SWR 750X Loses Power on E String

[Rushfan-nLV]

Very strange but here is the history.

Picked up the 750x to run through my Ampeg 8X10. Everything works great for half a dozen practices with the amp in an 8 space rack (with fans) set next to the amp. We purposely did not set the rack on the cabinet to avoid vibration to the amp. We play our first gig and due to space constraints, we set the amp rack on top of the cabinet. All works fine until the last few songs when I switch to my drop D bass and I hear some rumbling, farting sound coming out of the speakers.

Three days after the gig we set up for practice with the amp on top of the cabinet again. As I'm warming up, I play a descending line ending on the E string and as I hit the E, the rig starts the farting rumbling thing again and cuts down to about 1/3 power. ***? I hit the speaker switch, check all connections, check my bass and then turn the speakers back on....same thing. Maybe it's the pedal board. Run straight into the amp...same thing.

We are thinking the problem is a loose component on the circuit board caused by the vibration from the cabinet or maybe a bad connection in the speaker cabinet.

Anyone else have any suggestions regarding where to start running down the problem?

 

[BasSWhite]

I know this is an old post, but if anyone else has this problem...

I have had the exact same thing happen. My 750x would fart when it was on my cab at loud volumes. I noticed it more when in the passive input as opposed to the active/passive input. In other words, with a hotter signal. It was much more noticeable with lower notes than with higher.

I suspect it is rattling the preamp tube.

The reason I say this: I have two 750x, one that works, one that doesn't. I had this problem, and had no idea what was going on. Then I decided to swap tubes between the two amps (different tubes) to see if there was a tone difference. The tube in the dead 750x had one of those silicone anti-microphonic rings on it, so now it was in the working (sometimes farting) amp. It never farted again.

So, either the tube itself had something to do with it (bad tube), or the tube was picking up the vibrations from the cab and doing bad things with it (microphonics). Lower notes are harder hitting for stuff like this, so it explains why the drop D and E's were doing bad things for you (and me). I have a 5 string, and the B string sounded like a slow-motion explosion.

I have heard that some tubes are better for microphonics than others, which may have something to do with my problem going away. That with the anti-microphonics ring did the trick. I highly recommend those rubbery rings for all tubes. Be sure and place them at the top of the tube, not in the middle.

Although I've been told numerous times putting your amp on top of your cab is perfectly fine and will not cause any problems - I recommend not putting your amp on your cab if you have somewhere else to put it. Or put some sort of cushioning material between amp and cab (like a flat seat cushion "stadium seat" type thing, or good thick rubber feet maybe?)