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Thread: DC Inject onto lifted heater center tap

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    DC Inject onto lifted heater center tap

    Just verifying that I can do this with this type of Hammond PT and rectifier?

    thanks.
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    Senior Member trobbins's Avatar
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    I'd suggest posting a link to the entire amp circuit - that will help identify where you what to go and what it will affect. This snips don't show what HV you are using, or where in the heater circuit you are connecting, or what valves are in the amp and what their heater-cathode voltage will be.

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    Not sure what "0V Heaters" means in your drawing.

    For heater elevation you need a floating heater winding. So you would have to disconnect the winding's center tap from ground and inject the DC to the center tap (as your headline says). Otherwise the low DC resistance of the winding would short your DC to ground.

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    Last edited by Helmholtz; 10-08-2019 at 02:51 PM.
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    Your drawing isn't consistent with the use of dots to indicate circuit connections, so it's unclear whether crossing lines are connected or not.

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    Aha, ok well here's the whole amp then
    The DC source to inject is the common circuit type, two resistors and smoothing cap.

    thank you...
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  6. #6
    Senior Member trobbins's Avatar
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    I could suggest the simplest means to do heater elevation in that bassmaster circuit is to disconnect the heater CT from the 0V ground, and connect the CT lead to say a 63V electrolytic positive lead, with cap negative to 0V ground, and connect a resistor from the +39V common cathode point on the phase inverter stage over to the CT lead and new cap positive terminal. The +39V node has a pretty stable dc voltage that is not too high. The input stage triodes and V2B triode may achieve a lower hum, and V2A triode will have a reduced heater-cathode voltage stress on it.

    The new resistor can be something like 100k, and the capacitor something like 1 to 10uF (and doesn't really need to be electrolytic if you have a suitable film cap). The RC filter is then at most 2Hz, so will filter any mains frequency.

    If there were a short/arc on output stage socket pins 2 to 3 then the new cap may become collateral damage, and hopefully there will be no other stressed parts.

    If there was a short from a heater side to chassis, then no collateral damage due to the 100k resistor.

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