DB751 high frequency noise & ticks

[DriesG]

Hey guys,

After my latest gig with my DB751, I get a lot more high frequency noise (it's not hiss, I guess...) and get ticks (tk tk tk) when I turn the gain or master knobs. I've never pushed the amp above 60% on the master volume, so I'm not sure what happened...
I've tried my DB112's with my MB LMII, and I haven't got this problem with this amp. Is it possible that it comes from bad current in my house?
I used the DI output on the gig. Is it possible that the amp got damaged by phantom power coming from the mixing desk? My amp is like brand new, so I'm a bit worried. Hope someone can help me with this. I've contacted Aguilar, but I'm still waiting on a reply. Need some expert advice... thx

[Jeff Scott]

Always suspect suspect tubes first.

The DI should be protected from any harm due to phantom power being applied to it, at least, this is how it is on my Tone Hammer 500.

[craig.p]

The problem really could be anything. But I'd recommend going through the usual troubleshooting steps: changing instrument and speaker cables, connecting the effects send and return jacks with an instrument cable, running all the controls all the way up and down a few times (with the amp off, obviously) to clear any possible contaminants, trying a different outlet in a different part of the house (despite the LM's success on the same outlet), and trying just one cab. I agree with the tube theory. Reseat them and see what happens, or swap them around if they're identical. Regarding the DI, you won't know whether the circuitry behind it is isolated from phantom power until you see a supporting statement in the manual to that effect, or a schematic that proves it. If you don't know for certain, then you should always, always, ALWAYS assume that the XLR cable a sound man hands you has or will have +48V on it, and be prepared to isolate your amp from that voltage, either by means of a short F-M XLR patch cable that interrupts pin 1 and shield (i.e. passes only pins 2 and 3), or by some other method such as an isolation transformer. Many of the lower-priced and even some mid-priced boards use global phantom power that will have to be enabled if there's even one condenser on stage, unless an individual phantom supply is used for that mic. And of course some sound men forget to turn phantom power off when it's not needed. I've also seen cases where global phantom was enabled on a board after all the snake connections were made and tested, even despite its button being recessed.

[JGR]

The ticking is almost certainly a preamp tube.

[DriesG]

Let's hope it 's only the tube... Thanks guys