300b lovers

2. I use interstage transformers and separate PS for driver and input tubes. As a result, I have two separate GNDs: 1st - for output circuit and PS 2nd - for input/driver circuit and PS. Should I connect both these GND together to the amplifier chassis at one point? Or can I leave one of these GND floating?

@alexberger I can confirm everything @lynn_olson mentioned regarding your post in his above. Pay attention to all those issues!

Regarding this question which has to do with grounding.

You’ll want to ground your chassis if its metal (and if you’re running single-ended, ferrous materials will provide audio-frequency shielding). So the ground connection of the power cord should be tied to the chassis.

The audio circuit, if tied directly to chassis, leaves you open to ground loops. To avoid this, lay out your amplifier circuit so all the points that go to ground do so to the power supply (star grounding is nice) without touching the chassis.

Grid and cathode connections for each tube section (the ground side of the grid resistor and ground side of the cathode resistor) should be common and use a single wire going to the star ground. This forces noise imposed in that wire to be common mode and so will reduce noise.

The spot in the power supply to which you tie your grounds can be a buss tying all the filter cap grounds together.

Once you have all the grounds starred together, at that point you need to reference the chassis ground. That can be done several ways: you can use a simple resistor from the audio ground to chassis ground. You can use a pair of rectifiers in parallel, each opposite of the other, with a resistor in parallel with that. This higher impedance is presented to ground currents between chassis and the audio grounds which otherwise set up the ground loop.

Be sure to have the input connectors also isolated from ground. Most RCA connectors are supplied with shoulder washers for this purpose.

In this way, the chassis can be a relatively quiet shield for the audio circuit. You’ll find the audio circuit to be quieter with this practice.

Hi @lynn_olson ​​​​@atmasphere ,

Thank you for useful information.

Does aluminum provides any shielding? I will use an aluminum chassis.

There is a beautiful picture of Blackbirds. But I don't see inter-stage transformers. Are they under the hood?

Does aluminum provides any shielding? I will use an aluminum chassis.

@alexberger Yes, but only at radio frequencies. No shielding at audio frequencies, and for that reason steel is likely a better choice if you're running single-ended.

We use aluminum chassis in most of our stuff, but its balanced and so does not need shielding to be quiet.

Alexberger, the Cinemag input and interstage transformers are under the top plate. These are the third generation of the custom Cinemag transformers we started with. The two Monolith transformers are visible on the rear of the top plate.

The other things under the top plate (right side) are the B+ and filament regulators, and the master soft-start circuit, with an external 12V trigger. The point-to-point and turret-board wiring under the audio side (left) is very simple, with only transformers, tube sockets, cathode resistors and bypass caps. No current sources or other gizmos.

Wow. Those Blackbirds are stunning looking! They're just a wee bit larger than my Statement 300b amps from Don.

[How does one post photos here?]