Oct 31, 2006 12:18 pm does it still sound this way?
either bad tube or speaker.
cab becomes distorted after 2 hours
Posted on Oct 31, 2006 12:14 pm
jackson5
Member Since: Nov 22, 2004
I noticed towards the end of our show last week the speakers in my 1960A weren't sounding as crisp and clear as they were at the start. The gain was there but it started getting a little too distorted and they started farting a bit (for lack of a better word. You guys behave yourselves ) and sounded like crap. At this point my gear and head (JCM800 with EL34 GT Mullard re-issues)had been on for a couple of hours, could it be that the output tubes got too hot and needed to cool?
I thought maybe it was my settings with my new gear but it sounded fine at rehearsal the week before.
Any ideas why it would sound fine in the beginning and then fade towards the end?
Tubes going bad? I got a set of JJ KT-77's I'm putting in this weekend in case it is the tubes.
Anyone know?
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Oct 31, 2006 12:18 pm does it still sound this way?
either bad tube or speaker.
Oct 31, 2006 12:28 pm Make sure to keep plenty of ventilation space around your amp head while playing. It sounds like something is overheating, and the only culprit I could think of is the output transformer. Something in the output stage at least.
Just don't back it up against a wall...give it a foot or two so the amp head can breathe or else heat stores up in there and can do some nasty things.
Oct 31, 2006 12:30 pm I have also seen guys replace the front and back wood panels on amp heads with steel mesh or some other material that helps air flow around through the head.
That also often means the brand of the head gets removed which makes it less attractive to punk, drunk, losers in the club that like stealing ****.
Oct 31, 2006 12:41 pm yes tubes do go bad, and really i think this is where the problem is.....to troubleshoot, hook you amp up to a different cab for a few hours, and see if it starts crappin' out on ya...i've got friends that replace their tubes (usually power, not pre) about every 6 months. they swear buy it, i say "let me have the old ones!" hehe
i doubt you're cab is causing the distortion, the only think i think would do that is a blown speaker, which would distort all the time....cabs just don't 'warm up' like a tube amp does. are all your tubes glowing the same, or is one dimmer than the others?
Oct 31, 2006 12:46 pm well if there was a tear in the speaker cone that progressivly got bigger
Oct 31, 2006 12:50 pm Theoretically, there could be lots of spots that are getting too hot, and starting to open. Solder joints can weaken from years of hot / cold, from playing / not playing. Then they start increasing in resistance, which can affect the characteristics of the amp, giving unpredictable results.
You sound set in that you're replacing the tubes. Hopefully that's it.
I'd try a little troubleshooting if it doesn't get fixed by tubes. When you're playing, if it gets to the point of sounding bad again, point a fan of some sort to the amp, to see if it cools it down, and makes it improve. This will tell you if it's a heat issue.
From there, if it's heat related, re-flowing solder joints isn't a bad idea.
After that, physical inspections of caps, and resistors, and replacing anything that looks iffy. Sometimes the ends of caps and/or resistors can discolor, indicating that oxidization and/or degradation has taken place.
I had a bunch of this going on in my super reverb when I got it. It was full of iffy spots.
!BE Warned! Lethal High Voltages live inside your amp. Don't poke around in the guts unless you know how to get rid of the high danger juice. The Voltage lives on even when the amp is off, and unplugged, so go smartly, if you go.
Oct 31, 2006 03:14 pm okay cool. thanks guys. I haven't had it on since (got a bout with tendinitis :() I'm taking the head to my tech this weekend so I can have him check out the caps, joints and such. I might try to test it before I go to see if it happens again.
I never looked to see if the tubes were glowing together or not. Good call. Have an old computer fan attached to the rear grill to pull out the heat. (Though my lazy *** never turned it on.) Cab is close to new and has less than 30 hours of use.
I'm thinking it's a heat issue if it sounds fine in the first hour or so. It was hot in the room we were playing and tight quarters, might not have had good room air circulation.
I'll start with the tubes and have him check the other things. Hopefully that's the culprit.
Keep you posted
Nov 04, 2006 11:42 am Another possibility,'leads'I had a mystery problem with a Marshall head after it working for an hour.It was driving me crazy so I took it to the Marshall factory. After a long test they said its ok try chucking all your leads away and buy good new ones, if that doesn't fix it at least you'll have a new set. I reluctantly did this and it cured the problem. Doh!
Nov 08, 2006 03:06 pm I turned it on and left it running for a good 15 minutes to check the tubes. Both glow similar. I'm going to have the tech check the caps and such when my bias tool comes in but it sounds to me like it may have been a heat issue.
Jambo, what do you mean by 'leads'?
Nov 09, 2006 09:29 am not the chords. everything is new and I bought some George L's that improved the connection quality. Like I said worked fine the week before. I still think it was a heat issue
Nov 09, 2006 10:37 am Also check the speaker cable,but yes overheating looks most likely cause.If there is a fault in the amp it will happen every gig.Try reducing the input signal and increasing the master/main volume this should help it to run cooler.
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