Unwanted distortion in amp; possible preamp tube problem?

Whoopysnorp

New member
I play a Marshall AVT150 head into a Peavey 2x12 cabinet. I like the amp a lot, but lately I'm noticing a problem with my clean tone. One of the things I liked about it when I first got it was the smooth and rich clean tone. Starting about a month ago, though, I noticed that the clean channel would distort when I play chords with a strong attack using the neck pickup of my guitar (it's a humbucker). This is not a pleasant-sounding distortion, either. It is only audible on the attack of the chord (single notes don't seem to push it hard enough to cause the problem). Using both pickups, the problem is almost not there, but it's there. Haven't noticed the problem with the bridge pickup. Turning down the gain on that channel does not help at all, oddly enough. I think it may be worsening. This is a hybrid tube/solid state amp; it is possible that the preamp tube needs to be replaced, or do I have a deeper problem?
 
It's possibly the tube.....it's also possibly the p/u and if it's a hybrid where the pre-amp and power amp sections are connected by those flat, ribbon, computer type cables.......you could have fractured solder joints at the ends of those cables.

I've had that happen to every damned amp I've ever had that had those type cables.
My Ampeg re-issues have done it and my Hot Rod deVille does it about once every 7 or 8 months.
You just have to reheat the solder joints if that's it although on my Fender, it's so hard to get to the joints that I've quit fixing it. I'm gonna yank them out altogether and replace them with individual wires. If that's what it is.....the reason it would do it only on the neck p/u is cause there is more bass to make it vibrate.
 
Well, I ran the guitar in question through an ART Tube MP that I have set up going directly into my Delta 66 for when I want to record bass, and I couldn't get it to distort, no matter how hard I played. I think I can rule out the guitar as the cause. I opened it up, and the preamp and power amp don't seem to be connected by those ribbon cables you mentioned. I've only had the thing since February, so it seems weird that the tube would be bad already, but who knows. Maybe I'll email Marshall about it.
 
Well.....tubes can just be bad. Kinda like a guitar string can break the first time you play it. Also.....one thing that I see lots of git players do.....is move the amp and jostle it while the tubes are still hot. I always turn my amp off and let it cool while I tear down everything else so when I do set it down on the floor or the bed of a truck or vibrate it on a dolly the tube is cool. If you think about it.....when you thump a light bulb while it's on, that's when it'll blow and in aircraft accidents, one of the ways they tell if certain instrument panel lights were on or not is if the filament is broken. When it's hot, it's real sensitive to vibration. So I'd check the tube. Who knows, maybe it was just a bad tube.
 
Right on, Lt. Bob. Is there any way to visually detect whether a tube is bad, or should I just get a new one and stick it in and see what happens? 12AX7s aren't that expensive, are they? Also, will I need to worry about rebiasing or anything?
 
No they're not expensive.....maybe $10.
I don't think you can tell from looking at it.
You do not have to even think about rebiasing. That's only for power tubes.
One last possibility..........could have dirty contacts in the tube socket. I think that's pretty unlikely though if the amps not that old. Probablly the tube. If you got it used though.....it might be the socket. You can plug and unplug the tube several times (be careful...it's easy to bend the pins on a 12AX7) or go to Radio Shack and get a little cleaner brush.
 
Thanks for all the help, Lt. Bob. I got it new, so I doubt it's dirt in there. One more thing I thought of, though: I have run the amp a few times without a speaker connected. I did it when I first got it because I didn't have a cab yet, and I ran the line out into my mixer and from there into a couple of speakers. Recently I used the line out to record a somewhat difficult piece that required a lot of takes. Both these times, I was careful to turn the master volume all the way down. I know with all-tube amps it's strictly forbidden to run the amp in this way, but how about if the power stage is solid state? I'd hate to have caused the damaged by my own flagrant disregard of the rules.

Anyway, I'll pop by the shop and see if I can pick up a new 12AX7.
 
Interesting that you bring that up.
I asked a tech just the other day about running solid-state amps without a speaker. He said that, in general, they won't be harmed by that but, you should always check with the manufacturer to be sure. So probably you didn't hurt it but what the hell do I know.
I, personally, never run any amp without a speaker but I'm a geezer from the days when all amps were tubed and, as you said, you cannot ever run tube amps without a load. So I'm sure I'm just biased in that way.
 
Are you getting any kind of a "hissing" noise when playing? Thats what was happening to mine when my preamp tube was going bad. I have the same type of amp, solid state marshall with a tube preamp. If you like your sound make sure you get the same brand preamp tube.
 
Lt. Bob said:
So I'm sure I'm just biased in that way.

LOL BIASED CUZ ITS TUBES AM I RITE?

Anyway, the manual says not to do it, but that might just be to cover their ass. I'm not getting a hissing noise when playing, but it certainly sounds like the tube could be my problem. So I'll replace it and see what happens.
 
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