Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Craftyjam on December 22, 2025, 03:15:00 pm
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Hello all,
I'm trying to understand the purpose of the 470k grid leak resistors to ground on the SSS #02 schematic that I've seen on the web.
I undertstand that E and F are about 300v +/- to allow maximum signal swing to drive the 6L6's.
A different arrangement can be seen on the TENA schematic which is a hifi amplifier using a similar scheme. It also has the bias voltage directly coupled through the CF, but does not have any grid leak resistors.
Since the bias voltage is coupled through the CF, why does the SSS feature grid leaks? It wouldn't protect the circuit in case the bias voltage somehow failed, would it? Was it simply an after thought, since every guitar amp has a resistor in that position?
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Follow up question:
What is the impedence seen by the cathode of the CF driver with no grid leaks? Is it just the grid stopper, miller capacitance, and grid to cathode "diode"? (positive grid current.)
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Yeah you don't 'need' to do it that way - it all depends on how the output tubes get their grid bias voltage. Other amps (Like Ampeg SVT) set the output tube grid leak resistors up so that they are one and the same as the CF load resistors (which in my view is better because it eliminates the need to worry about impedance bridging). There's not enough voltage information on those schematics to comment about the bias voltage
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Cathode internal resistance / impedance is low, that's why cathode followers are used as buffers.
Nothing specifically about cathode followers here, but plenty about cathode resistance https://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers
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Cathode internal resistance / impedance is low, that's why cathode followers are used as buffers.
Nothing specifically about cathode followers here, but plenty about cathode resistance https://www.aikenamps.com/index.php/designing-common-cathode-triode-amplifiers
I understand the cathode followers output impedence, but the input impedence of the power tube is another thing. My understanding is that with no grid leak resistor, it is "infinite" below the cutoff frequency from the miller capacitance. Things change when the grid is driven positive and the grid has a "negative" input impedence, and current flows from the grid to the cathode.
Am I on the right track here?
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I'm trying to understand the purpose of the 470k grid leak resistors to ground on the SSS #02 schematic that I've seen on the web.
Homebrew schematic. People often start with an existing schematic & modify it, and those 470kΩ resistors may have been leftovers from the donor-drawing.
Knowing nothing else, I would assume a drawing error and that the 470kΩ grid leak resistors were not present in the original amp.