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RE: Voltage margin on PS caps and unusual cathode resistors.

Standby B+ voltage is with the output stage tubes heated and loading down the amplifier. Given the amp uses ss diodes, there is 10-12 seconds when a higher DCV is applied to all caps in the amp.

At power turn on, a modern e-cap meeting typical performance standards can withstand +10% of rating for a short time and infrequent events (such as during power on of a guitar amp).

Unless you are prepared to confirm by measurement what the peak turn-on B+ voltage can reach, and are happy with the margin for the caps you want to use, and the local line voltage variations, then imho don't go there.

I'd suggest the different cathode resistor values in the output stage are to counter-balance the difference in output transformer primary half-winding dc resistances, given there is already individual EL34 idle bias level adjustment pots. However that reasoning makes no sense for normal operation as the EL34 plate resistance in pentode mode would dominate nominal frequency response. It may make some sense during gross over-drive operation, where each EL34 saturates in turn, and an imbalance in saturation current levels could noticeably offset the output transformer flux swing symmetry.



Edits: 10/01/20 10/01/20

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  • RE: Voltage margin on PS caps and unusual cathode resistors. - trobbins 19:31:18 10/01/20 (0)

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