Class A2 - heater hum

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ACPEN
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#1 Class A2 - heater hum

Post by ACPEN »

Hello
I've built several A2 amps over the years usind DC filament heating - no hum problems mainly using 811a and DA41 / TZ40 types.
Thought I'd try ac heating this time with humdinger pot. Results vary depending on type of valve used - with 6v 811a and 812a, no problem but with 7.5 types DA41 / TX40 some balance well with very little hum but others although working well in other respects, won't balance out with high hum levels.
Any ideas please?
Atb
David.
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Nick
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#2 Re: Class A2 - heater hum

Post by Nick »

When you say "won't balance out" do you mean there is no null or that there is a null but there is still more hum than you want at that null point?
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ACPEN
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#3 Re: Class A2 - heater hum

Post by ACPEN »

There is a null but still a loud hum with some valves noticeably worse than others - thats the bit I dont quite understand - I appreciate that the higher heater voltage may well contribute to this, but why only some valves.....
atb

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Nick
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#4 Re: Class A2 - heater hum

Post by Nick »

Don't know, a relative question would be why do some null at all? I guess it depends on how the gain of the valve across the cathode/filament matches the DC resistance of the filament to distribute the hum signal.
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ACPEN
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#5 Re: Class A2 - heater hum

Post by ACPEN »

Yes, thats about where I'd got to - but dont understand the mechanism of why it happens.
Thanks for your thoughts.
Rgds
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Paul Barker
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#6 Re: Class A2 - heater hum

Post by Paul Barker »

Sometimes power supply counters the filament hum , even though the use ofchokes shifts the phase a little I’ve had a 212 amp which removed hum unplanned, but very pleased about it. That was the first 212 amp in my dining room a few people in here might recall.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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Nick
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#7 Re: Class A2 - heater hum

Post by Nick »

the problem (IMHO) with this sort of cancellation is while the hum signal is removed from the output, the valve is still seeing the hum and so its inherent non-linearity means its gm is being swept at 50 or 100Hz, so while there is no hum, if you look at a simple 1k tone you will see hum sidebands. That's why the effect of good DC heating is to clean up the midrange and low level tone resolution (which is sometimes unexpected). That's why I don't hold to the "you cam hear past the hum" view on AC heating. As I said, IMHO.
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#8 Re: Class A2 - heater hum

Post by Paul Barker »

thanks for that its helpfull
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