Coupling cap and filter cap 6G6b circuit questions

cowmoo

I’ve been having issues with this amp, it’s a Marsh bassman clone, 6G6b circuit.
It started making thundery crackly sounds in the studio, I replaced the tubes for some known good and then some new to no avail, my techy mate had a look and we found the first two CC’s had bad joints but after resoldering found they were still noisy. (Moreso when tapped with a screwdriver handle) We temporarily put in two Mullards which seemed to cure the problem. I’ve bought some new orange drops as per original, .22@600v but he suggested replacing with .15@600v to lessen the bass response as I have to run the amp at zero or close to, with my Phoenix.
(I am using a 2x12 v30 cab but it is semi open back so could this cause too much bottom end?)
Is this ok to do, will it achieve that result, or will I need to meddle further? (Which I’m reluctant to do)
Also there is too much 50/60 cycle hum. He suggested changing the filter caps starting with the two Spragues.
These are atoms .20@500v
The rest are holy grails .22@450v and .22@600v.(joined)
Will this sort the hum? I only have two new blue atoms.
Please no really technical answers, I struggle to grasp the mathematics involved or the lingo used for parts!!
Yes I know how dangerous the caps are and won’t meddle without assistance from a tech.
Thanks in advance.

Modulus_Amps

0.22 or 0.022 coupling caps?
Changing from 0.022 to 0.015 will make a very small or possibly zero difference. Marshall changed it to 0.0022uf on the bright channel.  If you have a good tech he can change the parts while you are there to hear the difference. Having your cab handy is a must

Modulus_Amps

Crackling may be bad Jack sockets or bad plate resistors. With the holy grail caps in there you probably have other cheap parts too

50hz hum could be from the heaters. Does it go away when you turn the volume down

cowmoo

Thanks for the reply, The caps are 0.22 and it was suggested I change for 0.15. All other caps on the tag board are orange drops, but the filters are two atoms and the four holy grails. 
No the hum is present all the time. All sockets, jacks and pots have been cleaned at least twice. Cheers

martinw

Normal or bass channel?
I'm assuming bass as you say .220 caps.....the schematic shows .250 but that's near enough.
Of course the bass channel is very bassy, but the normal channel should be err........ more normal.
If you can get the sound you want with the bass turned right down, is that not ok? It's quite often the case using vintage style amps that the eq is not What we'd expect these days.
The filter caps should be fine, don't change them unless you're sure they're faulty. If The hum is present all the time, with volume off, Its not the filter caps.
It sounds like your amp would benefit from a visit to a good tech.

cowmoo

Thanks. 
This may be the best option in the end. I’m just throwing money away on parts I may not need. 
Yes the amp sounds sweet when it’s not playing up! 

cowmoo

Recap, possible pun intention, after fitting three new coupling caps, checking everything else multiple times, reflowing solder more than once, revalving more than once, I noticed if I pushed on the eyelet board in the first gain stage area the crackle would appear as I moved the board. I think it’s been suffering from capacitance between the upper and blank sub board. So I’ve put some cardboard between and for three hours last night it never made a squeak. 
It seems old fender fibre boards suffered from this but modern clone builds? Hmmm 

ICBM

Ah, the cornflake box trick . I’ve fixed a lot of old Fenders like that...

But you’re right, it’s very surprising on a new amp.