6G15 hum dagnabit

[tubeswell]

Well following Sir Cuitous' experiment with the grounding on his one, I re-wired the ground on mine to a star ground with only one connection point to the chassis as per R.G.'s concept. The input and output sockets are now isolated from the chassis. All the ground points have their own separate wires to the star connection point. There is a single wire from there to the chassis ground. The reservoir cap and the HT CT are hooked up together before hooking into the rest of the star ground. The reverb unit is beautifully quiet on the dry side of the mixer, but hums louder and louder as you turn up the wet mix. Using the footswitch kills the hum, so it must be coming in somewhere before the recovery tube I guess. I have swapped all the tubes to no avail. At the mo' I have a Sovtek 7025 and a GE12AT7WA and a JJ6V6s in there. The heaters are elevated to the 6V6 cathode (about 25V). If only I could get rid of that hum in the wet mix, things would be perfect.

[txstrat]

Hi TW
Do you mean the hum disappears just by plugging the footswitch in? How does the unit behave in the different positions of the footswitch? Same? No hum even with the mix on "wet"? Did you isolate the footswitch jack from the chassis as well?

[Steve Conner]

Hi Tubeswell

The pickup coil in the reverb tank is very sensitive to magnetic fields. It's just like a single-coil guitar pickup in that respect. If it's close to the PT of your reverb unit, it will pick up hum by magnetic coupling.

You might be able to turn the tank around so that the pickup end is furthest away from the PT. Also make sure that the steel case of the tank is between the PT and the pickup coil. I found these tricks helped a lot in my Ninja Deluxe build.

Then again, if it still hums with the pickup unplugged and the input to the recovery stage shorted, it's just another ground loop in your circuit. In this case, you might want to try isolating the reverb return jack from the chassis, and grounding it straight to the spot where the recovery tube's cathode bypass cap is grounded. Again, I did this because I thought it made sense. These measures worked OK, when I turn up the reverb I mostly hear hiss from the recovery stage's internal noise, with just a little hum left.

[tubeswell]

Thanks for those prompts fellas. The pan input is already at the opposite end of the cab/chassis from the PT and it is sitting on the bottom of the cab. (In fact the pan input is nearer to the RT than the PT).

It makes me wonder whether the PT and/or choke is possibly coupling into the Reverb tranny tho' (altho' the RT is physically about 20cm away from the PT and the axis is offset by 90 degrees), or whether the reverb tranny is coupling into the pan (although I don't know if that makes any difference, because the RT is closer to the opposite end of the pan from the pan output).

Neither the PT or the RT have end-covers (not sure if that would make any difference). I might try moving the RT into different locations/orientations. I do have another RT that does have end covers (if that makes any difference) but it is huge.

[txstrat]

I accidentally skipped the fact with "using" the footswitch.
You could take longer leads and clip the tank and/or the RT into the circuit from outside the cabinet. That way you could make sure if any noise is induced by the PT or choke.
If the hum is still there I would take a closer look on the reverb cables. Are they well grounded via the rca jacks?

[tubeswell]

Well the first thing I did today was I moved the pan well clear of the cab to see if it was induced hum into the pan's transducers, but it still hummed.

The reverb cables test good, as do the shield connections to the chassis ground. There is a good connection (but only 1 connection) between the pan box and the chassis.

So then I thought, 'maybe it is the RT picking up hum from the PT', but when I did the headphone test with the RT secondary I could barely hear any noise at all (in fact the teeny weeny bit of induced hum noise was so minute/inaudible, I really wasn't sure if I could tell whether there was any hum at all for the first couple of tries switching the amp on and off, but it seemed to be there, albeit very very faintly detectable, but almost to the point of being non-existant (and on the point of being confused with my imagination). I used very high quality headphones and made sure they were matched to the secondary impedance - even tho there was no load across the primary - or do I need to do this test across the primary?). (I wonder if this test is all that reliable? Maybe I should make up a little aluminium cover to bolt over the top of the RT? Or maybe its something else?)

[Diablo]

My Tore-Vibe was a little noisy when I turned up the wet mix too. Poking around the reverb pan and cables, I discovered that one of my shielded cables wasn't doing such a good job of shielding. Swapped out the cables and it improved the background noise by a good bit when the wet mix is cranked up.

[txstrat]

Maybe I should make up a little aluminium cover to bolt over the top of the RT? Or maybe its something else?)

A sheet of aluminum or tin slightly larger than the RT bolted to the chassis in between the PT and the RT should be enough of a shielding - at least for testing.
...somehow I think of the cables to and from the tank more and more. That's why I asked if they are properly grounded in the first place. But they could indeed pick up the hum if the inner shielding is of poor quality.

[Enzo]

The reverb transformer is on the drive end. Hum would come from the return end. You already listened to it, and it was hum free anyway. If hum did induce into it, the hum would then have to traverse the reverb springs and it would be a reverberant hum, not plain old hum.

Does it still hum when the reverb cables are removed?

And if you leave the return cable but disconnect the drive cable at both ends, does that make a difference? In other words does removong the drive cable only make a difference? Accutronics et al make a variety of jack grounding schemes for such reasons. Yoou may need a different jack grounding scheme than your pan has.

The footswitch kills the hum, but are the FS and return jacks sharing a ground? That is are they both on the chassis side by side grounded to it? Or if isolated from ground are they together? In other words, does stomping on the FS connect the return circuit to a different ground than the shielded cable uses?

[Sir Cuitous]

I isolated my tank send/return jacks and the footswitch from the chassis to conform to the RG scheme. The wet side has a little hiss and noise that increases when mixed in, but you have to get your head close to the amp speaker to hear it.

[tubeswell]

The reverb transformer is on the drive end. Hum would come from the return end. You already listened to it, and it was hum free anyway. If hum did induce into it, the hum would then have to traverse the reverb springs and it would be a reverberant hum, not plain old hum.

Does it still hum when the reverb cables are removed?

OK so the hum could be induced from the PT? I will make a little aluminium cover box for the PT tomorrow. (Too tired now)

I thought I couldn't remove the reverb cable from the tranny to the pan because that would leave no load on the tranny.

And if you leave the return cable but disconnect the drive cable at both ends, does that make a difference? In other words does removong the drive cable only make a difference? Accutronics et al make a variety of jack grounding schemes for such reasons. Yoou may need a different jack grounding scheme than your pan has.

I'll try it and find out

The footswitch kills the hum, but are the FS and return jacks sharing a ground? That is are they both on the chassis side by side grounded to it? Or if isolated from ground are they together? In other words, does stomping on the FS connect the return circuit to a different ground than the shielded cable uses?

The footswitch jack and the reverb return jack tips are wired on the same wire and both sleeves are connected directly to the chassis - so when the FS is engaged, the grid of the recovery stage goes to ground. There is no hum at this point, which confirms that the hum is coming from before the recovery stage - right?

Both shielded reverb cables are connected to the chassis at each send/return jack. There is/are no other return wire(s) connecting the jack sleeves to the rest of the ground returns (at the star grounding point). The pan is only connected to the main chassis ground at one of the pan RCA jack sleeves. The other one is isolated

Tonight I made a little aluminium cover box for the back of the reverb tranny (as this did not have any covers) - this seemed to lessen the hum a little bit, but I'll try this same trick with the PT tomorrow (as that doesn't have any covers either).

[guitician]

Is the recovery stage quiet without anything attached to the grid; except the grid resistor of course? Don't assume that the hum comes from before that stage just because you grounded the grid. Aluminum won't block EMI, that's why all end caps are ferric metal, like steel.

[Sir Cuitous]

Aluminum actually makes great shielding and does not couple magnetically with transformers.

[guitician]

Aluminum actually makes great shielding and does not couple magnetically with transformers.

That depends on what your trying to shield. Magnet interference(PT Core induced), requires something other than aluminium to block EMI.

[Sir Cuitous]

Yes, your right. The aluminum would not attract the magnetic field. Good electrical shielding though.