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Old 26th July 2005, 03:49 PM   #1
bigbulb is offline bigbulb  Canada
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Default 2A3 AC filament hum

Hi all,

I finished my 2A3 amp with AC filament + hum balance pot. I adjust the 100ohm pot carefully and I get 2.8mV of hum at 8ohm load.

Someone on one of forum says that the AC filaments can be adjust to 0.3mV of hum. It this true?

Any one of you guys who using AC filaments get lower than 2.8mV? how low it can go?

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Old 27th July 2005, 01:07 AM   #2
PRR is offline PRR  United States
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There will be at least three main source of hum.

B+ ripple
filament balance
grid-circuit pickup

You have to attack them separately.

Get a holder for two D-cells and a 0.2 ohm 2-watt resistor. This is a hum-free heater supply. It only runs for a few minutes, but it will tell if your B+ is clean enough.

On AC heat, your heater wires must be well twisted and tight to a metal chassis.

The point where your heater winding center-tap (or balance resistor) must not tap into a point in the B+ system with large 100Hz/120Hz capacitor current spikes.

Grid leads must NOT run near heater leads. Take them away on the other side of the socket. Keep them short and neat. A low-impedance driver helps, but drivers are usually picked for other reasons.

Some 2A3 heaters are better balanced than others.
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Old 27th July 2005, 12:03 PM   #3
bigbulb is offline bigbulb  Canada
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Thanks for the Idea, I have done what you mentioned . I'm familiar with star ground and also handling B+ hum, I had tried my 2A3 amp with 2V sealed lead acid battery, it reads 0.08mV of noise. AC filaments will increase to 2.8mV.

What is the possible cancellation value the hum balance pot can do?

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Old 27th July 2005, 01:21 PM   #4
kmtang is offline kmtang  Canada
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I think you can add two 33-ohm resistors parallel to the hum pot which would give you finer adjustment.

If you can't bare 2mV hum induced from AC filament supply, I think you would need to power it with DC supply.

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Old 28th July 2005, 12:59 AM   #5
PRR is offline PRR  United States
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Put a short across the grid resistor. If hum goes down, you are getting hum induced in the grid circuit.

2mV may be as good as it gets for some 2A3, but it sure seems you could do better with a good 2A3.
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