Ampeg SVT4 Pro noise coming through power section, but only present on FX Send jack

[K-Frog]

Is this possibly a tube issue?

A friend of mine called me over to help troubleshoot the nature of the problem he's having. It's making a noise through his speakers; sort of a the sound you heard when trying to use a phone outdoors on a windy day, not too "swooshy", but about that frequency or tone (not real hissy, not real bassy, just air blowing sounds).

It does it with nothing connected. Makes no difference when you do.

From the front panel, the only things that affect it are Master volume and Treble control.

I also unplugged from the cabs and went from the line outs to another combo amp. No noise.

I do get the noise when using the Effects Send as a line out jack to the combo amp.

Seems weird to me that I can find a combination that is quiet, while one is noisy.

It is out of warranty, and may have never had tubes changes.

Anyone experienced anything like this or offer other trouble shooting ideas?

[Flump]

If both the treble and master controls affect it then it's coming form the preamp. Just get some new tubes and go from there. Always the easiest solution, or at least get 1 new tube and start swapping.

[K-Frog]

The gain didn't have any effect, wouldn't you think that would amplify the noise if it's in the tubes?

It still may be the easiest and cheapest next step though.

One thing I haven't tried yet, because i didn't have anything to use, was to try and bypass the preamp, using the head as a power amp only. That would further confirm that it's a preamp issue correct?

[B-string]

Use the effects return fed by a separate preamp to eliminate the 4 Pro's pre. The tubes have two sections in a single bottle and each serve different functions in different places in the signal chain.
PM hodgy he works at the Heritage plant and knows the 3 pro and 4 Pro real well.

[K-Frog]

ah, see i know very little about tube operation. That makes sense.

I'll PM him

[B-string]

He is a very nice guy and very helpful (Employed at LOUD/Ampeg)

[JimmyM]

Does sound like it could be a microphonic pre tube.

[K-Frog]

It seemed to be the tubes. I re-tubed both of his SVT4Pro heads tonight and it cleaned up the one, and the other just got a new set of tubes cause that's how he rolls.

Thanks