Ampeg 1970s old school SVT humming LOUDLY!

[six el six]

I've rebuilt the power supply on this amp, retubed it, retensioned everything, checked everything.

The result? It sounds great EXCEPT it hums like crazy.

Interestingly, someone removed the hum balance pot on this amp at some point in the past and put in 2 x 100 ohm resistors that are presently grounded on one of the can cap grounds.

The heater leads are not twisted.

Could I get some opinions, advice, experience, on remedies for the hum?

Thanks guys,
six

[stratele52]

Not easey to answer I don't know how you rebuilt power supply . how it look , ground , wire...

First thing ; heater wire must be twisted. Not twisted = hum for sure. Probably is the only mistake.

Ok for artificial ground with 100 ohms resistor each side of the heater and going to ground. IMO it is better than a pot wich can turn an make hum.

Send us some close up pictures , this will help greatly.

[xm52]

Oh man, this is a can of worms.

Although I agree that twisting the heater wires is a good idea, and I have done it on SVT's, they are normally not twisted. But I suspect that you are hearing a loud hum and not the type that one hears with untwisted heaters.

Can you identify the frequency of the hum? 60Hz? 120Hz?  60 Hz is more than likely related to the heater circuit; 120 Hz could be the high voltage power supply circuit.

Recheck everything that you worked on. You could have a bad solder joint which could cause the hum so check that all the joints are solid and wire contacts are good.

SVT's vary a bit in their grounding, depending on the revision. For the most part there are no or possible one ground point on the power amp chassis. The cap cans should have fiber isolation washers between the can and the chassis. Do not let the can body come in contact with the chassis or you will have a ground loop and loud hum. All the power supply cap returns are tied together on a bus and connect to the shield of the gray wire that runs to the pre-amp. Make sure that this is the only ground wire running to the pre-amp. The hum pot center tap, or in your case the artificial center tap from the 100 ohm resistors, should be connected to the power supply ground bus at the C node ground point. The circuit board ground goes to the C node ground point. The test point ground (on the chassis) goes to the C node ground point as well. The common from the secondary of the output transformer also connects to this bus at the A node ground point. The orange wire from the power transformer secondary center tap also connects to this bus at the A node ground point.

Sometimes the external amp jacks, J1 and J2, are isolated from the chassis with fiber washers, sometimes they aren't. This is often the only chassis ground point. When isolated, the jacks sometimes have a 10 ohm resistor to ground. This acts as an offset.

Hope that this helps.

[six el six]

Thanks xm52 and stratele.

I have the fiber insulation washers in between the can caps and chassis. The nuts and bolts holding the caps to the chassis, of course, still connect the cap to the chassis.

The ground wire to the preamp was broken on this amp.

I'm going to see if changing the spot where it connnects (presently) fixes this.

What's the best way to differentiate between 60hz and 120hz? It seems to me 120hz is a low frequency and 60 hz is twice as low. But they're BOTH low frequencies so how do you know?

[HotBluePlates]

I have the fiber insulation washers in between the can caps and chassis. The nuts and bolts holding the caps to the chassis, of course, still connect the cap to the chassis.

A does not equal B.

Sometimes can filter caps have to be used in a series arrangement to get the needed voltage rating (just like series filter caps in some blackface Fender amps). But the can is the negative connection of the cap. You can't use clamps to hold the cap to the chassis unless it has an insulating sleeve. This may be plastic (in the case of JJ, F&T or LCR caps), or in the old-school twist-lock cans it's a cardboard sleeve that slides over the can.

I don't know what you used, or if you used it right.

What's the best way to differentiate between 60hz and 120hz? It seems to me 120hz is a low frequency and 60 hz is twice as low. But they're BOTH low frequencies so how do you know?

If you have a guitar, 120Hz is in-between B-flat and B, assuming you're tuned to A440. The guitar's low E is a bit higher than 82Hz, so 60hZ hum is lower, 120Hz hum is higher. With a bass, 60Hz is in-between 3rd string 1st fret B-flat and 3rd string 2nd fret B. Octave higher for 120Hz.

But a buzzy 60Hz hum can fool you into thinking it's a much higher frequency, because there's extra harmonic content. I have on occasion encountered a buzzy 60Hz hum due to no 100Ω resistors or filament CT. That was just due to how the hum manifested.

[six el six]

I used can caps the way I always do. They always work. I'm not an amp builder. I fix them. So I have a hard time believing I screwed up rebuilding the power supply though it's certainly possible. I'm not being defensive. I'm just communicating that I understand the concept. These are ce can caps w/the fiber insulator boards. I replaced them exactly as they were with the exception that I had to attach grounds from the other filters where I could. They are attached at the grounds of the can caps.

There are two caps in each of the can caps in this unit.

Is it possible that the hum is being caused by poor placement of the preamp ground? For those of you not familiar with svt's they have a seperate chassis for the preamp. The ground wire that comes through the molex type connector from the preamp was broken. I attached the ground to a PT bolt.  Problem?

[xm52]

Remove the amp's ground from the PT bolt. That's your problem I think. If you have a three-conductor power cord, put the chassis ground (often the green wire) on the power transformer bolt. Don't ground anything else there.

The ground bus from the power amp, node C end, connects to gray shielded cable that goes to the molex connect and out to the pre-amp. The other end of the gray cable connects to the pre-amp ground and terminates at a ground point on the pre-amp chassis near or at the input jacks. The ground bus runs from Node A to the instrument input jacks. The center conductor of the gray cable carries the the signal from the pre-amp to the power amp.

What I should have added in my last post is to check the polarity of the PS capacitors. C8, 90uV 200V, is reversed and has the (+) to ground.

[xm52]

Another ways to distinguish 60 and 120Hz is to put a speaker on a signal generator and listen to how it compares to the amp hum. There are signal generators that you can run on your computer but you'll need a good speaker or headphones to hear the output.

[six el six]

Thanks xm52.

It was the ground coming from the preamp.

Had to ground it over by the external amp ground.

[xm52]

Great, glad that you resolved it!

I found what I was looking for in my notes. Normally the braided shield on the gray wire which is the power amp ground, goes to pin-4, the white center conductor of the gray wire goes to pin-1. The other end of the gray wire shield goes to the external amp ground, as you mentioned. Here are two pics, one end at the molex, the other at the external amp jack. The cable that plugs into the molex connector that runs to the pre-amp has another gray shielded wire that caries the ground and signal to the pre.

[stratele52]

You should replace the power cable by one with ground .

[xm52]

Yes, a three conductor power cord is always a good idea.

[Willabe]

Yes, Yes, Yes!!!!

Are you kidding me?      With the Voltage and Current avalable from that amp it is a must!


               Brad        

[six el six]

I already replaced the power cord.

Thanks guys.

Amp turned out great.

I think i'd like to build one for myself sometime. It makes playing bass a blast.