60 hz hum problem

[anson]

Hey folks, does anybody know about 60hz hum being picked up by ground wires? My homebuilt guitar amp hums quite a bit and the hum can be inceased/decreased when I move the wire that grounds the cathode resistor of one of the 12ax7 preamp stages. That leads me to think that wire is picking up an induced field from the PT, but I'm not sure. I've heard of PT's causing an induced current in chassis, but not individual wires. Does anybody have any ideas or suggestions?
Thanks, Anson

[cbarrow7625]

Are you sure it's 60Hz & not 120Hz? 120Hz could be ripple on the DC supply.

60Hz hum can also be picked up from the AC heater wires (especially if they are not twisted together tightly). Are your AC heater wires anywhere near this other wire? Are they twisted tightly?

Is your PT anywhere near this wire? i would assume that your preamp & PT are on opposite sides ofthe chassis. If not, that could be it.

[keysersoze808]

hi. i always install 2x100R from heater to ground/or chatode(if chatode bias) power tubes.. helps me a lot..

http://bp2.blogger.com/_gxUH1-DYi2I/...h/P1010180.JPG

[Enzo]

No the wire is not picking up hum from a field. The point of grounding matters. If that preamp stage shares a ground path back to the power supply with somehting drawing more vurrent, then the ripple will be shared with that stage. That is why we creat "star grounds." Look it up.

Move the ground for that stage to wherever it hums least.

[anson]

Well, I have a wire wound 100 ohm pot to create a "virtual" center tap. Also, I've conected that center tap to a positive (+35 volts) voltage to help keep filament hum from occurring and that hasn't helped a bit. Also, the heater wires are wound tightly and the hum isn't effected at all if the wire with the problem, the cathode resistor wire, is brought near them.

I'm not 100% sure that the hum is 60 Hz. I'm most suspecting that the hum is a mix of both 60 Hz and 120 Hz, because when I move the cathode resistor wire around, a lower frequency seems to dissapear and a higher seems to stay.

I don't think that the hum is coming from poor filtering, because I've got pretty high capacitances (100 u) on the screen, the the OT, and the preamp stages. Also, I've replaced the filtering caps already out of suspicion of their fault.

When I make the cathode resistor wire long, like 2 feet, and connect it to ground, I can really move it around alot. When I place that wire near the PT then the hum gets huge, and when I twist it in certain loop shapes outside the chassis, the hum seems to go away completely.

I have a star ground scheme, with the preamp stage grounds all connecting to their respective filter cap ground, then going to a star ground. But I'm gonna take Enzo's advice and move the preamp ground around to other grounds and see if I can't find a quieter place for it... Thanks guys....
Anson

[Twist]

This trick may help and it was passed down to me by an engineer and it sounds like you've already discovered it. If you've resolved that the hum is not ground related or in the power supply try this. Isolate which stage is humming....take a jumper wire with clips and attach one end to one side of your filament supply....now take the other end (being careful not to touch the bare end to any components) move the insulated part of the wire around the grid and cathode pins of the stage in question.....what your looking for is a humbucking effect at that point. If it doesn't help then reattach your jumper wire to the other side of the filament supply and repeat the process. If you notice any reduction in hum then solder a small short wire to the filament pin on that tube and lay it beside the cathode and grid of that tube and fasten it where it wont move. I've built dozens of high gain tube amps with very detailed attention to filament wires and star grounding and still ocassionaly I have to do this to one or two stages. I've actually noticed on PC boards in some manufactured tube amps where they run a copper trace around a grid and cathode to quiet it down. With a little experience you can build a tube amp with little or no hum.

[Enzo]

Perhaps I misunderstood. You are not moving where the wire is grounded? You are instead moving the wire around but leaving its connection point alone? My earlier comment was aimed at the former.

One comment comes to mind either way. having a lot of excess wire length to move around is a potential problem right there.

usually it is grid wires that pick up the hum, not cathode wires.

[cbarrow7625]

I would guess that there is no cathode bypass capacitor on that stage. Is that correct Anson?

Without a cathode bypass cap, the cathode also will act like a secondary input to the tube stage and can amplify any hum / stray noise that is picked up in the ground wire and/or cathode resistor.

If there is a cathode bypass cap, what is the value? If it is small, you may not be going low enough in frequency to help filter out 60-120Hz.

+1 of what Enzo said. You absolutely cannot have really long ground wires on anything. Wires & resistors are nothing but antennas, the longer they are the better the antenna they make. Wires can pick up all kinds of stuff you don't want that is floating around inthe chassis.

Make that wire as short as you absolutely possibly can. Try to ground it to the same point as the grid leak resistor for that same stage if you can (but being as short as possible will be of greater benefit if that causes you to make it longer).

[anson]

Hey guys, thanks for all the awesome replies. Sorry it took a month to respond to my own thread, I've just moved to Australia.

Anyway... I am deeply interested in the advice that Twist gave me regarding the use of a trace connected to the filament in the way he described. I'll try it soon, and then give my results to the forum. Why does this trick work? From your description twist, i would guess that the trace induces a 60hz hum out of phase on the grid and cathode that are in question. Is that it? My amp is a 4x EL34 setup with a pretty big PT and OT, but neither one of them are shielded. In other words, there are no bell housings. Would this make a difference?

And one more thing... unfortunately no amount of cathode bypass capacitance seems to help my hum situation. Actually it makes it worse. My guess is that this is because the bypass capacitor allows a higher gain to be produced in the tube stage which produces hum. Also, I've tried an assortment of decoupling capacitor arrangements and none have helped the hum problem.


...once again thanks for all your input.
Anson