NASTY distortion from my Ashdown ABM 500. Help please!

[Faderoktopus]

Hi guys, I'm having some trouble with an unpleasant distortion from my rig. I have a preamp/pedal setup that goes DI to a weak little PA and 1/4 line out to my Ashdown ABM 500 EVO II feeding an SVT 8x10 pro cab.

I have been working on taming my G&L 1500's rather hot pickups so I can basically bring my pedals to any gig and feed my favorite sound into any cr@ppy stage amps as well as the FOH without unwanted overdrive. This is working well, and at rehearsals I am not even clipping our rather weak behringer mixer or overdriving the monitors. Most stage amps respond well to my setup, too. However, my own rig has a nasty tendency to distort at what should be acceptable volumes. I really don't know why.

I am fairly certain it is not the bass or my pedals, but just to give as much info as possible, here is a description of my complete setup:

G&L 1500, passive, series -

Stageclix wireless and Boss TU2, I haven't encountered any problems with these units yet -

MXR 10 band EQ, I boost the low mids a bit and scoop a bit around 2k, nice curve, nothing drastic. I lower the output from this unit about 3-6db after EQ'ing; G&L's are hot even when passive -

Mad Professor Compressor, volume at noon (slightly above unity gain), comp. at 9:00, tone select off. Switched on comp., not sustain -

Aguilar tone hammer, slight boost to bass and treble, mids and master at noon, gain at about 10:00

XLR to mixer. No problems there, though admittedly the Behringer doesn't let a lot of lows through to the otherwise decent monitors -

1/4 to amp. I pretty much just use the ABM as a power amp, letting the pedals do the talking, so it's EQ is set flat, valve drive off, octaver very much off, no active input cut. Input volume about 10:00, output about 11:00, sometimes lower.

1/4 speaker jack to cab

This is not exactly a low volume setting, I know, but a 500w amp and an 8X10 should be able to play reasonably loud and keep a good tone. The distortion seems to be around 2k-3k Hz, but I'm not sure. I really hope it's not the drivers, but IME the SVT's can usually take a lot of abuse.

I think that's all the info I can come up with right now, I know it's a lot, but if anyone has any inputs on how to sort, or even just diagnose this problem, I would be very grateful.

[rodl2005]

have U tried going IN from a pre like a tone hammer to the FX return????

- that way U can tell if it's the power amp &/or then it might let U know it's tube or something else in the pre amp that's needing attention

[Faderoktopus]

No I haven't, will this mean that I completely bypass the ABM's preamp?
I'll definitely try it. Thanks

[rodl2005]

Let us know what happens

[Rune Bivrin]

I would start by plugging the bass straight into the amp. Nothing inbetween except a cable. Work your way backwards from that, adding stuff until the problem reappears.

The biggest mistake in troubleshooting is starting with the asumption that "I know unit X is OK." That invaribly leads to unit X being the culprit, something which you only realize after a long and painful hunt.

[Faderoktopus]

Originally Posted by Rune Bivrin

The biggest mistake in troubleshooting is starting with the asumption that "I know unit X is OK." That invaribly leads to unit X being the culprit, something which you only realize after a long and painful hunt.

This is solid advice, no doubt, and it's the reason I posted my whole signal chain. I really can't see how the bass or pedals could produce that kind of distortion and not send it to the PA, unless it comes from an overload of low frequencies that the PA simply doesn't reproduce. Anyhow, it'll probably be worth the effort just to eliminate some suspects. Thanks.

[rodl2005]

DO the FX return thang. It's MOST Likely just a pre amp tube gone futsie. A CHEAP n E-Z fix.