Ampeg SVT-3 Pro (Weird Hum Problem)

Nathan Warwick · #1 07-03-2011, 03:53 PM

I couldn't find any answers on here for my problem, so I'm hoping someone can help... As a note, the amp is a USA model, purchased used (No Ampeg Warranty) and it has been looked at locally with no help. What is happening is I'm getting a 60hz hum from the amp. It is audible only through the cabinet at about 5 feet up to the cabinet. I have tried multiple cabinets/power sources with the same result. The amp does have 3 JJ tubes (2 12ax7 and 1 ua7) with the original other 2 12ax7's. This is where it gets a little weird. The hum only happens when any sort of instrument cable is plugged in (Have tried multiple cables and it doesn't matter whether a bass is plugged into it or not) The really weird thing is that the hum will go away only if the master volume is around 70-80% - any position before that or after that and the hum returns. The hum also goes away when a cable is plugged into the Power Amp In on the back. Both output plugs have this hum. Any help that can be provided would be great. (As a side note, I also need a few knobs and the frequency selector switch but have had no luck locating any - if anyone can point me in a direction for those as well it would be great) Thanks!!!

-Nathan

**B-string** · #2 07-03-2011, 04:21 PM

The DIY? Buy one 12AX7 and a 12AU7 and swap one tube at a time to eliminate a tube concern. Beyond that it could be a grounding issue at the jack all the way up to the last stage in the preamp, to a faulty cap in the PS to the preamp.

Nathan Warwick · #3 07-04-2011, 09:30 AM

I'll try out swapping the tubes out one at a time - I have extra tubes that I know are good. Any other ideas?

BassmanPaul · #4 07-04-2011, 09:59 AM

The hum is being generated in the pre-amp - plugging into the power amp in disconnects the pre. Do as Mr B suggests, heater cathode leakage can cause all sorts of weird and wonderful effects. Let us know the results.

Nathan Warwick · #5 07-04-2011, 10:02 AM

Thanks BassmanPaul, that is what I was trying to find out online and was having no luck. I appreciate it. I'll test here in the next few hours and let you all know. Also, having mixed brands of tubes JJ's/Sovtek's - Can that cause it as well. If so, I'll swap the other two factory tubes over to JJ's.

**B-string** · #6 07-04-2011, 10:57 AM

Mixing tube brands won't cause it. Many mix tube brands for different effects.

Nathan Warwick · #7 07-04-2011, 12:33 PM

Okay, swapped out all the 12AX7's and still no luck - hum is still present, but this time the master volume doesn't seem to stop it. The only tube I haven't swapped out is the au7, but that tube is literally 3 days old. I do have the au7 in socket: J12 - its the once closest to the front of the amp of the back two..... Any other ideas for testing?

Nathan Warwick · #8 07-04-2011, 12:37 PM

As an FYI - the mute and the -15db pad are also making a popping when pressing them... don't know if that helps at all.

**B-string** · #9 07-04-2011, 12:44 PM

Three days old or three minutes old, new tubes can fail. If you don't have a spare 12AU7 you can sub a 12AX7 for testing (the tone will suck though).

Nathan Warwick · #10 07-04-2011, 12:52 PM

Thanks, tried that and no luck. Still hummy... But still dead quite in the power amp in section even with my bass plugged in, so the problem is def in the pre-amp. Are there any tubes I can completely remove and then test to maybe help isolate? I'm swapping tubes with my Fender GT out of my HotRod Deluxe. I know those tubes work absolutely fine.

Nathan Warwick · #11 07-04-2011, 12:53 PM

I really do appreciate your help in this!

Nathan Warwick · #12 07-04-2011, 01:25 PM

Okay, I was playing with the input plug and if I wiggle it to the right and hold it, the hum goes away. It comes back when I release. This happens with several cables so I'm guessing its a grounding issue with the input switch?

BassmanPaul · #13 07-04-2011, 01:28 PM

That would do it. Tech time!

Nathan Warwick · #14 07-04-2011, 01:37 PM

I'm pretty handy with things, but I'm not sure how to fix it? I can pull the board and check for cold solders, but that's about all I know how to do. Is there something special I need to look for?

GizmoTheOne · #15 07-04-2011, 01:38 PM

Quote:

Originally Posted by BassmanPaul

That would do it. Tech time!

+10. Take it to a tech and tell him what you suspect (something wrong with input jack, probably grounding issue). He'll fix it.

Nathan Warwick · #16 07-04-2011, 01:41 PM

Good deal. Thanks for all the help guys!

**B-string** · #17 07-04-2011, 01:45 PM

Just reflow the solder on the jack, fault my not be visible. Be careful as the power supply may still hold a nasty charge. See BassmanPaul above!

Nathan Warwick · #18 07-04-2011, 11:04 PM

Reflowed the input jack tonight and all is working great. Dead quiet, just the way I like it. Thanks for all the help, saved me tons of money by not having it looked at again. It's always the simplest things....

-Nathan

**B-string** · #19 07-04-2011, 11:07 PM

Awesome!