Help needed with squealing Marshall

rawk100

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?...

Maynehead

My first thought was a microphonic preamp valve. Try putting your finger on each valve when the ringing happens and see if the ringing changes.

ICBM

Does it do it at the same settings with the guitar plugged directly into the amp and no effects?

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.

rawk100

ICBM said:

Does it do it at the same settings with the guitar plugged directly into the amp and no effects?

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.

Yes it still does it with just the guitar plugged in. I will try your suggestion tomorrow regarding grounding the socket with the spare cable.

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....

rawk100

So I plugged a cable into the Recording Compensated output on the back and earthed the ring of the cable on the chassis, it actually made the squeal worse. I tried it on the Direct Out output and again it made it worse.....

ICBM

Hmmm... well that does at least imply that it's something to do with the grounding! Although exactly what, I'm not sure without being able to have the amp in front of me.

fretmeister

Stop playing Zakk Wylde tunes?

rawk100

Thought I'd update this in case anyone has the same problem in the future.......

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!.....

sgosden

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.  

ICBM

sgosden said:

I'd not really got the ideal of point-to-point amp building, until now.

It would be a mistake to assume that. Old Fenders are "point to point" - eyelet board in fact, although in the guitar world it's taken to mean the same thing - and often suffer from the same issue.

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.

impmann

rawk100 said:

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.

Yep... sounds like modern Marshall build quality to me...

sgosden

ICBM said:

sgosden said:

I'd not really got the ideal of point-to-point amp building, until now.

It would be a mistake to assume that. Old Fenders are "point to point" - eyelet board in fact, although in the guitar world it's taken to mean the same thing - and often suffer from the same issue.

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.

Good quality PCBs in everything eat in to profits. Sad times, but business is business I suppose.

martinw

sgosden said:

Good quality PCBs in everything eat in to profits. Sad times, but business is business I suppose.

That's bad business. Quality doesn't cost, it pays.

ICBM

sgosden said:

Good quality PCBs in everything eat in to profits. Sad times, but business is business I suppose. 

Oddly, Bugera - Behringer's valve amp brand - can manage to use decent-quality PCBs and not run traces too close together. It is true that they did use an under-spec jumper connector on some of their early amps, but this was quickly fixed. Marshall had at least three goes at the DSL/TSL100 and *still* didn't get it right.

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.

rawk100

sgosden said:

Just got my Marshall back from a tech with a similar problem and diagnosis.

What model of Marshall have you got? Is it the SLX as well?

sgosden

rawk100 said:

sgosden said:

Just got my Marshall back from a tech with a similar problem and diagnosis.

What model of Marshall have you got? Is it the SLX as well?

DSL100