With cathode biased amps that are idling too hot, the root cause is often that the HT voltage is somewhat excessive. That seems to be the case here.
The problem with cooling the idle point by increasing the cathode resistor value is that when the HT supply is poorly regulated, as with most guitar amps, as the cathode resistor value is raised, the decrease in HT current draw results in the HT voltage increasing. The end result being that the actual idle dissipation is little reduced, but the tone and response of the amp may be worse, due to reduction in the conduction angle and consequent increase in bias shift.
A better approach may be seen to address the root cause by reducing the HT voltage a little, eg add resistance into the HT supply, rather than the power tube's cathode return. A few volts off the HT will reduce the plate voltage and plate / cathode current. So adding x ohms into the HT supply has a double whammy effect on plate dissipation and no effect on the amp's tone / response, whereas adding x ohms into the cathode resistor has a 'half whammy' on plate dissipation and may worsen the amp's tone / response.
Hence rather than increasing the cathode resistance 21 ogm to 68 ohms, I suggest to add a suitably beefy 22 ohm resistor between the rectifier and reservoir cap.
Which accords with some other versions of silicon rectified AC30s, eg https://el34world.com/charts/Schemat...x_ac301986.pdf
The problem with cooling the idle point by increasing the cathode resistor value is that when the HT supply is poorly regulated, as with most guitar amps, as the cathode resistor value is raised, the decrease in HT current draw results in the HT voltage increasing. The end result being that the actual idle dissipation is little reduced, but the tone and response of the amp may be worse, due to reduction in the conduction angle and consequent increase in bias shift.
A better approach may be seen to address the root cause by reducing the HT voltage a little, eg add resistance into the HT supply, rather than the power tube's cathode return. A few volts off the HT will reduce the plate voltage and plate / cathode current. So adding x ohms into the HT supply has a double whammy effect on plate dissipation and no effect on the amp's tone / response, whereas adding x ohms into the cathode resistor has a 'half whammy' on plate dissipation and may worsen the amp's tone / response.
Hence rather than increasing the cathode resistance 21 ogm to 68 ohms, I suggest to add a suitably beefy 22 ohm resistor between the rectifier and reservoir cap.
Which accords with some other versions of silicon rectified AC30s, eg https://el34world.com/charts/Schemat...x_ac301986.pdf
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