What is the best place to add a 1 ohm bias resistor in #124?

[Joost]

Will it matter on which of the two pairs of 6L6s I place the two 1ohm resistors on pin 8 (cathode) for biasing?

The thing is that between pins 7 and 8 of V4 there is a 120 ohm resistor to ground to create a virtual ground for the heaters. On the cathode of V5 there is a connection to the 50/100W switch, on V6 pin 8 goes straight to ground and on V7 pin 8 goes to the 50/100W switch again.

I can't think of the best solution here. How and where would you mount the 1 ohm resistors between cathode and ground on these two pairs of 6L6s?

[martin manning]

Schematically it looks like the attached. Where are the ground wires for the ground connections marked 2 and 3 in the attached drawing? You can just replace those two wires with resistors like the red ones. I don't think it will matter if you leave the virtual center tap for the filaments tied to one of the cathodes.

[Joost]

Schematically it looks like the attached. Where are the ground wires for the ground connections marked 2 and 3 in the attached drawing? You can just replace those two wires with resistors like the red ones. I don't think it will matter if you leave the virtual center tap for the filaments tied to one of the cathodes.

Thanks!!! What are the blue resistors in this schematic?

[martin manning]

Schematically it looks like the attached. Where are the ground wires for the ground connections marked 2 and 3 in the attached drawing? You can just replace those two wires with resistors like the red ones. I don't think it will matter if you leave the virtual center tap for the filaments tied to one of the cathodes.

Thanks!!! What are the blue resistors in this schematic?

The usual set-up is to have a separate current sensing resistor for each tube, so that would be the blue ones.

[Joost]

Schematically it looks like the attached. Where are the ground wires for the ground connections marked 2 and 3 in the attached drawing? You can just replace those two wires with resistors like the red ones. I don't think it will matter if you leave the virtual center tap for the filaments tied to one of the cathodes.

Thanks!!! What are the blue resistors in this schematic?

The usual set-up is to have a separate current sensing resistor for each tube, so that would be the blue ones.

Thanks. Is there a disadvantage to using 2 instead of 4? Maybe a slight unbalance between the tubes? Or will those two resistors make no difference?

[Structo]

It's best to have them on every power tube so you can see if there is any huge mismatch between power tubes.

[Joost]

It's best to have them on every power tube so you can see if there is any huge mismatch between power tubes.

Just curious but how was the bias set on the original 124?

[Structo]

He probably used the transformer shunt method or simply measured the negative bias voltage.

Or perhaps he used the scope.