Hey Jim,
No schematic, and no discussion of fixed/cathode switching, but it might be useful. Go down to "rewiring the bias system"
https://el34world.com/charts/fenderservice5.htm
This ;
Rewiring the bias system:
Looking at the AB763 super reverb layout diagram here, we will examine the fixed bias network. The power transformer has a Red wire with a blue stripe. This is the AC bias voltage winding. The bias winding leads to a 470 ohm one watt resistor. This winding is the winding that will be rectified into a negative voltage compared to chassis ground. First the AC voltage coming from the winding is reduced a bit by the 470 ohm resistor. The AC voltage then is rectified into a pulsating DC voltage by the diode. This diode is inserted so that it rectifies the bottom half of the AC sine wave thereby creating a negative DC voltage compared to chassis ground. If the diode were reversed it would rectify the top half of the AC sine wave and the DC voltage would be positive compared to chassis ground.
After the diode, there is a 25 uf / 50 volt electrolytic capacitor connected to chassis ground. This capacitor is there to smooth out the pulsating DC voltage. It is installed with the negative end towards the bias voltage and the positive end connected to chassis ground. This is because the bias voltage is negative compared to chassis ground. The capacitor charges and discharges slower than the pulsating DC voltage and holds the DC at a more constant DC voltage level. It also has the effect of raising the DC voltage level a bit because the valleys and peaks of the pulsating DC voltage have been averaged up to a smoother less pulsating level. After the filter cap there is a 10k bias pot. The negative DC voltage enters one end of the bias pot and exits the other end through a 27k resistor. The 27k resistor is connected to ground on the other end. A voltage drop occurs across the bias pot and then across the 27k resistor. The center tab of the bias pot is able to sweep across the voltage drop of the bias pot and select different negative voltages. The 27k resistor sets the lowest point on the pot that the bias voltage can go. If the 27k resistor were not there the entire bias voltage would be dropped across the bias pot and the bias voltage could swing from zero volts to the maximum volts produced by the bias network. The bias voltage could short out directly to ground if the 27K resistor was not there and the bias pot was set to zero ohms. This could cause harm to the bias winding if the bias voltage was shorted directly to ground and the tubes would go into runaway current.
The center tab of the bias pot then goes to the junction of two 220k resistors. where it passes through the resistors then through the 1500 ohm control grid resistors and ends up on the control grids of the power tubes as a negative bias voltage. This is basically the way all fixed bias systems work. If your amplifier does not have a bias pot then a fixed value resistor is substituted for the bias pot and the 27k resistor. If the 10k bias pot in the super reverb was set at 6k resistance to achieved proper bias you could substitute a 33k resistor for the bias pot and 27k resistor. (27k + 6k = 33k).
Topic: Everything Bias (Read 2664 times)