Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on April 15, 2025, 10:22:52 am
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Using a PT with no CT and i need to power the bias from one leg of the HT secondary. The PT i'm using the same as used in the Marshall Artist. In this schematic it appears they have set the bias with a fixed resistor which is fine. I am more interested in what they've done before the bias rectifier diode. HT leg series into a .047u ? cap then a resistor to ground then into the diode. What is this doing? can i series into a resistor instead and not use a cap? Like a standard Marshall that uses a PT with a CT?
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the un-educated explanation; :icon_biggrin:
the cap insures only AC gets in, so no "offset DC" between the circuits
the R "fixes" a ground reference for the bias circuit, then it all gets converted to -DC bias, "fixed" to chassis as a reference point.
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so the resistor to ground is necessary? the PT with a CT doesnt do it
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the PT with a CT doesn't do it
apples don't taste like oranges either :icon_biggrin:
my circuit analysis skills have been traded for farmer stuff so i might be off;
the PS WITH CT is the AC reference to ground
the PS WITHOUT CT uses R1 to reference BIAS supply AC to ground.
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It’s an AC-coupled bias supply. Yes The resistor is necessary- See explanation at bottom of page in link
https://valvewizard.co.uk/bias.html (https://valvewizard.co.uk/bias.html)