Power supply node decoupling

Discussion in 'Amp Tech Center' started by Nickfl, May 16, 2019.

  1. Nickfl

    Nickfl Tele-Afflicted

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    How do you know you have enough? I've seen plenty on how to calculate power supply ripple, but not much on decoupling other than a few things saying that using dropping resistors that are too small and feeding too many valves from the same node can cause oscillation, but nothing about calculating for this in the design stage.

    I am wrapping up a recent build that should have been pretty straight forward as it is pretty much just a 5c3 with an additional grid leak biased input channel and switchable NFB. However, upon startup testing I discovered it was motorboating with the phase inverter tube installed.

    After doing some reading on the causes of "motorboating" and checking all my solder joints and trying different tubes, I determined I had insufficient power supply decoupling. The power supply was the only place I had really deviated from the Fender design because I used a field coil speaker and transformer set from an old radio and had to adjust the dropping resistors to accommodate. I ended up increasing the preamp dropping resistors by a few K and adding an extra node to seperate the input stage tube from the PI tube and these changes took care of the issue.

    So... how would one go about designing for this before the amp is built? Calculations, rules of thumb, anything? It wasn't the end of the world to have to add an extra power node on the fly since this was a semi PTP build anyway, but it was certainly less than ideal and I would like to avoid having to do it again.
     
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