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90's Ampeg SVT 2 PRO problem

Andybart

Andybart

Nov 29, 2012
49
2
Spokane, Washington
#1
I have been experiencing a problem with my amp making this really obnoxious crackling/fuzz sound. At first I thought it was my old worn out cab so I replaced the speakers in it with brand new ones. Didn't notice the sound after that for a few weeks. Then it started happening again so I did some research and every single thing I read made it seem like it was a bad tube. I bought the unit used so I just replaced all 6 of the power tubes that way I know all of those are good. Seemed better after that. A few more weeks and it is making the noise again. Started fiddling with the knobs. Ran the preamp tubes super hot to see if they might be the culprit. The bad sound definitely gets louder but that's because the amp is louder. I noticed tonight that my midrange and mid/frequency knobs weren't working as they should. If I pushed either one of those knobs to the side and held it there, the noise seemed to stop. I'm guessing it's one or both of those pots. If so, is this something I can replace myself safely? Or does anybody have any other ideas as to what might be going on. Either way the midrange knobs are definitely not working properly and/or shorting out.
 
#2
Try to avoid replacing pots. Usually you can spray them with a cleaner made for pots such as Deoxit and fix them. And it also may not be a pot but a loose connection to the pot. However, I think you might have a bad preamp tube. One of the tubes in there is for the mid control, I believe, maybe V2. Have you tried tapping your tubes lightly with a chopstick or the skinny end of a wooden spoon while it's on and plugged into a cab?
 
#3
I'd try technique, get/buy a 12AX7 of known good quality, pop the top off the amp, starting from left to right, pull one of the tubes and replace the tube in the amp with the new tube, run the amp and see if the noise goes away, repeat for each tube that call for a 12AX7, the 3 small tubes at the back are the splitter & driver tubes, and the driver tubes are a different value, and if memory serves me right the is one different tube in the front row ... don't have a tube layout chart in front of me ,so I'm going from memory
 
#5
@Anderbass, do you mean patch a cord going from the send directly to the receive jack?
@jastacey and JimmyM, I had no idea some of the preamp tubes were for the eq controls. I will try that also. Like I said though the noise definitely stopped when I was pushing the mid and/or frequency selector knob to the side which really leads me to believe it's a connection or the pot. This is my first all tube head though so I'm a newbie to this kind of stuff. I'll try a new 12ax7 and I've also already tried spraying the knobs out but I will give it another shot. I just want to try everything I can myself before I possibly have to take it in and spend a ton of money for something I could've fixed myself. Thanks for the help guys
 

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