Re: Hum pickup on inter-stage coupling transformer
I've been loaned a small 6VA 12+12:220V mains transformer to try in place of my damaged microphone transformer. It is the common unshrouded split bobbin type. Unsurprisingly, if anything the hum is even worse.
However, when I said "Using a separate power supply makes no difference either" I lied. When I did that test I placed the external HT supply downstream of the onboard rectifier and paralleled the external 6.3V AC heater supply across the audio amplifier valve. What I failed to realise is that this allows the external heater supply to energise the onboard mains transformer via its 6.3v AC winding. Doh! Disconnecting the 6.3V wiring at the onboard mains transformer cures the hum. I'm still a bit puzzled why with onboard power the hum persists for a few seconds after I switch off the mains but perhaps the big power transformer is coupling to general electrical smog within in the house?
So now the problem becomes how to avoid coupling between the mains transformer and the interstage audio transformer. Maybe a steel screen around the mains transformer made from a baked bean can is the next thing to try? If all else fails I suppose I can use an external PSU but that would be a last resort.
73, Alan G3XAQ
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