6G15 build ground loop hum question

[Enzo]

Dai, WHAT higher current returns? The setup hums EVEN WITH THE POWER OFF ON THE REVERB. ALL the ground loop currents are AC. His two units perform exactly as I predicted. We are not dealing with currents IN the chassis.

txstrat, if we did that, then there would be no ground reference for the whole circuit. It would be come a much worse example of like when the ground wire to the bridge comes off in your guitar.

[txstrat]

Maybe I put it improperly.
I mean the ground reference of the 5e3 could become the ground of the 6g15 too, via the guitar cords ground. Couldn't it?
I know, all in all that wouldn't be the best solution.

[dai h.]

Dai, WHAT higher current returns? The setup hums EVEN WITH THE POWER OFF ON THE REVERB. ALL the ground loop currents are AC. His two units perform exactly as I predicted. We are not dealing with currents IN the chassis.

okay, the other amp is hooked up and even if the power on that is off, since you've given the return an alternate path back (through the powered off amp through its safety ground, through the powered on amp's own safety ground towards/through the noisy end since--noisier end returns are on the chassis) there should still be noise since you've screwed up the grounding scheme. Looks like still the same problem hence the suggestion to not use the chassis for the higher current returns to prevent a common impedance.

[tubeswell]

Look at your guitar cord, and consider the tiny currents the pickup sends down it. How much do you think a 47 ohm or 470 ohm or 4700 ohm resistor in series with it would affect the amount of signal the amp gets? The 68k series resistor at the amp input doesn;t knock it down much. In short, it will have no effect.

Sorry - I thought that by having a mixer/voltage divider on the cable shield side of the connecting cable (inside the buffer box) , that had the 'ground' leg of the divider going, via its own connection, to the same mains ground as both sets of amps (whereupon the box would be shielded and the ground leg would be ground-referenced) would act like a hum balance pot in a heater circuit.

And I thought it was the ratio of the resistance in the series/leg setup of a voltage divider that determined how much voltage was seen at the junction of the series/leg (and not just the actual value of the series resistor alone). However having said that, might a bigger value (say 1M) pot and 'ground' leg resistor work?

By still having the ground loop hum even with the reverb turned off, we can see that the internal grounding arrangements are not the deal. It is more about minute leakage in the chassis wiring and induced voltage from mains wire placements.

Remember, the ground loop is a relationship between the two units, ther is nothing wrong with either one of them. They disagree on chassis potential, you have three pieces of gear (in the discussion), and two of them have the same chassis potential and one does not. How do we determine which one is "wrong?" What if we got five more amps and three more reverbs and discovered that the odd unit out now actualy agreed with more of them than the other two?

There is an experiment for you: your two amps disagree and if conected together they hum. You can turn the one off and it still hums, what happens if you turn the other one off instead? I bet it still hums out the one left on. Obviously that won;t work on the reverb-only since it has no speaker. But on the two amps?

As long as we have a difference in potential, it doesn't really matter which thing is which, it is the difference that matters.

Or pulled the mains plug for the reverb out of the wall socket or lifted its ground. Remember there has to be a loop for a ground loop. The loop is up one earth lead, across the signal cable ground, and back down the other earth lead. Break any of those legs and the loop disappears.

Yep gotcha loud and clear already - I understand that the loop is caused by the two sets of mains ground connections being joined by the reverb output/amp input cable shield. What I was wondering about in my previous post was, is there a simple way of equalizing/balancing the voltage difference between the two amps' chassis potentials so that the hum is stopped.

[tubeswell]

Both guitar amps (5E3 and 5G9) work quite humlessly without the (6G15) unit.

But one guitar amp (5G9) also works quite humlessly with the (6G15) unit.

So when the 6G15 is plugged into the 5E3, the hum is the result of the 'ground' voltage on the 5E3 chassis being at a different potential to the 'ground' voltage on the 6G15 chassis.

How much different - I haven't measured it yet, but from what I gather it might well be a very small difference, because I have read that it only requires a small difference to induce lots of hum.

So I was wondering whether the hum could be resolved by somehow balancing the voltage difference between the two chassis with something like a hum balancer that had its own ground reference? (Has this point been missed?)

[txstrat]

As far as I understand you built the 5e3 and lifted the heaters to cathode level to eliminate the hum, right? If not, don't read further...
The 5g9 and the 6g15 don't have lifted heaters right?

I think it might be a good idea to eliminate the hum in the 5e3 in a different way.
That'll probably stop the hum problem with the 6g15 in front as well.

I consider any additional device for reducing hum as a soothing, not a cure (in this special case).

I wouldn't say so if there wasn't the 5g9 that is not humming with the 6g15 in front of it.

Hope I haven't mixed up some facts.

[tubeswell]

I had the heaters in the 6G15 elevated when I first built it. It didn't make any difference - the 5E3 still hummed when plugged into it.

[TD_Madden]

I had an Uncle Spot reverb unit that did that with both my Deluxes. I ended-up just using a HumX on the Deluxe. Not cheap, but effective.

[tubeswell]

I had an Uncle Spot reverb unit that did that with both my Deluxes. I ended-up just using a HumX on the Deluxe. Not cheap, but effective.

Thanks TD Madden

D'ya think this one of those things that elevates the ground with a combination of a parallel low value resistor, high voltage cap and 2 arse-about-face high-current diodes (that R.G. was saying probably isn't safe?)?

http://www.ebtechaudio.com/humxdes.html

[Enzo]

I think it might be a good idea to eliminate the hum in the 5e3 in a different way.

The hum in amps is not some monolithic problem. There is not one large pool of hum and the more anti-hum measures you take, the less hum you have. The hum you hear comes from various sources. Each source would have its own cure. The hum you would have from not grounding the heaters is a totally separate thing from a ground loop hum and totally separate from a poorly grounded guitar, and so on. Improving the grounding inside the guitar will have zero effect on the hum from the heaters.

Imagine you wiped out water skiing. Your arm hurts, your leg hurts, your head hurts. Putting a better bandage on your arm won;t make your leg feel any better.

The sources of chassis potential are things like the transformer mounting bolts and the steel chassis forming a large single turn within the power transformer field. The transformer then induces our curents. If they are not the same in two pieces of gear, then there is a potential difference between chassis.

[tubeswell]

Lost question

Hi Enzo

I think my above question to you got lost in the several branches of reply posts in this thread. I wonder whether you could please clarify for me whether the concept of a mixer/divider 'hum balance' pot that had its own ground reference point (from the leg of the divider) might work in principle to balance the different voltage potentials in the two chassis?

http://music-electronics-forum.com/s...91244#poststop

[Sir Cuitous]

As far as I understand you built the 5e3 and lifted the heaters to cathode level to eliminate the hum, right? If not, don't read further...
The 5g9 and the 6g15 don't have lifted heaters right?

I think it might be a good idea to eliminate the hum in the 5e3 in a different way.
That'll probably stop the hum problem with the 6g15 in front as well.

I consider any additional device for reducing hum as a soothing, not a cure (in this special case).

I wouldn't say so if there wasn't the 5g9 that is not humming with the 6g15 in front of it.

Hope I haven't mixed up some facts.

As Enzo said, it's not about the hum that you might find in the individual amps, it is the hum caused by the ground loop when the ground of the reverb unit is connected to the amp. The amps individually might be very quiet requiring no modification of the heater wiring. I think the isolation transformer IS the solution here. Lifting the heaters won't have any effect on the hum that results from connecting the 6G15.

[tubeswell]

Hi Sir Cuitous

That wasn't my question - that was txstrat's question. I know that lifting the heaters is not the solution - I already tried that last week.

What I want to know is if I can make a hum balancing pot (like what is sometimes used in a heater circuit) to balance the different potentials in between the chassis?

The thing I have in mind is a mixer/divider that hangs off the signal shield cable (in a little shielded metal box), that has its own reference to the mains ground. The input end of the pot connects to the 6G15 side of the signal cable shield, and the output end of the pot connects to the guitar amps side of the signal cable shield, and the pot wiper connects to another resistor (which is the leg of the divider) that goes to the mains ground via an independent wire. (Please see my schematic in my previous post)

[andrew_k]

On the topic of the transformer solution, Enzo can you please suggest a suitable high-Z 1:1 transformer to use in this situation? Preferably cheap...

[Sir Cuitous]

Yeah, Tubeswell, I just thought the point needed re-iteration.

I saw your schematic, but I wonder if a third connection to ground is just another ground loop. Would any impedance in the ground connection help to break the loop? At that point why not just break the ground connection between the two units?

I'm all set to start my 6G15 with isolation at the output, but I won't get time to begin for about another two weeks. Otherwise I would post the result. I will start a post under "Conversions" once I'm under way.