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RE: Diode Tweak

Posted by trobbins on April 28, 2021 at 19:45:49:
Eli, there really isn't any switching transient in a vintage valve diode application, due to the relatively high winding resistance and self-capacitance, and the lowish diode peak current pulse (dictated by the valve diode). The situation would be different for a typical ss diode rectified supply with much larger filter capacitance, and a modern power transformer with single secondary winding for full bridge or doubler use.
The 1N4007 has a measured PIV of about 1.5kV (I've measured a few, and seen a few threads that presented measurements), whereas the UF4007 has a measured PIV of about 1.2kV.
The 1N4007 has very low (pretty much unmeasurable) leakage current out to the PIV level. The UF4007 has a measurable and increasing leakage out to its PIV. Not that leakage current is really an issue, as the valve diode and ss diode will both conduct the same leakage level and the reverse voltage with apportion accordingly but can't push either diode in to breakdown as the other diode will limit the current to just its leakage level.
Aging valve diodes can start to show measurable leakage below their rated PIV, and the leakage of any diode in a valve is typically unrelated to the other.
Adding in an ss diode effectively prevents a valve diode from arcing over (and hence damaging itself) during stressful events (eg. a bad output stage valve, or at turn-on), and allows nominal rectifier operation if any valve diode in the valve starts to exhibit end-of-life leakage increase, which is likely to result in a longer than normal service life (although pretty much nobody maintenance tests their diodes and wouldn't know if a diode was degrading until it continuously arced).