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Question on Cap Voltage Rating - Printable Version

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Question on Cap Voltage Rating - davemc - 04-03-2022

I have two Philcos on my bench: a 37-630 and a 38-60. The tone control circuits and associated capacitor values are identical in these radios. All caps are original. <https://philcoradio.com/library/download/service%20info/wiring%20diagrams/vol2/split/Part21-Philco%20Wiring%20Diagrams%20Vol.%202.pdf). The in the 37-630 the capacitor from the output tube plate to the tone control is 0.008mfd at 400 volts (that point is shorted to ground in one tone control setting). The capacitor from the tone control to ground is 0.03mfd at 600 volts.
 
In the 38-60, the 0.008mfd capacitor is rated at 1600 volts. The 0.03mfd cap is a bakelite block (8328 SU), and thus no published voltage rating. I have read in the Philco Repair Bench article on on rebuilding bakelite block caps that all of these caps are rated at 400 volts so that they can be used in any position.

For both radios I was planning on using a 0.0068mfd at 1600 volts for the 0.008 (what I have in stock - 15% low), and a 0.033mfd at 630 volts for rebuilding the 0.03mfd cap. My theory is that in the later radio Philco increased the voltage rating of the 0.008 cap because of the possibility of damage to the output transformer if hit by a large transient. Any feedback? Thanks!

RE: Question on Cap Voltage Rating - morzh - 04-03-2022

The tone ctl caps, if used in the tone-compensation scheme against the volume regulator, or in any circuit other than direct conenction to the plate of the output driver (across the primary of the audioxfmr) could be low voltage, and the sole reason I use 630V ones is because I do not want to have various voltages while I can have just one for every use.

However those used across the primary of the output xfmr, use higher voltage to withstand a possible flashover when a speaker disconnects.

My personal take on it, if the speaker is pluggable, I would use 1000V (1600V is good, but 1000 is enough).
However in a hardwired speaker this is even less likely to occur (not highly probable in the first place) and so I use 630V without thinking twoce about it.