> This is the PSU schematic
1) WHY just 330 ohms? A larger resistor is better filtering but more voltage drop. Doesn't the mike suck just a few mA? Can't that resistor be 1K, maybe 5K, without much drop?
2) Even so, a single filter stage can only do so much. For LARGE ripple reduction cost/bulk increases directly as "large" increases, and with exponential hearing this really means 3X or 10X cost/bulk for every little bit of buzz reduction. (And there are limits with ESR and stray wire resistance.)
Here's rule-o-thumb. Use as many R-C filter stages as there are amplifier stages from here to the loudspeaker in a simple system (like guitar-amp). This mike might drive a Fender directly. There will be 2 or 3 small stages and a power stage, at least 4 amplifier stages. Even though your mike lacks all the later stages you need about this many R-C filters to get the buzz down.
Here's rule-o-thumb. If not gasping for supply voltage, allow 10%-20% drop in the filter. Any benefit from a mere 20% rise of B+ is likely to be hidden behind buzz.
(If you like math and can compute raw ripple and mike-amp's PSRR, assume 20db-30db is a good target for each R-C stage. Less for high current low loss (EL34 screens), higher for low current (preamps).)
Below I've drawn your supply with several more R-C filters. I used your "330r" value and cut it in fourths to spread it around.
However 330r seems teeny. What is the mike's actual draw? There's 100K so the whole thing can't act like less than 100K. The 100K "fights" a tube so the sum is probably more like 200K. The supply is an astonishing 260V. Total demand is near 1.3mA.
By the 20%-drop rule we should use around 50K total drop resistance. Say 10K per R-C. Or since caps are cheap, maybe less. 4K7, 6K8, 8K2, whatever is handy.
Another rule-o-thumb. Use 1uFd per mA of current, to 10uFd/mA for small loads and low-cost caps. 220uFd is absurd. Five 220uFd 350V is large, and the mike will stay-on a very long time after AC is cut. R-o-T suggests 1.3uFd-13uFd. 22uFd is convenient and cheap. Since you already have two over-size 220uFd on board, I only used 22u in the middle, but you could re-do with only 22u and 5K.
Another rule-o-thumb. 3,000-pixel images don't fit most PC monitors.