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RE: Seldom Needed

Posted by cpotl (A) on March 1, 2015 at 16:24:58

In Reply to: Seldom Needed posted by Triode_Kingdom on March 1, 2015 at 14:49:56:

"There have been several in-depth discussions on this topic here. Not only does the SRPP require a specific load Z to work in push-pull, that load is typically much lower than is practical for our use...

Below is a link to one of the earlier discussions. Search for SRPP under my name and you'll find more."

Thanks for the link. I'm embarassed to find that I actually participated briefly in that earlier discussion, but had somehow missed some of what was covered in the rest of the thread! My only excuse is that the dates coincided with my annual migration back to Texas from the UK.

After a quick skimming of that thread, I notice that the issue of what difference the bypassing of the cathode resistor makes to the behaviour was addressed. Perhaps my experiments in Spice today added just a little more to that; indeed, spurred by Lew's remark about omitting the bypass capacitor, I found also that it didn't change the "critical" load impedance values much. But I did find an enormous difference in the distortion figures, and especially, the quite impressive cancellation of the 2nd harmonic distortion at the critical value of load impedance that balances the antiphase tube currents, but *only* if the cathode resistor is not bypassed. (I just noticed that Lew's ValveWizard article mentions this too.)

Just one other question: Even though the load impedance required for balanced push-pull operation is unaturally low from the point of view of the kind of impedance the next stage in a tube amp would normally present, might it be that giving the SRPP such a low, and precisely adjusted, load might make sense? It does look as if the distortion could become a lot less under such conditions.

Chris

Edits: 03/01/15 03/01/15 03/01/15

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