[Amps] Problems with 3cx3000 amp

[Jimmy Jones]

Hey Guys
Hope you can help me.
The circuit:
1) 1-3cx3000a7
2) plate voltage=4200 vdc (choke input)(4 uf capacitance)
3) plate choke=123 turns of #21 mag wire on 1" dia. form 5" long
4) c1= 5-250pf 15kv vac. var.
5) c2= 50-1200pf 7.5kv vac. var.
6) blocking cap= 1200pf @ 30kv
7) L1= B & W COIL 10 Thru 80m
8) High Voltage is bypassed at the entrance to the plate choke by 3 caps in
Parallel values: doorknob type 500 @ 20kv, doorknob type 500 @20kv and
ceramic disc type 4700 @10kv.
9) no Parasitic Suppressor

The amp works pretty good on 40 but not as good on 75. When I remove the
4700pf bypass cap the amp becomes a monster on 75 and will hardly work on 40
at all. I have yet to be able to tune thru the peak on c2 on any band. C2
likes to be at full capacitance. When I add fixed capacitance to c2 it
doesn't change the way it wants to be set.(fully meshed)c1 tunes fine. I
have noticed some welding type arcs down around the bottom of c2. There
isn't anything down there. Could I have a bad c2? I don't fully understand
what effect the plate choke and bypass caps have on the tank circuit and why
the removal of that one little cap would have such a dramatic effect.
Any Help would be appreciated.

Tnx,
Jay