SVT Hums

[gibsonlp]

Hi Guys.
I finally got all the parts for my SVT refresh, this includes a power cap kit from fliptops and a fresh set of tubes, all NOS and all checked again by my tech on his hickok.
Additionally - my tech removed the polarity function and relocated the standby function to the polarity switch as per my request (I can't reach the back of the amp with my bass on me)
After the "surgery" the "cracking" or "boiling" noise that I could hear when the amp was idle is gone, my tech said that this noise came from my old capacitors that were leaking (you could see their mess on the chassis).
To make a long story short - there is a really annoying and pretty load hum @ 100hz (our AC runs at 50Hz) as soon as the amp goes out of standby, my tech said that he thinks it's a ground loop due to different potentials between the preamp section and the power amp section. He removed the 12DW7 tube from the power amp section (effectively isolating the power from the pre) and took it off standby. The hum was gone so it shouldn't be anything in the power section but rather a signal that is being amplified from the preamp section or indeed some sort of a ground loop between the power and the pre, when my tech installed the capacitors he made sure to wire the new ones to the same spots where the old ones were connected to in order to avoid this kind of looping.

Any suggestions? my tech is still looking for it, he is going to check where does the hum begin using a scope.

Thanks!

[winston]

The information contained in this link may be of some assistance Gil.

http://www.lynx.net/~jc/hum.html

[johnallg]

100Hz hum would be the rectified AC not being correctly filtered by the new capacitors. I'd have him look there.

[soundmasterg]

The biasing procedure for SVT's is pretty specific, and if it hasn't been done right, the amp will hum, and can self destruct itself, so even though you think it is in the preamp area rather than the power amp, I would make sure the amp is biased correctly as far as each side being in balance with the other.

Aside from that, check grounding resistances...should be zero ohms or very close to it at every ground point.....check wire routing.....check for filament hum being amplified by a preamp stage.....pull tubes further up in the preamp one at a time and try to isolate which stage stops and starts the problem. Once you know which stage the hum is coming from, then you can troubleshoot it a lot easier because there are only a few parts around that stage to troublshoot.

I don't envy your tech trying to hunt down the problem......SVT's are a PITA to work on, are very heavy, and can be very dangerous too. I have to rebuild my SVT once I get some other projects done, and I am not really looking forward to it....haha.

You may also post some questions on some of the good amp boards around.....Ampage, Hoffman, AX84, Weber...etc.

Greg

[gibsonlp]

Thanks guys.
Biasing an SVT is really easy if you have a good multimeter, both my tech and I have good multimeter (fluke) and the amp is biased correctly.

The hum is almost completely gone.
It turns out that Ampeg had a reason to put the standby toggle at the rear of the head and not in the convenient front. Moving it to the back eliminated the hum almost completely.
The hum I have now is not stronger than the fan and it does not go worse on input 1 when the volume is turned up (it does get worse on input 2 though...)
Not the best thing in the world but it's so much weaker compared to the signal that I can live with it.
Of course, with my luck I noticed something new: Every note I play is having an annoying side-noise that sounds like cracking, very similar to the sound of nylon bag being played with, the side-noise is not audible when I actually hit the string but less than a second into the sustain I can hear this "sqsh-sqsh-sqsh-sqsh" staying on the same level while my original note is fading away, the noise continues for as long as my note is playing.
Any suggestions?

I noticed that the NOS 12dw7 tube (GE) that we installed on the power amp is introducing more hum than the original mullard tube that was there (we a/b tested them), I am going to return all original preamp tubes one by one and see if it goes away.

[gibsonlp]

GOOD NEWS!!!

It turns out that my "scratching sound" was nothing more than the bridge pickup pots that needed some cleaning in form of turning backward and forward a few times!
The amp sounds incredible.
I can't explain how relieved I am, the last thing I wanted was to carry this monster back to the tech, I am not moving it anymore, ever.

[jps]

I am not moving it anymore, ever.

Yeah, right. They all say that at one time or another.

That's why Markbass was created.

[soundmasterg]

Glad to hear the problem has been resolved! I should have caught the standby thing....moving that to the front of the amp would for sure cause a problem, because now your noisy DC is going into the previously quiet and isolated preamp chassis, and then back again. It could possibly work ok if it was located on the front of the power amp chassis, but that would end up being where your grille cloth is, which wouldn't look right for one. Mine is on the back too, and its annoying, but it works so I am happy with it there.

Greg

[gibsonlp]

Well, we replaced the polarity toggle (which controls the phase/0 which is in my case - 220v AC) with the Standby (which controls the 220v AC that goes to the PT), on the paper it's the same, but in reality it caused a weird ground loop.
Thanks for the support!

[JakeK]

Greg, maybe you can help me...my Deluxe Reverb recently started humming badly. What do I do?

[scott_s]

Teach it the words?

Ah that one never gets old... if the hum is present with the controls down, I'd suspect either the output tubes or the filter capacitors. To rule out a preamp tube, pull all of them out and power on the amp again. If it's not that, I'd swap in a set of fresh, matched 6V6s (you *do* have a fresh backup set, right?) and see if that clears up the hum. Failing that, I'd have a tech look at the filter caps.

- Scott

[JakeK]

Apparently the amp doesn't like uneven floors

I propped the amp up on a chair, and all was good. Yep, I don't get it either

[scott_s]

Now that's odd! Does it resume humming when you set it on the floor again?

Are any of the tubes loose in their sockets?

- Scott

[gibsonlp]

Perhaps is some sort of electromagnetic interference?

[johnallg]

Perhaps is some sort of electromagnetic interference?

I thought of that too - AC wires/light fixture under the flooring?