Ampeg SVT Classic and noise

[Tumshie]

This thing's pre-amp is so hot it really amplifies anything the bass pick-ups attract, noise wise. Sometimes, it buzzes like a hornet when an instrument is plugged in but often there is a general mains hum going on in the background and this is exacerbated by sending the bass' radio signal from the DI to input 1 normal. Because I'm using radio packs, I figured it would be ok - not ideal, I understand - to lift the ground on the mains plug. This cures all noise. However, I know this is potentially ('scuse pun) dangerous so I'm wondering if you guys can recommend something, maybe an isolation transformer or whatever. The amp head is regularly serviced and PAT tested.

Signal path is: Bass TX -> Radio RX -> DI box -> FOH/1/4" to Ampeg.

[The Dude]

Is there a ground lift on the DI?

[Tumshie]

Lifting ground does nixi. Happens all all basses, irrespective or whether they're on RX or straight in on jack.

[g1]

Happens all all basses, irrespective or whether they're on RX or straight in on jack.

Straight into the SVT without the DI being used at all? Or only if the DI is involved?

[Tumshie]

Straight into the SVT without the DI being used at all? Or only if the DI is involved?

Either way. And, as stated, lifting the ground on the DI solves nothing.

It's not always bad but most of the time there is a hum; sometimes, it's really loud. The amp is relatively quiet when on but nothing plugged in.

[lowell]

It's not always bad but most of the time there is a hum; sometimes, it's really loud. The amp is relatively quiet when on but nothing plugged in.

If this is 100% accurate...it must be the instrument, cord, or EMF. With nothing plugged in...and quiet...the amp seems to be functioning well.

[Enzo]

Plug the guitar in and get the noise. Now turn the guitar volume knob to zero, does that turn the noise off?

[Tumshie]

It may be stuff being picked up by the bass itself, I dunno. I've tried various leads (cords) going to the amp and nixi.

[Enzo]

That is what turning the volume to zero tells you. If it turns down the noise, then the guitar is picking it up.

Guitars are notorious for picking up fluorescent lighting for example.

[Tumshie]

Happens even on outdoor gigs when no fluros around.

Somebody told me that the input stage of those SVTs are really hot. Would changing the pre-amp valve mitigate any of this?

[Enzo]

Fluros was just an example, the test takes only a second and tells us a lot.

The preamp of an SVT is no hotter than any other preamp. Changing the tube would help only if the tube is what is causing the problem.

[drewl]

Is this a problem that just occurred or was it like this all the time?

How about the low gain input jack?