Marshall 2558 problem, any help appreciated!

[trguitar08]

Hi all
I have a 1988 black tolex marshall 2558 jubilee, ive had it for around 18 months now and its a beast of an amp (albeit a noisy bugger) but at rehearsal on sunday it started making this god awful crackling noise, turned it off for a while then tried it again and it stopped for about 5 mins and started again this time my gain was going, sounded like a old plexi dimed rather than the full rich gain i get on my settings, first thought was a tube problem but a quick look round the back the power tubes at least were glowing their usual brightness so i though leave it unplugged for few days and inspect the preamp tubes too.
Yesterday before we rehearsed i took all the tubes out and checked if any burn spots nothing took re-jigged the pre amp tubes and turned on. All the tubes are glowing well so i thought is that it? plugged my LP in and there it was the gain was back and it sounded the biz then that crackling sound came back after 5 mins of playing, so now ive just left it and thought id ask around for any suggestions.

I know it really could do with a service and re-tube but ive no job atm from being made redundant and ive got gigs at the end of the month, anyone any suggestions to what is going on and if theres anything i can do sort it out?

any help greatly received thank you!

[ICBM]

Valves glowing when the amp isn't misbehaving proves nothing. Have a look at them when the crackling is actually happening and you might see sparks or arcing inside one of them - most likely a power valve. Try knocking (gently...ish) on them with your knuckle and see if you can induce the crackling, if it doesn't happen straight away.

If nothing is obvious, get one new 12AX7 and use it to substitute for each of the preamp valves in turn. You should really have a spare set of power valves too, but if you haven't it may be a good time to think about getting some. You don't need a full set of preamp valves, one is enough - I know that won't save you a lot of money.

There is an outside chance it could be something else - although these amps are extremely well-made and reliable normally - but you need the spare valves anyway really. Don't worry about biasing yet, at least not until you've found the fault.

[trguitar08]

ive looked at the power tubes while crackling happening and they were just glowing the same.
to note the crackling still occurs wether iam playing or not and when i hit it on standby the crackling comes through for about 20 secs after, ive tried plugging my guitar into the return of the fx loop and still happens then.

[ICBM]

Does it do it in both the 50W and 25W modes?

[trguitar08]

yes it does, from another forum i got this response

That could be a electrolytic cap going. If they haven't been done they are due anyway. I changed mine myself but if you are not comfortable with high voltages or are inexperienced working on an amp, take it to a tech. You can get killed in there.

[ICBM]

Yes, that is certainly possible - although the caps Marshall used then were high quality and don't normally need replacing this early, despite some of the nonsense you may read about doing them "every ten years" or such - which doesn't mean one can't have failed though - but it's definitely worth eliminating the valves first before checking for anything else. You really do need spare valves anyway if you're going to use a valve amp for anything serious, so now is probably the time to get them.

[trguitar08]

yeah i agree pal and thats top priority. is there any makes you recommend?
2xel34 and 3x25ax7 is whats in there