Merlin Blencowe / Valve Wizard Grounding - questions

[DF96]

That depends on the details of the wiring. A humdinger will help if you have capacitive coupling from heater wiring to the audio circuit.

The symptoms you have seem to be consistent with a ground loop somewhere. You need to double-check that everything which is grounded has one and only one ground connection. Also worth double-checking that the charging pulse loop does not include the chassis or the signal ground bus as part of the loop - there should be one wire from this loop to the ground bus.

 

[Malcolm Irving]

... when I connect the probe and it’s ground together I get a flat line.

If I connect both the ground and probe very close together I still get about 7 or 5mV of buzz. I also tried to connect the probe ground on the star ground where the circuit buss is connected to chassis and then I touch the alligator clip directly with the probe and I still get the buzz.
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This makes me wonder if a ground loop including your amplifier and the scope might be involved. This loop would be signal ground of amp - safety earth of amp - safety earth of scope - signal ground of scope - signal ground of amp.

Having the scope and the amp on the same mains outlet would tend to reduce this. Or (to test this theory) plugging the scope and the amp into widely separated mains outlets might increase the buzz.

 

[Malcolm Irving]

… With all controls to zero the hum stops and I can only hear a very quiet hum from the speaker, but the hum from the PT itself is louder than that.
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To be honest, that's typical of guitar amps, which have higher gain (and usually lower quality transformers) relative to hi-fi amps.

 

[Malcolm Irving]

...I have a different question. Is it wrong to twist together the OT CT with the OT primaries? I get a kind of blocking distortion on the trail of guitar chords when the amp is on 10 but I probed the circuit with a 0.1uF cap blocking the DC into a solid state power amp and discovered that in a superlead most of the distortion is happening in v2a and the cathode follower. It was eye opening. If I cut the bass before going into the amp it goes away, or if I roll the volume back and I let the bright cap do the work. So I think this is normal but I still wonder if twisting the OT wires can be an issue in terms of oscillations or other things.
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Twisting the CT and primaries of the OT should be a good thing. It helps to prevent them radiating magnetically and electrically to other parts of the circuit. It will add a bit of extra stray capacitance across the primary windings, but I think that would be small compared to the intra-winding capacitance already present inside the transformer.

 

[overtone85]

Twisting the CT and primaries of the OT should be a good thing. It helps to prevent them radiating magnetically and electrically to other parts of the circuit. It will add a bit of extra stray capacitance across the primary windings, but I think that would be small compared to the intra-winding capacitance already present inside the transformer.

Thanks! I was afraid I had to undo all of that! Ahah! But I guess the blocking distortion is just too much bass in the first stage when the amp is on 10. And the OT CT is not a problem. Thanks for the explanation.

When I’ll have time next weekend I’ll try and plug the scope in a different outlet.