Crackling, splattering in amp...

[Bob M.]

Hi,

I have a SF Princeton Reverb that I'm fixing/updating for a friend who just bought this amp. This amp sounds good but when I hit/pick a guitar note really hard (make a big transient) I hear a crackling, splattering distortion noise in back of the note. At first I thought it might be a speaker distortion so I tried the amp with several different speakers and it does the same with all. This problem isn't note-dependent, it does it on all notes but is most noticeable with notes in the mid or low registers. Of course, I tried different guitar cables, different guitars, different speaker cords, different power tubes, different preamp tubes with no change in this problem. It is more noticeable when the amp's volume is 4 or above, where the amp likes to be used normally. Before I start changing out bigger things like filter caps and transformers, I'd though I'd ask if anyone has encountered anything similar to this problem and if a definite solution was found.

Thanks for your time and expertise,

Bob M.

[52 Bill]

Turn down the reverb control and play your guitar through the amp. While the signal is going through the amp give the top of the case a good solid Enzo Whack with your hand. Any reaction from the amp?

A bad tube or a broken solder joint will cause the type of noise that you are describing.

[Bob M.]

That didn't do it for this particular problem. I've changed out all the tubes (on set #3 of full tube compliment). All new electrolytics, new plate load resistors. Tested with reverb on, reverb off, with reverb tank unhooked up and the result is the same. I've been over the circuit board with a magnifier and spot soldered many eyelets. It's possible I have an unseen bad solder joint but I'm doubting that. I'm suspecting some sort of capacitor problem but I'm not really sure.

Thanks for your insight,

Bob M.

[Jazz P Bass]

You may be at the level where the amp itself is distorting.

If it is only the bass notes causing the problem, I would look at:
A: lower the first tubes bypass cap.
B: try twiddling the value of the feedback resistor.

[Enzo]

Bob, when he suggested the Enzo whack and turn down the reverb, he wasn't suggesting the reverb was involved. We turn the reverb down so it doesn't make a huge noise when we whack the amp. I am not sure if you did that test of not, but just visually inspecting joints is no guarantee they are OK. We are looking for some possible connection issue. Connections are not limited to the solder joints. The wire leads on resistors and capacitors go inside and connect to the guts of the part. Intermittant component leads can be the loose connection, hence the whack test. or use a rubber mallet.

If you have a reason to suspect a cap, then go through the amp looking for DC voltages offset from where they ought to be.

If you turn the volume down, does the noise stay about the same loudness or does it turn down with the music? If you turn the treble control up and down, does that affect the TONE of the noise? If so, then the noise is in the first stage.

Try this, unplug the reverb pan and inject a signal into the reverb return jack on the amp. Use the reverb control as a volume. Doing this bypasses most of the preamp. The preamp controls will not function that way, but we want to know if the noise is still present or is it now gone? If the noise is no longer there, then the problem is in the first tube - the first two stages of the preamp. If the problem remains, that means it is in the power amp or the last preamp stage.

This is classic troubleshooting - isolating the problem.

You can also use the trace and inject method. Make a signal tracer and move it stage by stage through the amp listening. We want to know what stage you first hear the noise. The opposite way is to inject some signal into the amp stage by stage to see where along the circuit the problem appears.

[Mick Bailey]

I've just has the same symptoms with an amp, though similar faults can be caused by entirely different reasons. In my case it was a bad coupling cap solder joint off the second preamp stage. Played fine until you dug in and then it distorted like a bad speaker. Couldn't get a result from thumping the cab, but chopsticking showed up the problem. Reflowing the joint cured it.

You can get this type of problem anywhere there's a large enough voltage swing when a note is played hard. Sometimes it can be caused by internal component failure where there's marginal contact.