are resistors directly soldered at the tube sockets at the grid pins.
Use forward and return path wired close to each other, twist them. The inductance of a conductor is larger, the more area you have enclosed with it.
Use little ceramic capacitors anode to ground at the power tubes, soldered directly at the socket pins

[bepone]

Using the chassis for gnd, you can just make the worst scenario..
I would remove all the gnd wires, connect them only to electrolytic minus terminals where they belong, and ground the chassis only at the input jack. With the bad or random layout oscillations are just normal, telling you how good you are, or not

Speaker ground is tricky here, need to scratch the head little bit. Guess where it is going. It is easy.

It is going to gnd side of 1k resistor in PI (6SL7) where it belongs (not to the chassis).

[Mark]

I think I’m nearly there. The noise on the output jacks is 10mV.

Thanks for your help, your insight and your patience.
Much appreciated.

[bepone]

this is good, enjoy

[TUBEDUDE]

10mV is well minimized, but what is the source? When that is identified it can be not just reduced, eliminated. Even low level oscillations will heterodyne with signal to make the signal less pristine.

[R.G.]

I think I’m nearly there. The noise on the output jacks is 10mV.

That's great! Good work. Can you tell us what worked best for reducing the noise?

[Mark]

I think I’m nearly there. The noise on the output jacks is 10mV.

That's great! Good work. Can you tell us what worked best for reducing the noise?

The shielded cable was the game changer. It made the most difference.

[martin manning]

The shielded cable was the game changer. It made the most difference.

Which was the offending lead, or did you replace more than one?

[Mark]

The shielded cable was the game changer. It made the most difference.

Which was the offending lead, or did you replace more than one?

The usual culprit, the wire from the wiper of the volume pot. This wire would normally be on my hit list, but I thought the builder was on top of this. Everyone makes mistakes.

[Raja_Kentut]

great!
now you can eradicate the HF, have you experimented with a gridstopper?