Marshall problems!

[oncesolemn]

Hello people, new to the forum today and would love your views on this!  I have a Marshall JCM200 DSL100 head with some issues.  The thing is, I'm not getting much out of it!  When I'ts switched on I have to turn the volumes up full on both channels to get a very small return.  Tried different guitars so no problems there.  All the valves seem to be lighting up normally and all connecting cables are in perfect working order.  The cab is a 1975 Marshall 4x12 with appropriate celestions.  I bought this amp new in 2005 and have gigged it extensively since without a hint of trouble till now.  I also noticed the output gradually reducing during rehearsals, accompanied by a farting alien sound coming from the cab!

Anyone know what this could be?  I will obviously take it to a good tech, if it's not something I can fix myself.
Thanks.

[Twinfan]

I'm assuming you've tried different valves?  Even just swapping them around?  You've tried a different cab too, just in case?

It sounds like a transformer issue to me, but I'm no tech!  I'd find a local one....

[oncesolemn]

I've tried a different cab [modern 1960A] with the same result.  How would swapping valves about potentially alter the result?  Both pre and power valves?

[oncesolemn]

I should add that I mean swapping the existing valves about, I have'nt tried new valves yet, other than a new ecc83 in each pre position.

[oncesolemn]

I should also say, that was a single ecc83 I bought that I swapped into each position in the pre-amp section, and tried that with the existing pre-amp valves each time/position!

[Twinfan]

In case one particular valve is only putting out a really low output, swapping them about may make the problem more or less.  If it's always the same, then it's not the valves.

If you've tried a new ECC83 in each preamp position, try pulling the outer two EL34s and halving the impedence on the amp:

e.g.

Pull the two outer EL34s, set the amp to 8 ohms, connect a 16 ohm cabinet.  Then try again, swapping the two EL34s for the pair you've just removed.

If you've still got the same issue, then it's a trip to a tech I reckon?

[HTH AMPS]

have you tried plugging a patch lead between the send and return jacks? - effects loops commonly lose a firm connection through the send/return jacks causing the problem you're describing.

[hunter]

Also plugging the guitar directly into the return of the amp could give some indication.

If it's loud and powerful through the return then the issue is before the poweramp (i.e. a bad connetcion, poti or preamp tube).

[oncesolemn]

Thanks for the tips guys, really apreciated!  Will be trying them all out as soon as I can. The one involving the send/return jacks is interesting as I always use the head with rack effects.  Will let you know!

Cheers.

[Catalyst77]

I had exactly this problem and it turned out to be a faulty pre-amp valve.