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felipemuniz 5th December 2012 02:20 AM

Vox ac30 HUM B-2
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I grounded the input pin and now I can hear clearly what's going on, and it's seems that is a B-2 hum.
It's from the pre amp because, how I more turn up the pot, it get more louder and when the pot is almost or totally down, the hum dissapears. The filter capacitors are new.. I also made a little test: I connected the heaters to a 6V/4Ah lead-acid battery, and nothing has changed. I changed the transformer to test too, and nothing has changed too.
Sometimes the hum change a little bit the frequency (sorry I not got it on the record), like Bb2, I dont know, happens very fast, 5 ou 6 secs.

What can be making this? You can find the diagram here: VOX | Vintage Circuit Diagrams

Is the: "AC30 top boost - 1986"

cyclecamper 5th December 2012 08:38 PM

Did it start when you replaced the filter capacitors? Sure sound suspect...

Put a scope on the B supply and find out.

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