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Old 18th January 2012, 02:10 AM   #11
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Is the heater voltage ac or dc?
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Old 18th January 2012, 02:12 AM   #12
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Quote:
Originally Posted by trobbins View Post
Is the heater voltage ac or dc?
In my application I am using 6.3VAC.
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Old 18th January 2012, 03:01 AM   #13
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I assume you have a humdinger pot on the heater and are elevating the wiper. I would recommend the elevated bias voltage capacitor bypassing be substantial, such that its impedance relative to the bottom cathode resistor is small, and the capacitor connects to the bottom cathode resistor 0V, as that is the loop the conducted heater hum can take. Connecting to another point on the circuit/chassis will introduce any chassis noise between those points in to the bottom cathode circuit.

From heater cathode conduction leakage versus dc bias characteristics I've seen, as long as the heater DC bias level is more than about 20-30V away (pos or neg) from the individual top and bottom cathode levels (at idle) then conduction leakage is likely to be about as low as it can get.

Last edited by trobbins; 18th January 2012 at 03:03 AM.
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Old 18th January 2012, 03:21 AM   #14
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Thanks for the reply. The voltage divider for the low-level signal tubes is using a 33uF cap to ground very close to the first cap point. This is not a DHT amp. I am using 6CA7 as the outputs in PP. The 6CA7 heaters are sourced from an independent xfmr and will be referenced to ground on the center tap. My question is only with regards to the heater elevation of the mu-follower input section.

Last edited by scott17; 18th January 2012 at 03:27 AM.
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Old 18th January 2012, 05:16 AM   #15
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That's fine, as my comments were about the bottom input triode of the mu-follower, and heater hum and chassis noise being injected into that high gain stage.
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