Filament "0" Volt Center Tap?

[Mcentee2]

True, bit it might capture the stray ac magnetic field from the power transformer, not?

Yes, that's what I thought you were getting at, and if in a JTM it is of significance Vs a very high gain amp - I assume not, is practically a small improvement, but thought I would check with you.

I remember doing some DIY dummy coils for my strat a while back, they worked with area as a basic component for how much em noise each turn picked up, so can see it is worth thinking through.

 

[Pete Farrington]

A sense of scale / proportion seems to be missing here.
If using AC heater power, at the preamp end of the chassis, it seems to me that the fields arising from that will surely be massively greater than any other source of 50 or 60Hz EMI?

 

[tschrama]

I had to re-think about it for a bit. I focussing magnectic/inductive coupling. That happens due to a large AC current through a loop causing an AC magnetic field. This happens in the power transformer and a bit around the AC wires from the power transformer.

Such a AC mag field can couple into a another conductor loop, causing a AC current. Such loop exist in the amp between HT-capacitors. The capacitors can be thought of asa straight wire for AC current. Thus a loop is created e.g. around the PI capacitor , the red and back wires and the next capacitor. Any AC captured mag field will cause a AC current. This current can cause a AC voltage in the ground bus that get added to the guitar signal.

solutions:
Minimized the captured magnectic flux by minimizing the area of the loop (keep the red and black wires close to each other, even beter: twist them) and , or,
Use thick ground wire.

 

[tschrama]

I think the magnetic flux caused by the AC heater is low because it is only a single loop. While the power transformer has many windings.

Furthermore, the HT AC wires contain high peak current, representing high frequencies, but the heater wires and low frequency without current peaks.

 

[Pete Farrington]

I suspect this ‘induced interference from the PT field’ maybe a red herring. Mains frequency hum generated within the amp just isn’t an issue, unless valves with leaky heater to cathode insulation are fitted (and there’s no circuit mitigation for it).
The hum issues we do note are at twice mains frequency, and are mitigated effectively by the use of a well designed 0V common arrangement.

 

[tschrama]

You could be right, but the PT radiation is in sync with the current slike, that is twice the mains frequency.
On top of that, I never had a amp build that suffered from 50Hz heater interference, wether I used marshall style heater wirkng or Soldano style wiring. But I have experinced rectifier induces buzz many times, and solved it with carefull bundeling of current supplying and cureent returning wires. PS these are very high gain amplifiers, but I do manage to keep the Hiss from the thermodynamic noise of the hummbuckers as the dominant background noise source.

 

[danfrank]

The hum issues we do note are at twice mains frequency, and are mitigated effectively by the use of a well designed 0V common arrangement.

Twice the mains frequency "hum" is from the B+ supply, 50-60Hz hum is from the filament circuit.

 

[Pete Farrington]

…50-60Hz hum is from the filament circuit.

Or, if applicable, the bias supply, if, as is generally the case, half wave rectification is used there

 

[danfrank]

Yes, I forgot about bias supply. Thanks for bringing it up.