Original WAD 300B pp Question

[david]

Ok you guys with the original 300b pps, I ahve a question for you:

Clive Meakins and I met up last night to take our scratch builds forward. I also got hold of a pcb via Clive (thanks Greg!).

If I look at the circuit diagram, then the heater circuit for the 6072 and 5687 is simply connected to the floating ac from the mains tx. However, the pcb is clearly designed so that the heater ac is earthed via Pin 9 of the 6072, thus sharing the same earth run as the input valves. This seems a sure fire recipe for hum to my mind and I gather some of you building the amps did have problems with hum.

So, my first question is why not leave the heater for these valves floating as it is in many other designs and as indicated by the circuit diagram?

I would certainly like to try this option rather than opting for a DC heater solution in the first instance.

Thanks
David

[Greg.]

Hi David,

The below is courtesy of Neal Gibbons' own modifications.

As standard the 6072/5687-heater ground return and input and driver stage
earth return is provided by the signal input wiring earth shield. I do not
like this arrangement and so cut the earth track on the PCB from the heater
wiring and installed a potential divider consisting of two 100R 2W resistors
across the heater winding and took the junction of the two resistors to
ground via a separate wire. The two resistors ensure equal balance and
noise cancellation in the heater winding.

DC heater circuit may be of interest.



I hope this helps.

Best wishes,

Greg

[david]

Thanks Greg but I'm still interested if anyonme tried the flaoting ac heater route.
David

[NickG]

Try it and see, but I think you will find a floating heater supply will induce noise and hum. You are assuming it will float at 0v, it wont, it will float where it feels like, which may be +200v or -200v or both depending on the time of day. If the heater supply has a center tap, earth that, othewise create a virtual center with a couple of 100R resistors and earth the center of that.

[david]

Thanks Nick, makes a lot of sense to try ac like that then dc if any noise present. I now it can theoretically float anywhere but in practice with those amps I've tried with floating ac heaters there doesn't seem to be noise or hum issues.
David

[John Caswell]

Hi all,
At the risk of offending NealG a better solution may be to replace the 2 x 100R resistors with a 200R pot and twince for minimum hum. This would possibly allow for any anomalies in the 6072 & 5687.

John aka Dr John

[NealG]

No offence John, that's what I did on both of my 300B amps. It was easier to recommend 100R's as on both my amps measuring the WW pot's I used gave equal 100R readings.

The reason I went to DC was due to rectifier switching noise that seemed to be capacitively coupled somewhere in the cct on both amps. I tried various things to trace it but eventually gave up and went DC which killed it at once.

[James]

Please bear in mind that in the final production run of PCB's for the WAD 300b PP clone project, I removed the link from the PCB ground plane to the heater connections. This was done to allow the use of Neals DC heater circuit. (which I incorporated onto the heater PCB)
I believe you may have an early "prototype" pcb, in which the artwork clean up was not complete and this link is still in place.
Please check carefully for suitable clearances on these PCB's as extensive work was done in this area when creating the final artwork.

James.