fastdave
Member
I became aware of a hum, more of a buzz, coming from the JCM900 4100, and being intent on playing, politely ignored it.
A few nights later, I could smell burning, and ignored this also, until I realized I was some way away from the amp, and it wasn’t the normal ‘valve heat I was smelling, but something more sinister. I checked the ‘fuse fail’ indicators on the back, to discover one fuse o/c. searched everywhere, and in my stock, found 2 x 500maT fuses – replaced that one and switched on, holding a mirror round the back (amp was against the wall, pedals on floor, cables trailing, etc., and to my horror, witnessed the plates of V4 glowing red and becoming white.
I switched off, cleared the area, and pulled out the chassis. A 1.5K perhaps 1W resistor, strung across 5 and 6 of the 5881 furthest from the mains switch was completely fried and o/c (open circuit – I don’t know the nomenclature for the American, if there is any).
I managed to source a 3W wirewound 1.5 k from my stock, suspecting the wattage was perhaps higher than 1/2W, duly replaced the resistor, and switched on, having replaced the valve with an old EL34 which I knew worked, this having been one of the ones I replaced with nice new matched quad 5881’s.
The amp started, but seemed to burp intermittently until it warmed up, and it played ok, but when I tapped the cabinet or valves (tubes), it appeared to be microphonic (this may have been the reverb coil, but I don’t think so), and the noise was not from dirty pots.
I put it back together, and played for 10 mins or so, (one gets involved, you know the feeling), and then checked the back with the mirror again.
This time the valve closest to the mains switch seemed to be getting redder and hotter.
Throughout this I was still getting a buzz – better described as a buzz, because it wasn’t smooth like 50 Hz ( British mains frequency ) through the speaker.
I haven’t checked any voltages thus far, since my valve theory is more than rusty, but I had set the bias when I got the new 5881’s and had played successfully every other night for the past year or so – always set on high output.
One transformer seemed different from the other, so it may have been replaced, and I am reading that Marshall stock trannies aren’t very good.
Any ideas? Can anybody give me a quick solution or speed diagnostic action?
Dave.
If I was near you, Mr Wilder, I would be on your doorstep!!
A few nights later, I could smell burning, and ignored this also, until I realized I was some way away from the amp, and it wasn’t the normal ‘valve heat I was smelling, but something more sinister. I checked the ‘fuse fail’ indicators on the back, to discover one fuse o/c. searched everywhere, and in my stock, found 2 x 500maT fuses – replaced that one and switched on, holding a mirror round the back (amp was against the wall, pedals on floor, cables trailing, etc., and to my horror, witnessed the plates of V4 glowing red and becoming white.
I switched off, cleared the area, and pulled out the chassis. A 1.5K perhaps 1W resistor, strung across 5 and 6 of the 5881 furthest from the mains switch was completely fried and o/c (open circuit – I don’t know the nomenclature for the American, if there is any).
I managed to source a 3W wirewound 1.5 k from my stock, suspecting the wattage was perhaps higher than 1/2W, duly replaced the resistor, and switched on, having replaced the valve with an old EL34 which I knew worked, this having been one of the ones I replaced with nice new matched quad 5881’s.
The amp started, but seemed to burp intermittently until it warmed up, and it played ok, but when I tapped the cabinet or valves (tubes), it appeared to be microphonic (this may have been the reverb coil, but I don’t think so), and the noise was not from dirty pots.
I put it back together, and played for 10 mins or so, (one gets involved, you know the feeling), and then checked the back with the mirror again.
This time the valve closest to the mains switch seemed to be getting redder and hotter.
Throughout this I was still getting a buzz – better described as a buzz, because it wasn’t smooth like 50 Hz ( British mains frequency ) through the speaker.
I haven’t checked any voltages thus far, since my valve theory is more than rusty, but I had set the bias when I got the new 5881’s and had played successfully every other night for the past year or so – always set on high output.
One transformer seemed different from the other, so it may have been replaced, and I am reading that Marshall stock trannies aren’t very good.
Any ideas? Can anybody give me a quick solution or speed diagnostic action?
Dave.
If I was near you, Mr Wilder, I would be on your doorstep!!
