Amp gurus! Intermittent crackling question..

[uncle looie]

I've got a '64 Vibroverb (original, not the reissue) with an intermittent crackling problem. It seems to be related to bassier notes, like lower notes on the three wound strings. Doesn't happen with higher frequency notes. The coupling caps are brand new, filter caps are about five years old, but good (Sprague Atoms). Power tubes and PI tube are brand new (and the amp was doing it before the change). All preamp tubes have been swapped out one at a time, and it still happens. I know it's not a speaker issue because I can unplug the onboard speaker and plug in an extension cab and it still happens. It's also not a reverb issue, it happens on both channels. The output tranny has been replaced and I don't know the manufacturer, but I'm not sure of the replacement trannies quality. As best I can tell, it is a "TF". The numbers on it are "TF-125" followed by what I guess is the serial number- "1052-9542". Could it be an output tranny problem? If not that, then I'm figuring either a resistor or cap somewhere going bad or a loose solder joint somewhere. Any ideas where to look first?

One other thing, it seemed to quit when I plugged a speaker cab into the extension jack and ran the onboard speaker and extension cab, changing the impedance. That's why I was wondering if maybe it was the output tranny?...

 

[wilder]

The last couple of times I ran into this problem it was a solder joint that would melt once the amp heated up. I pilled the chassis and hooked it up to my bench speaker and proceeded to poke every connection with a plastic toothbrush until I found the one that moved/made the crackling noise. I removed the old solder and re-soldered it. Problem solved.

Chris

 

[uncle looie]

That's one thing I was suspecting, but the weird thing is that it went away when I plugged in an extension cab, like instantly after.

 

[Mook]

I've had that problem with:

1) loose solder joint
2) bad power tubes.

Try new 6L6s first, and bias correctly.

Mook

 

[uncle looie]

Originally posted by Mook
I've had that problem with:

1) loose solder joint
2) bad power tubes.

Try new 6L6s first, and bias correctly.

Mook

Could be a bad set of tubes, I suppose, but it's got brand new JJ 6L6's in it, with a new PI tube, and it was doing it before the tube swap, although not as bad as now. That still leaves the loose solder joint as a primary suspect though...

 

[Swarty]

FWIW, unlikely that an amp will get hot enough to melt a solder joint. But a bad solder joint is the most likely culprit. When you hit the bass notes, it is causing the amp to vibrate and causes the wire to vibrate in the loose solder joint...thus crackle. This is why it is fine with the extension cab, the chasis does not vibrate. You may be able to get the amp to crackle by smacking the amp. First place I would check would be the speaker connections. Second place would be the 100ohm resistor that grounds the phase inverter/feedback loop. This is in a spot on the circuit board that is prone to warping and often they come lose.

 

[uncle looie]

It will happen with an extension cab if I disconnect the onboard speaker and run the cab out of the primary speaker jack. It doesn't happen if I run both the primary speaker AND an extension cab at the same time, which changes the impedance. That's what had me thinking "output tranny".... still, it doesn't rule out a loose solder joint, which maybe just wasn't acting up while the extension cab was plugged in too.

 

[Swarty]

It could just be that with the extension speaker plugged in, it takes enough of the load that there is not enough vibration from the amp's speaker to cause the problem. What happens when you wack the amp with your fist?