Wiring seperate cathode resistors for each pair of tubes..

[johndandry]

I am installing 2 x 125ohm cathode resistors, 1 for each pair of EL84s in a Rocket. When wiring the tubes this way I noticed that
1. each pair must be connected as if it were the only tubes, meaning
one pair must have a 1.5k from each side of the PI, a plate winding from both sides of the OT and a bias resistor/cap. Then I should connect the 2nd pair pf EL84s off the 1st pair. Does this make sense??
It looks like on the Liverpool layout that the OT plate windings are only connected to one side of the PI..then jumped from there.

[martin manning]

To separate the Rk's, my put would be to do it like this. Each left-right pair has a common plate lead, a common signal feed from the PI, and common Rk, but each tube has its own grid stopper (no change to the PI signal feed). As a first cut, you should double the RK and halve the Ck to hold the bias point and frequency response of the Rk-Ck combination.

The Liverpool has a common Rk for the inner and outer pairs of tubes, meaning that each Rk is shared across a push-pull pair, not separated left-right. I'm not sure exaclly what effect having two shared Rk's vs. a single shared Rk would be, but my first thought is that the sound quality wouldn't be much (if any) different, but that arrangement might balance idle current better with unmatched tubes.

I'm not sure what your goal is here. Are you just trying to use two shared Rk's like the Liverpool, or separate the Rk's for the tonal difference that will produce?

[johndandry]

I keep thinking that the cathode bias resistor/cap should be like the liverpool. One Resistor/cap for inner two tubes, resistor/cap for outer two. That way it would be set up with a the bias across a push/pull pair.

[martin manning]

I keep thinking that the cathode bias resistor/cap should be like the liverpool. One Resistor/cap for inner two tubes, resistor/cap for outer two. That way it would be set up with a the bias across a push/pull pair.

The wiring would be as I show above then, except for the cathode connections, where the jumpers would connect the inner pair and outer pair instead of left-right pairs. I'd make the Rk's 2x the Rocket value (goes with the Rocket Plate voltage, which according to the documentation is lower than the Liverpool's), and halve the Rocket Ck.

[Clyde]

I'm thinking that Cathode cap values should not be halved.

[martin manning]

I'm thinking that Cathode cap values should not be halved.

Your rationale?

[fishy]

Probably thinkng the balancing of the RC. Doubling the resistance and halving the capacitance maybe......

The obvious reason for splitting the cathodes might be to pull two tubes.

[Clyde]

My rationale is that the load has no effect on the value of the cap, unlike the cathode resistor. If I were doing this, I'd retain the same value of bypass cap. I stand to be corrected of course.

[martin manning]

I'm not sure what you mean re the "load." To hold the cathode bypass corner frequency the RkCk product must be maintained. Parallel pairs with 2x the Rk and half the Ck should respond the same way as when all four tubes share a common R and C. I don't know if KF was thinking about pulling two tubes in the Liverpool when he configured it this way but you certainly could. I also note that the Liverpool schematic has two 220uF, but it's a different amp, and that value was chosen for it specifically.

[Clyde]

Again, I'm no expert but my understanding of the bypass cap on push-pull output stages is that it's a different animal than the one used on preamp tubes, probably because they run in class A, and usually output stages are AB. The cap on the output stabilizes cathode voltage swing, keeping it stable for higher output, rather than establishing a corner frequency as it does in combination with the cathode resistor when used on preamp tubes. It still has some effect on frequency (it is a capacitor after all). With each output pair independently biased, would the capacitor values sum at all and have the same effect as they would in a preamp tube connected parallel?

[martin manning]

In a class AB stage there is more to it, but in both class A and AB the capcitor is still bypassing AC and stabilizing the voltage at the cathode, and the range of frequencies over which it is able to do that effectively depends upon the RC time constant. In a class AB stage the average voltage at the cathode will rise when a signal is introduced, and that will happen whether the cathode resistor is bypassed or not. The rate at which the cathode voltage rises depends upon the size of the capacitor (given a bias resistor value), with a larger cap resulting in a slower rise. If you start from a case where there is a common Rk and Ck for four tubes and you want to split the inner and outer pairs as discussed here, the response will remain the same only if you double the R and halve the C. Using the original C value for each pair will produce the same effect as doubling the C in the original configuration.

[Clyde]

I guess it really doesn't matter a whole lot as I can't seem to find anything in the literature on it. I did find this old thread from this board, FWIW. Thanks for your insights Martin, always trying to learn something new.

https://tubeamparchive.com/viewtopic.ph ... 17dec0ec5a