Re: Capacitors for Trio 9R 59D
C40, C41 & C42 were only originally rated at 250V. This is reasonable commercial practice in a price-sensitive design.
Since electrolytic capacitors are much cheaper in real terms than 50 years ago, we can allow much more generous margins:
The transformer's output is 160V. Multiplying by SQRT2, adding 10% for mains being 10% over 230V, and another 10% for surges due to regulation of the transformer during valve warmup and good luck gives around 294V, so 300-V or 350-V electrolytic capacitors are more than adequate.
Another thing, looking at the circuit, the AF Gain is directly coupled to V7b's grid.
This makes it prone to being "scratchy."
When changing the pot., I'd suggest adding a capacitor (say, 10nF, only needs to be rated at a few volts) in series with the wiper and a resistor of 2M2 -4M7, from the capacitor's junction with the wire to V7b's grid, to the low end of the track(screens).
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