AB763: pros and cons of shared cathode bias vs separated cathode bias ?

[Lambertini]

Hey!

In Fender AB763 circuit, cathode bias resistor and caps for V1B is shared with V2B, cathode bias for V3A is shared with V3B, and cathode bias for V4A is shared with V4B.
It makes sense to me for V3A and V3B as they are serialized parallelized, processing the exact same current, but all the others are located at very different stage of the circuit.

What are the benefits of such practice? What are the cons?

Would separating them avoid useless interactions between triodes and gives more stable current to theses tubes?

Thanks.

[sluckey]

The only benefit is lower cost. Saves the cost of two caps and 2 resistors, about a dollar. Makes no sense to an individual or a small boutique builder, but it adds up to thousands of dollars to someone like Leo.

It makes sense to me for V3A and V3B as they are serialized

Not serialized, parallelized.

I don't see any issues with sharing for V1B and V2B since they are completely unrelated circuits. However, sharing V4A and V4B is a potential problem since they are consecutive gain stages. As long as the cap is doing it's job, there is no issue. But if the cap begins to lose capacitance due to age, then you have a situation for some positive feedback and that 'may' lead to oscillations.  Best to avoid shared cathodes in consecutive gain stages.

If you look at Hoffman's AB763 you will see that he used separate cathode components for all the mentioned tubes.

[Lambertini]

Not serialized, parallelized.

Oops, sorry ;)

If you look at Hoffman's AB763 you will see that he used separate cathode components for all the mentioned tubes.

Right away!

Thank you very much sluckey!

[stratomaster]

Splitting cathodes opens up the possibility of partially bypassing to shape tone.  I'm personally a fan of doing this at the first stage, but it opens you up to some noise.  Splitting the 2nd stage and partially bypassing, combined with tonestack and plate/cathode resistor mods allow multiple voicing options for each channel that would otherwise be unavailable with shared cathodes.