Stand Alone Reverb - ground loop busted

[Sir Cuitous]

Hi y'all. Finally, I have built this thing. This time I have to specifically thank the following members for their input.

Tubeswell: schematic, rectifier advice.
Enzo: isolation transformer mod.
Steve Conner: parallel cathode follower.

It is not really a conversion, but I did gut a broken Fender SK100-B for the cab and chassis. I did not save any time since I had to patch some holes in the chassis and make my own layout, but I suppose I saved some money. It works, with one problem - hum. I have attached some pics (you can see the isolation transformer has been temporarily removed from the circuit).

It's a 6G15 with a full-wave bridge rectifier. But then the output goes to a 12AU7 wired as a parallel cathode follower with 0.1uF coupling caps before and after, and then to a 600ohm:600ohm transformer before the output jack. The output jack is isolated from the chassis and connects only to the isolation transformer.

Everything functions as expected: Dwell, Mix, Tone. When the isolation transformer is not in the circuit I get hum through whatever amp I'm using. It does not matter if the Reverb Unit is powered on or not. I believe this is the expected ground loop hum caused by connecting the Reverb Unit to an Amp through the patch cable. Tying the filament circuit ground to the 6K6 cathode or changing the grounding layout have not made any difference in the hum.

When the isolation transformer is in the circuit I get no hum.... until the Reverb Unit is powered on. Then it sounds like the same hum I get without the isolation transformer.

At least while the Reverb Unit is not on, it seems like I have broken the ground loop to the guitar amp. But why would it return when I "power on" the Reverb Unit? Is there something simple that I missed?

[Enzo]

You broke the ground loop and eliminated the hum it caused. Now when power is applied, you are hearing hum from the circuitry. I sounds like the ground loop hum because all the hums will be either 60Hz or one octave up at 120Hz. They will sound alike.

For testing, plug the reverb into a ground lift adaptor so the third prong on the power cord is lifted. That will also break a ground loop. Now see if the hum remains. I'd bet it does. You could also find a battery powered amplifier and connect the reverb to that. That would insure to ground loops. Then any hum you hear is from the circuits.

And at that point you are down to routing out the hum as you would in any amp build.

[loudthud]

In one pic I see you have the ground side of the bridge rectifier and the power cord ground going to the same screw in the chassis. Bahahahad !!! The power line ground should always have it's own screw as far from any power supply ground as practicle but away from any circuitry like in a corner of the chassis. It's ok where it is but remove the wire that goes to the minus side of the bridge and run a short wire from the minus side of the bridge to the minus side of the first filter cap. That should get rid of most of the hum. If that's still too much, make a diagram of every ground connection and post it.

[Sir Cuitous]

loudthud, good eye. I know it's wrong, but I left it there out of frustration since none of the grounding changes I have done have made any difference. But, I will return it to the layout you describe as soon as I can get to it again (+ Enzo's test recommendations). I wanted to see if moving the rectifier ground away from all the others would make any difference. It did not.

[Sir Cuitous]

I started trying different grounding methods checking at frequent intervals to see if I could hear any changes in the hum. Nothing made any difference in the sound or volume of the hum. It is a very low tone which might not exactly be a hum.

With the isolation transformer in place at the output, it failed Enzo's "ground lift" and "battery amp" tests. Without the isolation transformer it passed. In other words, without the isolation transformer, the ground lift adapter quiets it up to nothing but a faint hiss.

Is this not hum, but a low level tone coming from the isolation transformer? The isolation transformer worked when the unit was powered off - the path to earth through the Reverb Unit was broken. But it seems like it creates a tone when the unit is powered on - path to earth still broken. Is all that possible?

[loudthud]

If the isolation transformer is not shielded, it could pickup a 60Hz tone from the power transformer. This can usually be confirmed by twisting the transformer around in relation to the PT. The other way for 60Hz to enter the signal path is signal wires in close proximity to wires from the PT including the heater winding. Twisting wires with oposite polarity reduces this. A center tap on the heater winding helps balance the voltage on each side of the heater winding to reduce noise.

[Sir Cuitous]

The isolation transformer is about 14 inches away from the PT. I want to repeat my tests to make sure so when I have it connected again I will check the orientation, but it's pretty far away from the PT.

The heater wiring is twisted and grounded through 100 ohm resistors. I did try tiying the heaters to the cathode of the 6K6 which made no change at all.

The only change has been when the isolation transformer is removed and the ground lifted. I am going to repeat my tests to make sure, and I have a different isolation transformer to test.

[Enzo]

You are n0t grounding any lead from that transformer are you?

[Sir Cuitous]

Not on the secondary, but I did ground the primary. I'm guessing I should not ground any lead from that transformer?

[Sir Cuitous]

Here's the way the transformer is wired:

http://www.music-electronics-forum.c...9&d=1228384912

I re-tested today. Same results through the test amp.
With the isolation transformer:
low tone when the reverb is on.
no low tone/hum when the reverb is off.
low tone when the ground is not lifted.
bad buzzy hum when the ground is lifted.
Without the isolation transformer:
hum (no tone) when the reverb is on.
hum (no tone) when reverb is off.
hum when the ground is not lifted.
no hum (quiet) when the ground is lifted.

The last result had me convinced that the isolation transformer I was using was not right for the job. But, I tried a very different 1:1 transformer - a 10K interstage transformer - and I got the same weird tone. I'm kind of stuck for what to try next.

[tubeswell]

Hi Sir Cuitous

I don't recall ever getting any feedback about that schematic from other people here as to whether it'd work, but I believe you need quite a hi-impedance 1:1 tranny to any any chance of success at reproducing the frequencies appropriately. In my experiments to-date I haven't found one that is high enough (although I am still hunting for a couple of decent matching hi-lo Z microphone trannies that I can put back-to-back). I think your result with the isolating transformer connected but the unit switched-off, suggests that it works to bust the ground loop, but that there is possibly something else causing hum when you switch the unit on. Also maybe the tranny you have doesn't have a high enough impedance to deliver a good tone. (Maybe the only other option is some sort of active isolation device?, or otherwise maybe re-wiring both the unit and the amp in order to elevate the return path from the chassis and use Fender RI concept of back-to-back 6A diodes in parallel with a 10R flame-proof resistor and parallel high-voltage cap)

[Sir Cuitous]

Tubeswell, you're right, no one did sanction that schematic. I'm not showing it to hold you to any design obligation. I just have the time and some resources to devout to some experimentation here. I want get to the bottom of it and see what works. I just wanted to show that I grounded one side of the primary.

So far I have tried a 600:600 tranny and a 10K:10K tranny. Surely the 10K would be high enough to exceed the impedance from the reverb unit? Don't forget that I have a 12AU7 as a cathode follower to lower the output impedance going to the transformer.

Both transformers exhibit the same problem which is a high fequency hum or low tone whenever the unit is powered up. It is possibly a 120Hz sound that I am am hearing, but I am not experienced enough to tell you if that is certain. It is more likely that I need to change wiring or there are some resistors or some other components that this circuit needs to make it work.

Yes, it seems like with the transformers the ground loop is broken, but the tone occurs. I do want to make it clear that this sounds different than when the transformer is out of the circuit. When the transformer is out, it "hums" - plain and simple. Until I break the ground with an adapter and then it quiets right up. So with the cathode follower, but no transformer, it behaves exactly as expected - the ground loop can be lifted. With the transformers a low tone or sound is constant and lifting the gound loop does not help.

I will have some time soon to try some things. Maybe DC is getting to the transformer through the ground connection on the primary. Maybe I need to ground through a resistor and a cap? There is probably something simple that I am overlooking.

[tubeswell]

I think I recall Steve Connor suggesting that maybe it needs a min of 15k if it is a passive isolation transformer. (Was that a number he plucked out of thin air?)

My understanding (limited as it is) is that you should aim for an input impedance at least 5 times as high as the source impedance if you don't want the signal to attenuate too much. If I know how to work out the source impedance from the back of a reverb unit I would know what isolation transformer to get made up.

I figured out how to work out the output impedance from an ordinary bypassed gain stage from the Valve Wizard site (is that Merlin B?). And I guess that you have to work out the output impedance from the CF stage (which I'm not quite sure how to do), and then maybe put the two together as parallel impedances, in order to arrive at a source impedance for driving the isolation transformer. But since the 'dry' and 'wet' sides of the reverb unit circuit are blended with a mixer pot that you can adjust, I'm not at all confident that you can calculate the output impedance from the reverb unit in this way.

[Sir Cuitous]

I'm not sure how to really get the correct impedances either. I took a shot here. I thought it made sense to drive this transformer with the cathode follower. I wish I knew just how much the cathode follower is lowering the impedance. Without really knowing I took a chance that it would work.

I'm not done with it, but for now what I have is a nice unit that functions without the transformer. I'm going to try a couple more things to get it to work, but if not, I will just play it for a while without the transformer.

[KingCameron]

Persistent hum in Kendrick Reverb Unit

Guys,
Thanks for this information. I have recently had a customer bring in Kendrick Reverb Unit (6G15 Clone) that has a persistent hum, like you have described.

I will be interested to see how this develops

Thanks,
Cameron