Amp Cutting out intermittently..

andyoz

My favourite little Marshall JCM1h practice amp is cutting out intermittently. Seems to do it only after half an hour or so and once it starts to do it it gets worse.

It happens with multiple guitars and cables.

Should I just buy another power valve? Do I need to do any simple diagnostic tests?

Arkless_Electronics

'fraid that if you've ruled out cables, guitars and any effects/pedals then in the case of an intermittent fault like this it could be a wide variety of things causing it... It could be the output valve yes but I would expect it to happen much sooner than half hour if it was.... It's about the only thing you can try changing yourself before taking it to an "amp tech" but I reckon less than 50% chance of it being the valve in this case.

ICBM

It's not going to be the power valve because this amp uses both halves of the valve in push-pull, so a fault in either half will not stop the amp working - just make it quieter and more (probably badly) distorted.

It could be a preamp valve, since the signal passes through all the valve sections in series and a fault in any one will stop the amp working. You could try replacing them one at a time with a known good valve and see it that cures it.

But, I would still expect it not to be a valve - this kind of thing is not typically a valve fault, more often a bad solder joint (often on a high-power resistor which has overheated and partially desoldered itself) or some other signal-path connection problem.

Arkless_Electronics

I'd assumed single ended output.... not familiar with this amp!

DJH83004

Sounds like a very similar design to the Blackstar HT-1R where they use two halves of a triode (ECC82?) in push pull with a standard centre tapped O/T. Think you guys are right though, a time / heat related problem, such as a higher wattage resistor. Wouldn't rule out a valve going low emissions over time, I had to change the output valve on a HT-1R recently, but in fairness it was completely dead. Happy hunting :-)    

andyoz

Thanks all. Sounds like it's more complicated than I thought. These have 3yr Warranties but I assume Marshall would need valves ruled out as a cause before looking at it angle then there's courier fees. It would probably be easier to get a local amp tech to look at it.

I think it only uses the second ECC83 preamp valve with the Boost circuit. So switching preamp valves might help me rule out a bad pre valve?

ICBM

No, both valves are in use all the time. The Boost switch activates a Jubilee-style diode/LED clipping circuit.

andyoz

I think I'll buy a pair of ECC83 values.

I've got a JMP1h as well and that uses the same 2 x ECC83 preamp valves so having spares won't hurt if it turns out to be something other than valves (which seems likely).  Then it will be off to a pro tech.  There isn't much inside these so I'm sure the issue would be tracked really fast.  Great little practice amps though.

Jeez your guys are some help!

Andy

57Deluxe

Sometimes this can be easily solved if the  amp has a send return just jumper that if not in use. Sadly not applicable ehre it seems...