5F1 hums, quiets down when volume at maximum

[motzu]

Hello, I've built a Tube Amp Doctor 5F1 kit and I'm pretty happy with it's tone, but recently I've added a 25uF/25V bypass capacitor for the 1K5 preamp resistor, and that increased it's overall loudness and made hum an issue.

The hum was there before (and the problem I'm about to relate to you), and I would hear less of it, but now it's become a problem. This is what happens: after I turn on the amp and the tubes warm up and start to tickle the speaker, I hear humming at about 4/12 on the volume pot. All this happens with no input (no guitar or anything else plugged in). Then as I rotate the volume pot more and more towards 11/12, the hum gets louder, and at 11 it's quite something. Then, between 11 and 12, as the pot starts to behave like a full-on conductor, the hum goes away. Some negligible hiss remains, and if I plug a guitar in it works fine (in fact it works fine at all volume levels if I ignore the hum, but between 11 and 12/12 there's almost no hum).

This leads me to believe that the hum is not caused by lead dressing issues. I've also swapped tubes around with no effect on the hum (all the tubes).

I'm suspecting either a grounding issue or some defect of the volume pot. My grounding scheme is pretty much based on the Weber kits:

https://taweber.powweb.com/store/5f1_layout.jpg

Specifically, the 1K5 resistor goes to G on the INPUT2 jack, and the volume pot, and second 8uF from the left (along with the 25uF bypass cap next to it) go to G on the INPUT1 jack. The jacks are non-isolated Switchcrafts connected mechanically to the steel chassis.

The 1uF, along with pin 1 of the 6V6GT go to ground to a solder lug held with a PT screw on the chassis. The same solder lug holds the twisted legs of a couple 100R resistors that are used as an artificial PT center tap (actually they're closer to about 120R each - they're carbon composition 10% - maybe that could also be a factor in the hum?).

The AC power cord and PT grounds go to another solder lug held in place by a PT screw.

There are no bad solder joints. They all look fine and I've tested everything for continuity.

So now I'm thinking about replacing the volume pot (but I don't think it's likely to solve the hum problem), or moving the 8uF capacitor ground away from the preamp ground (the jack ground), and moving all the preamp grounds to one single jack, not both of them like it is now.

Have you encountered this problem before? What could it mean that when the volume pot is full on, the humming goes away?

Thank you for reading my post!

[jmaf]

Seems to be classic ground loop. When it's not a short between input and grid, it's letting the ground current modulate that grid via a loop that is also passing by that volume pot, consequently the grid circuit.

Do you have a single point of ground? Or is it grounded all over? For shielded leads, don't ground both ends, just one. Each ground must have a path to the central ground bus.

The rectifier current to the first reservoir cap is the noisiest, it should be the central ground point for most of your amp.

[Bruce / Mission Amps]

Pull the positive lead off the 8uF cap and stick a new 10uF-450v cap right at the junction of the two 100K plate load resistors of the 12AX7 and ground it right at the input jack ground... you can use clip leads to rig this up... then try your same test.

[motzu]

Do you have a single point of ground? Or is it grounded all over? For shielded leads, don't ground both ends, just one. Each ground must have a path to the central ground bus.

No, I don't have a single point of ground, unless you count the chassis itself as a single point of ground. The preamp part of the circuit is grounded on the lugs of both input jacks (the volume pot is grounded there too), and one 8uF cap is grounded there as well. Then, the 16uF, pin 1 of the 6V6 and the 100R resistors (the PT center tap) go to one solder lug screwed on to the chassis on a PT screw. Finally, the third ground point is another solder lug attached to a PT screw and the chassis, and holds the AC cord ground wire and the PT ground wire.

There's no shielded wire in the amp, only "authentic" cloth-covered wire. I don't think the type of wire is the issue though, since I would hear that when the volume is all the way up too - or am I wrong about that?

[motzu]

Pull the positive lead off the 8uF cap and stick a new 10uF-450v cap right at the junction of the two 100K plate load resistors of the 12AX7 and ground it right at the input jack ground... you can use clip leads to rig this up... then try your same test.

Thank you for the post! I'd try that, but the only spare cap I have around is a 25uF/25V electrolytic bipolar capacitor, and unfortunately where I am right now I'd have to order one online and wait for it for a few days to get here. I guess I ought to get some anyway, as the 16/8/8 filtering caps do seem a bit on the insufficient side. But for now I'm afraid I'm stuck with moving ground points around.

Just to make sure, you're talking about the second 8uF cap from the left, the one grounded on the input jack lug?

I'll go see if I can find a 10uF/450V cap somewhere local by some stroke of luck.

[tubeswell]

Check this out

http://www.freewebs.com/valvewizard2/Grounding.pdf

[motzu]

Check this out

http://www.freewebs.com/valvewizard2/Grounding.pdf

Thank you, that's a very good read. In my defense (though it's probably rather like pleading ignorance) I followed the TAD layout diagram exactly, save for 2 things: I've put a 25uF bypass cap across the 1K5 resistor, and instead of wiring the OT primary wire to the junction of the 10K resistor and 16uF cap, I've sent it directly to pin 8 of V3 (which somebody on a different forum claims is what Victoria does in the 518 5F1 clone, and can't possibly hurt anything, ground-wise or not).

Unfortunately I'm doing all this word-dance and I'm linking to the Weber layout because TAD in their infinite wisdom decided that the layout is "(c) TAD, for private use only, do not copy". So I've decided against making it public, and instead talk about specific differences or similarities between my build and public layouts.

[MWJB]

"Finally, the third ground point is another solder lug attached to a PT screw and the chassis, and holds the AC cord ground wire and the PT ground wire." If bt PT geroun wire you mean the red/yel B+ centre tap, this should be grounded with the 16uf filter cap (& the 8uf filter cap for the screen supply @ junction of 10K & 22K power supply resistors).

[motzu]

"Finally, the third ground point is another solder lug attached to a PT screw and the chassis, and holds the AC cord ground wire and the PT ground wire." If bt PT geroun wire you mean the red/yel B+ centre tap, this should be grounded with the 16uf filter cap (& the 8uf filter cap for the screen supply @ junction of 10K & 22K power supply resistors).

Yes, the red/yellow wire. I'll move it there and see if that helps with the hum. Thanks for pointing that out!

[motzu]

"Finally, the third ground point is another solder lug attached to a PT screw and the chassis, and holds the AC cord ground wire and the PT ground wire." If bt PT geroun wire you mean the red/yel B+ centre tap, this should be grounded with the 16uf filter cap (& the 8uf filter cap for the screen supply @ junction of 10K & 22K power supply resistors).

Moved it there, no change. That was not the solution to my problem apparently.

[tubeswell]

instead of wiring the OT primary wire to the junction of the 10K resistor and 16uF cap, I've sent it directly to pin 8 of V3 (which somebody on a different forum claims is what Victoria does in the 518 5F1 clone, and can't possibly hurt anything, ground-wise or not).

I think you're confusing windings and transformers here

The 3 x Power Transformer secondary windings consist of a High Tension winding (the 2 x red wires with a black centre tap), a rectifier winding (the 2 x yellow wires) and a heater winding (2 x green wires with a green/yellow centre tap). For heater elevation (i.e.: to raise the heater winding ground reference voltage above the rest of the chassis ground potential in order to reduce hum), you take the heater centre tap (the green/yellow wire) and connect it to the cathode (pin 8) of the 6V6.

On the other hand, the sole Output Transformer primary winding has a red wire and a blue wire. One of those wires must go to the 16uF/10k filter cap junction (in order to supply the OT primary with the necessary high DC voltage), and the other wire must go to the plate (pin 3) of the 6V6.

[MWJB]

Tubeswell wrote: "The 3 x Power Transformer secondary windings consist of a High Tension winding (the 2 x red wires with a black centre tap), a rectifier winding (the 2 x yellow wires) and a heater winding (2 x green wires with a green/yellow centre tap)." Depending on the actual PT used wire colours will vary a little, however the PT's supplied with TAD kits typically follow traditional wire colours - PT B+ centre tap is red/Yel NOT black. Champ style PT's may, or may not have a grn/yel CT depending on supplier.

Motzu - it's worth looking for a shield wire on the PT legend, sometimes there is an orange wire that also needs to be grounded with the B+ CT.

Does the hum disappear when the 12AX7 is removed?

Time for some pics of the build....

[motzu]

On the other hand, the sole Output Transformer primary winding has a red wire and a blue wire. One of those wires must go to the 16uF/10k filter cap junction (in order to supply the OT primary with the necessary high DC voltage), and the other wire must go to the plate (pin 3) of the 6V6.

That's right - and in my layout, the red wire goes to pin 3 of the 6V6, and the blue wire is supposed to go to the cap/resistor junction. From the cap/resistor junction, there's also a wire going to pin 8 of the rectifier tube (where one of the PT's yellow wires is also connected).

Instead of wiring the blue OT wire to the cap/resistor junction, I've wired it directly to pin 8 of the rectifier tube, all the other wires connecting as specified. I don't see how this could have affected any grounding issues, but then again I might be wrong.

[MWJB]

OT primaries - have you tried reversing blue & red wires? If it's easier you can try removing the NFB loop from the speaker jack first to see if you have a OT polarity issue?

[motzu]

Motzu - it's worth looking for a shield wire on the PT legend, sometimes there is an orange wire that also needs to be grounded with the B+ CT.

Does the hum disappear when the 12AX7 is removed?

Time for some pics of the build....

No shield wire. I've got: red/black - 240V, green/black - 230V, black/yellow - 220V, black - 0V, 2 x red and a red/yellow - HV, 2 x green - 6.3V, 2 x yellow - 5V. Furthermore, all the wires are connected somewhere as per layout, no wire is unused.

Yes, the hum dissapears when I power on without the 12AX7 preamp tube. I'm left with only the buzzing of the tubes and no change whatsoever in output level regardless of how I set the volume pot. I suppose this makes sense since the volume pot feeds the second half of the 12AX7 tube? So with no preamp tube plugged in there's no point in me wiggling the volume pot.

I should mention that this is not the preamp tube's fault. I've swapped two different JJ 12AX7 tubes and the default TAD 12AX7 with no effect on the humming.

You're right, I should have posted pictures from the beginning, but I don't have enough light and can't seem to find the camera. I'll post some as soon as I can.