Marshall squeals like a pig

[Monkeyfart]

Hey there, newbie here and need some help. My Marshall TSL 60 head is squealing like a pig at high volumes on all three channels and is even worse when using the effects loop and a processor. When I begin to play it sounds ok but even the slightest movement on the strings when muting will send shivers down your back. Am I correct in saying Microphonic Feedback?

I changed all the valves last night and it is still doing it. One thing I noticed, the preamp tubes didn't glow like before but the crunch and lead channel still sounded right.

Any info would be great. I have a gig Saturday night and I think I'll be using a friends amp but need to know what's going on.

Thanks,
Monkeyfart

[olddawg]

Have you tried a different guitar? You may simply have microphonic pickups. They could be replaced or wax potted. Is it worse when you stand closer to the amp? You could also have a preamp tube gone microphonic especially if they are still the stock tubes. Preamp tubes are cheap and require no adjustments to replace. You can take a pencil and tap each tube starting with V1 to try to isolate which tube(s) are microphonic or you can simply buy a couple and sequencially swap them out. There is also the possibility that you are oversaturating the preamp with the master volume turned down. There is no fix for that except learning how to set up your amp. This usually happens a lot when someone wants to play with a lot of gain at low volume through an amp much to large for what they are doing. If you have your master set on 2 and the preamps cranked that may be your problem. The solution to that situation is to get a smaller amp.

[Monkeyfart]

I've tried other guitars and ran my main guitar through another similar amp. No problem. I changed all the tubes last night but didnt' have time to set the bias before practice started. I will do that tonight when I get home.

Could a bad guitar cable or speaker cable cause this?

[olddawg]

I've tried other guitars and ran my main guitar through another similar amp. No problem. I changed all the tubes last night but didnt' have time to set the bias before practice started. I will do that tonight when I get home.

Could a bad guitar cable or speaker cable cause this?

You don't have to set the bias unless you replaced the output (power) tubes although it is never a bad idea to check the bias on an amp. A bad cable can cause this (rarely) because stated simply a cable has resistance and capacitance and can turn your preamp into an oscillator when combined with the inductance of the pickups. I have found it happens occassionally with cheap curly type cables and the cheapest of the cheap.

[Monkeyfart]

I use a decent set of planet waves cables for about the last 3 years. They seem reliable. Well, this is just the way it goes. I'll try tapping around the tubes tonight and see what I can find. I have a gig tomorrow night and will truly miss my Marshall head.

Another thing I noticed, when I unplug from the guitar and touch the tip of the cable jack, it gives me the same squeal sound as I desrcibed above. This never happened before this problem arose. Not to say I go around pulling cables out of my guitar all the time but it is something I noticed.

Thank you for all your great advice.

Monkeyfart

Tim Johnson
sidewinderdayton.com

[Enzo]

Just because the cord worked the last three years doesn't mean it works now. Everything works until it doesn't.

Try a different cord.

Get something between you and the amp. Use a really long cord and step around the corner from the amp and play. Alternatively, put the amp in a closet and close the door on it while you stay outside and play. Does it still do it? If not, it is the guitar feeding back. Just because the guitar doesn't feed back through some other amp doesn't mean it is not doing it here. That TSL has a ton of gain after all. Especially when turned up loud.

How about you don't plug anything into the amp, just turn the amp way up sittong there alone. Does it squeal? Does tapping the tubes do anything?

[Monkeyfart]

I've checked or done almost all of those things except isolating the amp and cranking it without being plugged in. I did however use it Saturday for my gig and all went well. I went in with the intention of trying it to see if it would work and if not I'd switch to my Hot Rod Deluxe and use the TSL to power my Heil Talk box. I had time on my side since we set up at 3:00 and didn't go on till 10:00 (the club was only 15 min from my home). I won't have another chance to check it till Thursday. I'm hoping that it won't rear it's ugly head but rather doubt it. My next gig is a couple of weeks off so this weekend will give me the time I need to check it more thouroughly.

Great tips, thanks.