Have you ever calculated the RF current in a 40 microhenry plate choke in a stage with say 2500 volts plate voltage operating on 80m? It's surprisingly high, and that current needs to flow the RF bypass at the B+ end. Which it makes it desirable to use an other choke and bypass to keep RF out of electrolytic B+ capacitors...
Agreed, BUT... the OP stated it is a MONOband 20m amp. 50uh = 4396 ohms (@ 14 mhz)
2500 X .62 = 1550 V. ( RF component). 1550 / 4396 = .35 amp.
90 uh won't series resonate on 20m, so lets try that. 90 uh = 7913 ohms of XL @ 14.000 mhz.
1550 / 7913 ohms = .196 amp.
I stocked up on 4700 pf @ 10 kv.....and also 4700 pf @ 15 kv disc ceramic caps a while back. Ceramite brand.... which appear to be no longer available. Typ 2-3 of em are used in parallel for a good bypass on 160m...( with a 225 to 400 uh plate choke). I see our German friends have found a source for .01 uf @ 20 kv disc ceramic caps.... and I have no clue where they are sourced from. They are huge things.
On 80m, with the same 50 uh, resulting current has now increased to 1.4 amps..which is a bunch. When real small bypass caps used at the cold end of the 50 uh plate choke, like 1000 pf, the 1000 pf cap won't have 1.4 amps flowing through it, more like 1/2 that....with the remaining 1/2 flowing into the series string of HV lytics.
TX doorknobs, like the HEC HT-50 / HT-57 variety would also work as a bypass cap on 20m, provided they handle the current....which is easy enough to see from their graphs. Typ TX doorknobs work better for upper HF bypassing, like 20-6m. The disc ceramics appear to work better on the lower HF bands, like 160-30m. Both can be used at the same time, and that is done on some commercial 160-10m amps like the 8K ultra.
The ameritron 225 uh plate choke works good.... up to 1.5 amps dc plate current ( CCS). Above that, it goes into thermal runaway..and fast.
W8JI has cooked up a bigger version. 1.3" OD x 10.5" long (including the 22 ga wire). It's 400 uh, and does not resonate on any of the 10 HF bands. It does resonate on 8.5 mhz...and one higher freq, think it was 16.3 mhz. Rated for 10 kv @ 5-6 amps.