Help! Ampeg SVT CL Phantom noise

[mike 54]

1999 SVT CL A quiet but annoying crackling noise that I have trouble trying to repeat. I've changed out all tubes power and pre-amp( done proffesionaly). I've ruled out the speakers,cables,and instrument. The only constant is yeah you got it, ME. But I have witnesses. It will play for a while and start this intermittent crackling, scratchy noise that is barely audible. Dosen't matter what volume I play but it seems to respond often to an open A string. Comes and goes and sometimes never happens. I turn all the pots and switches and nothing affects this sound. The tech checked it and found nothing wrong but I haven't duplicated this sound for him yet. Can anyone please help or guide me in the right direction? Thank's Mike

[tim]

It could be any number of things. Those amps are pretty basic as far as amps go but after I get done with one of those (or the original SVT's) I always feel like I just rebuilt a 454 V8. One of the things that can cause crackly noises is noisy plate load resistors in the preamp (a common problem in older amps but I've also seen it in brand new amps as well). Also cold/fractured solder joints can definitely cause noises on the output. Usually those are found under high use parts like input jacks, speaker jacks, etc. Also they will appear under parts that get hot like doping resistors, regulators, output transistors/fets, etc etc.

[mac1amps]

I've found it necessary to go over factory soldering most of the time with virtually all "mass-produced" P.C. board based amps made in the last 20-years (sound's eccentric, I know, but it's found a number of hard-to-find problems for me on many occassions, ESPECIALLY with respect to Bass amps). Another problem that occurrs quite frequently is the shorting contacts on the patching jacks (most noteably, the "return" jacks of effects loops and/or "power amp in" jacks) where the unit relies on those contacts in order to properly pass signal when those facilities are NOT in use. I've known alot of techs to replace the jacks, when, in my experience, a decent cleaner AND contact "enhancer" (like Deoxit D5 for instance), will alleviate the problem 98% of the time.

[Amp Kat]

I agree with you Mac 1 on the effects jacks but that usually causes drop of volume problems. I'm thinking that volume and gain pots are intermittently putting DC on the signal. Try cranking the amp a hair and turning those pots to see if you can recreate the problem. The first preamp section is in the front where 2 12ax7's are. Make sure all of the screws are in tact as that amp vibrates so much it will loosen screws and also check the transformer screws and tighten them if they are loose as I've seen them back out and fall on the main board. I use lock-tite and silicon them in.Check that green cable running thru the clear plastic and make sure it's tightened up good and make sure the two 12AX7's are in good. The main board is a bit more tricky to get to and you have to take the cables out of the front preamp board to get it out but there are 10 ohm resistors that can open up in there and cause some crackles. Make sure the (3) 12a?7's are pushed in good on that main board and you may want to even swap them if possible. Bias shouldn't be an issue as the lights tell the story if working correctly but I use flourescent lights to get the hum just right keeping the led's barely in the red at idle. From what your telling me I truely believe you have an intermittent volume or gain pot acting up. Could also be the standby switch as it takes a good bit of abuse. Be damn carefull in there because theres close to 700 volts in this amp on those 6550's

[Shellac]

I had a very similar problem with my SVT CL made in 2002/2003. The unit was producing a 'white noise' distortion, loud pops and crackling whilst playing. These sporadic noises would appear 15/20mins into playing and were VERY noticeable at gig volume. The unit also seemed to lack the power and punch of the old days. I also noticed that the front panel was very micro-phonic when altering the controls and using the switches. Initially I put this down to aging pre-amp tubes in the upper chassis; and the loss of power down to aging power amp tubes.

I replaced both 12AX7's in the upper chassis and fitted 6 new 6550's. The noises continued. I then replaced the 3 driver tubes. The noises continued. I handed the unit over to a tech at this point and he found multiple dry solder joints in the upper chassis and re-soldered the lot. Now the unit sounds rich and powerful again, producing a tight bass sound with plenty of head-room. I admit that this was an expensive route to fixing the issue, but I felt it necessary to rule out the ancient valves before bothering the tech.

Many thanks to 'mac1amps' for posting "I've found it necessary to go over factory soldering most of the time with virtually all "mass-produced" P.C. board based amps..". Which gave me something to aim for when I passed it to the tech. And I also agree with "effects loops and/or 'power amp in' jacks" creating un-wanted noise in these units. Contact cleaner and sliding a jack lead in and out worked for me in the past.

[Jazz P Bass]

Not to be ignored but they must be ruled out: leaking and or microphonic coupling caps.

[Shellac]

Not to be ignored but they must be ruled out: leaking and or microphonic coupling caps.

Thank you Jazz, I will keep that in mind for the next service!

[bob p]

I've never been inside of a CL, but I have been inside of a 1990s SLM-era 2 Pro in the past week. One thing that impressed me about the circuit is that the builders completely avoided the use of CC resistors as mojo-enhancers; the 2 Pro looks like it has only MF or CF resistors throughout all of the boards. This would eliminate CC resistor noise from the differential problem list. I don't know if your CL is built the same way -- it would be interesting to know if the CL-type amps also avoid the use of CC resistors, using MF & CF instead, or if they use CC resistors as some sort of retro-mojo quest for tone.

Assuming that your CL avoids the use of CC resistors, then I'd be looking for carbonization on tube sockets, bad solder joints, lifted traces, bad coupling caps, etc. Lifted traces are especially important if your amp has the tube sockets all mounted directly to PCB instead of mounted on the metal chassis. IME the noval tube sockets are tensioned so tight that it's really hard to remove/insert the tubes, and the PCB undergoes a lot of flexion during a tube change. The amount of PCB flex that goes with a tube change really scares me.