So my 50w Marshall SLX is acting a bit strange, it sounds great and plays well but starts emitting a high pitched squeal when turned up to around 8 on the master volume. Yes I know that's very very loud but I'm playing in a metal band the moment with a very loud drummer! It's not a feedback-type squeal either(so please don't tell me to get my pickups repotted, or turn down the gain, etc, etc!)....If I unplug the cable from the amp then the problem goes away. If I unplug the guitar from my effects but leave the effects plugged into the amp then the problem remains. So far I have replaced the output valves and rebiased, replaced the preamp valves one by one, cleaned the input Jack with switch cleaner and tried jumpering the effects loop with a patch cable. I don't experience this problem with my other amps when playing at the same volume with the same guitar, effects, leads, cab, etc, etc. Does anyone have any suggestions on what it may be?...
Comments
One feature of this amp is that the internal electrical ground is partially lifted from the chassis ground - the idea is to prevent ground loops when used with other equipment which may be grounded. But its also possible that by not being fully grounded it's allowing some sort of feedback loop to occur.
You can test this by plugging a spare cable into any of the sockets on the amp which don't cut off the sound (the second speaker jack is probably the easiest) and touching the outer metal casing of the plug (not the tip) on the other end to one of the bolt heads next to the mains input socket - if the squeal stops, it's that.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I also just found that turning down the treble and/or mids reduces the squeal, the bass or presence does seem to make a difference though....
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I’m so bored I might as well be listening to Pink Floyd
I ended up giving the amp to a local tech who went right through it and found multiple things wrong. The amp had been (badly) repaired in the past and many of the tracks on the pcb were damaged with components badly soldered. Some components were the wrong value and some were actually damaged and passing volts/current that they shouldn't have been. Some of the pots had dry joints and weren't working correctly, etc.
Basically it seems a lot of little things contributed to the problem rather than a single issue. Anyway it sounds even better now and the squeal has gone......best £50 ive spent in ages!.....
Just got my Marshall back from a tech with a similar problem and diagnosis. Although there hasn't been any dodgy repairs on mine, he said (or what I understood of what he said) because of the age of the PCB its starting to degrade and there is almost a voltage bleed going between tracks where there is eg.300v and 6v lines very close to each other. especially once everything is running hot at practice / gigging volumes.
I'd not really got the ideal of point-to-point amp building, until now.
The Marshall is to be retired, and likely Sold before I need to fork out on a whole new PCB.
You can avoid the problem entirely with a PCB as well - you just have to use a high enough quality board, and design it properly… something that Marshall have consistently failed to do. It's a particularly well-known problem on the DSL/TSL100, and they don't even have to be old for it to start happening. Sometimes they go beyond 'voltage bleed' and into an outright arc where the board catches fire.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
That's bad business. Quality doesn't cost, it pays.
There are many other valve-amp manufacturers at all price points who don't make the sort of stupid design mistakes Marshall did with this series too.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein