Line Filter Capacitor Across Heaters

Clb8484

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Would there be a benefit (i.e. noise reduction) in placing a capacitor across the ac heater lines?

Mr. Robinette mentions doing this on the transformer secondary where it enters the rectifier tube:
"50 or 60Hz buzz is sometimes caused by power line noise and can be addressed by applying a small, high voltage filter cap across the high voltage rectifier input wires. This .02uF 3KV (3000v) ceramic disc cap works well for this. I usually solder one across the rectifier tube socket where the two high voltage power transformer wires connect. For a solid state rectifier solder the cap across the high rectifier inputs"

Just curious if the same or similar benefit could be obtained doing the same across the tube heater wires. Perhaps at their first connection point - the pilot light.
 

Phrygian77

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Complete lack of necessity aside, that value coupled with the low inductance of the 6.3 volt secondary, and the heater load, isn't going to do much of anything at all.
 

Phrygian77

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Noise from heaters is primarily from leakage current between a tube's heater and cathode, and the EMF around the heater wiring.
 

Clb8484

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What about using something in the 1uf-10uf range? That seems to be the range that X capacitors across mains are around. I don't have noise in the audio path, but the tube heaters or power transformer are audibly humming even when the amp is in standby, i.e. the hum isn't coming through the speaker, its just a hum the amp itself is making. In standby nothing else should be getting power, so I'm assuming its something in the heaters or power transformer. A cap across the heaters seemed like something easy to try was all I was thinking. Alternatively, I could try an X caps across the mains and a Y cap from mains to ground. Or, I could just leave it be since it isn't THAT loud and isn't in the audio signal.
 

Phrygian77

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Transformers will hum and buzz under load. In fact, just about everything can make noise under certain conditions. If you inject a 1k signal in with a dummy load attached, you'll start to hear it from the tubes and output transformer as you start turning the amp up. You could try to isolate the power transformer acoustically by added some felt washers.
 




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