Ampeg SVT-II Loud Hum

[spforster]

Hello all! I've got an Ampeg SVT-II all tube bass amp that has a terribly loud hum at any volume settings. I've done a bit of poking and prodding around and have come up with the following observations:

1) I've found a signal with an odd looking waveform, which seems to correspond with the hum, present on the plate off the first gain stage into the phase splitter, and also on the grid, plate, and cathode of the phase splitter (cumulatively, V1 in the power amp section)

2) When I pull this tube, the hum/signal goes away

3) I've tried replacing, one-by-one, all of the caps around this circuit to no avail, and have measured good resistance values on the resistors as well

Any thoughts, or need for clarification? Thanks!
Best,

Shane

[txstrat]

Have you replaced the tube with a known good one?

[spforster]

Oops, I should have mentioned that! Yes, I did swap the 12ax7 for a good one. I also swapped out both 12au7s. I had the idea that it might be a bad filter cap, so I tried trying a new one in parallel with each filter cap, with no change one the hum.

[doctorfate]

I have a hum problem I can't figure out for the life of me too , but with a B15N('62 ) which I almost completely (re)built. I'm not sure if this is a dual channel in your case but if so does the setting of either channel's volume control have any effect on the hum? at least then you know the hum lies in those stages.

[dlplatt]

Did anybody a solution for this problem? I have the same issue in my SVT-2 Pro. Thanks!

[Zouto]

Did anybody find a solution for this problem meanwhile? I got the same troubles in a late SVT-2 Pro. Sounds like some unbalanced heater noise. Thanks!

[soundguruman]

Where you have ANY input output jack on this amplifier...input jacks, send return jacks, preamp out jacks, etc...

The jacks have internal switches. The switches must operate.
If the switch does not operate, it will cause all kinds of HUM and noise.

It doesn't matter if you are using the jack, or not. The switches on the jacks MUST be 100% GOOD.

The jacks corrode and the switch fails = hum.
The jacks have cracked solder connections = hum.
The jack is bent or broken = hum.

Cleaning the jacks does not always solve the problem. They must often be replaced.

Use CLIFF UK jacks. They fit right onto the board.
The Chinese jacks don't last, and the metal corrodes VERY fast.

What you are describing sounds like a defective input jack. resolder it, clean it, replace it.

[bob p]

I have a pair of 2-Pro that are dead quiet when properly balanced.

I'm quite surprised that nobody has asked the OP the most important question:

What is the frequency of the "hum" that you're referring to?

Technical types use terms like hum, buzz, and noise to refer to different sounds. Many people will use the term "hum" to refer to a number of different frequencies, so it's important to clarify what you're telling us.

When I hear someone say "hum" I think 50-60 Hz hum from the PSU. When I hear someone say "buzz" I think of rectified power supply buzz at 100-120 Hz. I don't apply those two terms to anything other than 60 or 120 cycle noise. (from a 60 Hz USA perspective, of course).

So are you hearing hum, buzz, or some other type of periodic noise? Or non-periodic noise?

[nevetslab]

I service a lot of the Ampeg SVT-CL's, AV's VR's, and the SVT2-Pro bass amps, as there's a good collection of them in our Backline Gear Rental Dept. I had one SVT2-Pro in the shop early on, and found the one Grounding Standoff that supports the back side of the large Power Amp motherboard wasn't getting proper continuity to the chassis. While I had that PCB out of the chassis (lots of fun in that), I removed all three standoff's, found no exposed metal at all under the one standoff that carries the ground reference to the chassis....and was NOT reading low resistance. With a carbide burr installed in the Foredom hand grinder, I ground away the paint to bare metal at that location, did the same on the ends of the painted standoffs, and while at it, carved away the paint on the inside of the chassis where the bias pots are located to give them ground continuity.

Then, I had solid ground on the top of the PCB mounting standoffs....there's one at the corner that has to be tight mechanically, as IT is your ground path to the chassis. When I re-installed the boards and re-connected all the harnessing, and finally powered it up, the hum was gone, as was some RF interferrence that was also present! No more vibration sensitivity anywhere on the amp, where before, you'd scare the dickens out of yourself banging on the amp's chassis from the explosive noises that sometimes came out of the speaker.

I've also found that hardware loose (long term cause from the power transformer's mounting hardware working loose from their sheer weight and mass), and have only had to tighten down both sides of that one grounding stud. I think the one that I had to grind off the paint didn't have the Avery label 'DOT' in place when the factory painted the chassis. Paint does a good job as an insulator sometimes.

A side note on SVT-CL's....I absolutely hate Ampeg's use of clear vinyl 'garden hose' material to protect their wiring between amp chassis and preamp chassis. I make it a point to replace them with Tech Flex, so you can easily position the preamp & power amp on the bench without that bloody garden hose dictating where it wants your preamp to be!