RFI or what?

GizmoTheOne · #1 10-14-2011, 03:49 PM

Hello guys,

I've been struggling for a while trying to find the source of my problem, and can't seem to put my finger on it.

The problem is that there is a weird distortion coming out of my speakers when I play. BTW I know it is not a speaker problem since I have tried on many speaker sets and it's always the same.

It basically sounds like if I was clipping my amp, but it does so even when gain and volume are at very low settings (before half).

I have noticed that there is a little bit of background noise coming from the amp, which sounds like and seems to be RFI or something like that. The noise is there even when the bass is not plugged in.

When I plug the bass in, the noise seems to go a little bit louder when I hit a note. When I play harder, distortion comes in. I was wondering if it was about the same issue (RFI going louder and sounding like distortion on peaks)... What do you guys think?

johnk_10 · #2 10-14-2011, 03:55 PM

it might help if you told us what type of amp it is.

GizmoTheOne · #3 10-14-2011, 06:27 PM

Oops, sorry, forgot that. Its a bass amp, an Ampeg SVT 3 Pro

rodl2005 · #4 10-14-2011, 06:34 PM

How old is the 3 pro?
Could be a tube? I know these pre amp tubes last for ages, but......

**B-string** · #5 10-14-2011, 07:52 PM

RFI not likely. Bad preamp tube, faulty ground, dirty effects return jack.....
Is the noise present a hiss or hum?

GizmoTheOne · #6 10-14-2011, 08:14 PM

Thanks for your input. The noise is more like a hiss... And I don't think it's a tube since the DI is working perfectly, no noise. It seems to be coming from the power section.

**B-string** · #7 10-14-2011, 09:23 PM

I believe there are 5 tubes in the 3 Pro. DI is before any tubes IIRC.

rickdog · #8 10-14-2011, 09:39 PM

Quote:

Originally Posted by GizmoTheOne

Oops, sorry, forgot that. Its a bass amp, an Ampeg SVT 3 Pro

As soon as I read the problem description, I was pretty certain that's what you'd say.

The solid-state output stage of the SVT-3Pro has a bias adjustment. If that's not set correctly (too little idle current), it can cause pretty much the exact symptoms you described.

You might want to check out the advice in SVT3Pro Bias Setting. Jerrold Tiers is pretty much The Man when it comes to Ampeg tech info.

If you decide to adjust the bias yourself, heed the advice about how touchy the adjustment is!

96tbird · #9 10-14-2011, 11:06 PM

One thing to check. Today is was getting weird hissing noises out of my v4b. It changed in pitch and frequency, crackled and sputtered a bit. I could not figure it out. I had just rolled in a complete new array of 50's and 60's NOS blacklate pre tubes the other day, so I thought one may be bad. It was distorting the notes and I could hear the hiss with the notes; it seemed to be louder with the notes. Then the calendar alarm on my smart phone rang and it was in my pocket. I took it out and put it on the table. Hiss gone, just the normal sweet hiss of the tubes that cannot be heard when notes are struck. This may not be your situation, but we all have cellphones now. Pickups pick network traffic rfi when cells are a few inches away. Something to keep in mind. YMMV.

johnk_10 · #10 10-15-2011, 02:52 AM

from the op's very first post i was thinking that the power amp's bias needs adjustment.

NKBassman · #11 10-15-2011, 07:50 AM

I was having the exact same problem with my 3PRO earlier this week. It's at the tech right now, so I'll let you know if/when he finds the problem.

GizmoTheOne · #12 10-15-2011, 10:05 AM

Thanks guys but the bias has already been adjusted by a good tech.
NK, I'll wait to hear from you.

NKBassman · #13 10-17-2011, 01:06 PM

Sorry, tech said he couldn't find anything wrong except a few preamp tubes that tested as "tired". Poweramp bias was ok, and he couldn't find anything else wrong with it whatsoever. We are replacing all the preamp tubes and cleaning a few connections, so I'll let you know if the problem persists when I have it back.

GizmoTheOne · #14 10-17-2011, 02:27 PM

Thank you NK, I believe my problem could lie in either a bad driver tube or a dirty connection.. Otherwise I really have no clue what it could be. Keep me informed please.

GizmoTheOne · #15 10-19-2011, 01:29 PM

Hello guys,

Upon further investigation, the issue is in the power amp section. Preamp out is clean, Power amp in is distorted. The resistors all seem pretty clean looking, everything seems to be in good working order. I am currently trying to swap the driver tubes with spare ones... I guess if that doesn't do it, it might be time for a bias adjustment, right?

johnk_10 · #16 10-19-2011, 02:26 PM

IMO, it was most likely the bias adjustment in the first place.

GizmoTheOne · #17 10-19-2011, 04:17 PM

That's what I thought, but since the bias had already been adjusted I thought it could have been something else.

1958Bassman · #18 10-19-2011, 05:09 PM

Hum is usually caused by the bias being mis-adjusted, although if it's balanced AND too high, it can increase noise. Hiss is usually caused by too much gain or noisy resistors. If the master volume is turned up too high and the other volume control is low, you'll hear hiss. If the master is backed off and the other volume control is turned up a bit, but not enough to cause clipping, it should remain quiet. If the treble control is goosed, you'll have more hiss, too.

Crackling can be bad connections, resistors, tubes or caps.

**B-string** · #19 10-19-2011, 07:24 PM

Quote:

Originally Posted by GizmoTheOne

That's what I thought, but since the bias had already been adjusted I thought it could have been something else.

Doesn't mean it was done correctly!

svtb15 · #20 10-19-2011, 08:03 PM

MOSFETs may need RE BIAS.. there are a few threads about that here.. but the search isn't too helpful..
I had the same issue with my old SVT3 pro.. rebiased the FETS and it was OK... Very common issue with the SVT3 pro .. at least from my experience..

If not done correctly it can SMOKE your FETs or at the other extreme , it will make the amp hum and sound real gritty and jagged