These medium sized combos from Marshall should be very cool. They are packed with features (like multiple voicings, low noise pre-amp, fab speaker) all in a small box.
Unfortunately they are not so cool because they run so hot and they suffer from it. Look at the picture left. What you are looking at is a power valve that has had its label “fried” off it.
The cause of this problem is that the amps were originally biassed to run a little too hot and also the ventillation is limited with heat rising up but with almost nowhere to escape to.
One of the main problems can be the bridge rectifer shown below. This is black square on the right hand side of the picture. These components supply the pre amp heaters with the super quiet DC. Orignally Marshall fixed these directly to the boards and they get up to around 70deg celsius. The problem with this is that the heat so close to the PCB is enough to start melting the solder joints.
Later Marshall realised this and raised them off the board by a few centimetres, the impact was tremendous and reduced the temperature by about 30%. I go a step further and add a little heatsink to them too.
The symptoms of the overheating is faults like cutting out, channel switching problems and general weirdness. Once modified to remove these issues you have one of the most well thought out practical gigging combos out there.
Thought I would share a couple of photos of Marshall JMP amps that I have worked on recently. First we have a 74 and then a resissue. They sound good as gold and the re-issue has a much better turret board design to boot.
Amp repairs seem to go in waves. The latest one being a string of TSL with what I call TSLitis. This increasingly common condition causes these amps to overheat and do serious damage the amp.
The problem is that the amps circuit board material is not doing its job of insulating the connections properly and what happens is that valves bias drifts which in turn drives the valves harder overheating the valves which causes destructive feedback loop. All of this places a strain on the power supply, valves and supporting circuitry; all of the most expensive components in an amplifier.
The ones I have seen seem to be from around 2000 to 2002 and are quite easily identified.
Things to look for:
Amp misbehaving after an hour or two and excessive heat
Discolouration of the valve bases due to overheating. This is usually visible only on the reddish brown bases where they go distinctly dark brown as they meet the glass envelope.
A unique electrical burnt Bakelite smell – see photo
The colour of the valve PCB (only visible by opening the amp and inspecting. Effected boards seem to be a light green colour and dark green boards
Red plating of the power tubes (that is the flat grey piece of metal you can see inside the glass envelope). It is supposed to be grey but when over heated turns a “cherry” red colour.
Amp transformer failure if left on for a prolonged period.
So what is the cure?
Some folks are offering kits to remedy this by separating the sensitive bias circuit from the main board. This is quite a big change and I suspect will have reliability issues in the future. It is best, in my view at least, to replace the PCB all together. It’s not as expensive as you may think and renews many other components effected by age like filter capacitors, valve sockets etc
It seems to me that Marshall really got it right here – the right mix of hard wiring and PCB. The Valve sockets are all carefully hard wired to the circuit boards and the. Layout is very neat and as a valve bass amplifier I think this takes some beating really warm bubbly typical valve sound. Not sure when this one acquitred its leopard skin appearance but it sure isn’t original.
Standard valve amp problems of course are always present in the classic EL34 tube amp but don’t let that put you off venturing into valve amps. Think of it more as a tool that needs sharpening from time to time. Lifetime of these amps will easily go on indefinitely as all components are replaceable and readily available.
Lovely combo as used by Noel Gallagher, Paul Weller and many others. It’s a simple formula 2 x 12″ speakers, 50 watts from a pair of EL34 and, my favourite point, as few controls as possible.
Construction of the cabinet is unusual in that seems very thin. I don’t mean the wood rather the front to back depth it can only be 10 inches. The amp itself is standard enough with one channel but with bass and lead inputs. Using a patch cable to link the inputs gives some interesting tone variations.
On this particular amp it would blow fuses occasionally and this was thought to be caused by the valves and a recap job. Certainly one of the EL34 was faulty. But that was not all.
Going through the caps I found that these amps feature a PCB for the capacitors and resistors linked by wires to separate valve bases. Unfortunately the quality of the circuit board is pretty low. Very thin tracks and pads make repairs difficult. My solution is to replace components onto vero pins lifting the component away from the board and making replacement a breeze.
Photos to follow of: Burnt board and Crap cct board
Even after all this, the amp worked but there was something in the tone that wasn’t quite right. On certain notes a little harmonic aliasing existed sounding like a second not played across the top. I was stuck. As always I went back to first principles. Check the anode voltages, check the voltage drops across the valves one by one, pin by pin. Just as I was beginning to get to the end of my fault finding idea list and lose all hope I found something odd. On of the B valves the cathode was sitting at about 40v – and not zero volts. How can that be? But it turns out that there was a broken wire from the valve base to the chassis- it was floating.
Quick fix and then powered it on. It sounds awesome. I think this is the amp that Paul Weller uses on the early solo albums and its a real modfather cracker. What happened next to the amp is a real joy to me.
After returning the amp to my client he had decided to sell the amp on. Following a quick sale it was snapped up by none less than Bobby Keys, best known as sax player with the stones but to me best known as Ronnie Laine’s sax man. Either way these amps sound great and are really flexible and if the pcb bothers you then it is easily replaced by a tag board alternative.
Sorry not taking any new client appointments for a while – Aug 2019.