What to do about hum?

[joebob]

I just finished an AX84 p1_ex and it had bad hum. After several hours of trying everything under the sun I scraped the ax84 grounding scheme. I then follwed the Hoffman style and the hum went from about a 2 to a .1 on a 1-10 scale. It is now extremely quiet.

http://www.el34world.com/charts/grounds.htm

['67_Plexi]

What do I do if I've tried all the tricks to reduce hum and none of them work? It isn't a low level hum but a significantly loud hum.

1. Created Virtual CT with 100 ohm resistors. Quieter but not enough.
2. Tried raising the CT to 50 volts, more noise than if grounded.
3. Moved leads around all over without one iota of difference to the hum.
4. Hum level doesn't change with volume control up or down..
5. Turn reverb level up and hum increases some.
6. Unbolted reverb xformer and moved it around. No diff.
7. Changed tubes to no avail.
8. Ground out v2 output after cap. Dead quiet. Ground anywhere on tone stack and no diff.
9. Redid PS with diff caps. No diff.

At wtt's end. Please advise before I throw it out the window.

Rut

If nothing being moved chages the hum, it's not an induced one from an external source (like the filaments). A hum like this exists because of a heavy ground current path in the chassis being shared with a ground path from a senstive circuit area such as the first gain stages.

The biggest mistake I see often that can cause hum is the OT secondary ground not being connected directly to the speaker jack, enabling all that current to travel half way across the chassis, pissing off everything in it's way. Connect it directly to the jack, then run a ground wire from the jack to the same point the prescence/PI circuit grounds. Only a very small feedback loop current will flow through this wire, all the heavy stuff has a direct path out of harms way.

['67_Plexi]

The last message posted twice, so I'll just edit and smile instead on this post, because it's not possible to delete a post...I don't think.

[jaysg]

I have an old Fender amp with a hum balance control...100 ohm, 1 or 2W CTS pot....whatever they used. Clever me decided to free up the hole by removing the pot and replacing it with resistors. To my chagrin, the lowest hum position wasn't all that close to the middle. I asked the sages at Ampage and was told...'yeah, that happens.'

[rutledj]

I just finished an AX84 p1_ex and it had bad hum. After several hours of trying everything under the sun I scraped the ax84 grounding scheme. I then follwed the Hoffman style and the hum went from about a 2 to a .1 on a 1-10 scale. It is now extremely quiet.

Just tried this scheme. No difference whatsoever.

The biggest mistake I see often that can cause hum is the OT secondary ground not being connected directly to the speaker jack

Mine is.

[rutledj]

One other thing I've really just noticed is that the PT gets very hot, as in too hot to touch. This is a hammond 372jx, One honking xformer size wise. Nothing seems to be drawing too much current and all the voltages look fine. Perhaps this is normal?

[rutledj]

Put my scope on the output of v1b cap. Got a 60hz signal there so I guess it is definitely heater hum. Pulled v1 and hum decreases considerably. Ground it out (with v1 in) and dead quiet.

Running out of ideas.


BTW, I don't have a separate ground wire running from the spkr jack to anywhere. The jack is grounded to the chassis at that point via the mechanical connection. Tried jumpering the gnd from there to diff points with no effect.

[heisthl]

Unless you are running a mismatch (like a 16 ohm speaker on the 2 ohm tap) your transformer should only run hot when really pushing the amp for longer periods of time.

[drz400]

Run a DC heater supply like this
$30 and no hum, no transformers either
237-1295-ND Digikey

[heisthl]

Run a DC heater supply like this
$30 and no hum, no transformers either
237-1295-ND Digikey

How do you find room to fit a 5" x 3" x 1.65" open frame P/S in a chassis?

['67_Plexi]

Put my scope on the output of v1b cap. Got a 60hz signal there so I guess it is definitely heater hum. Pulled v1 and hum decreases considerably. Ground it out (with v1 in) and dead quiet.

Running out of ideas.


BTW, I don't have a separate ground wire running from the spkr jack to anywhere. The jack is grounded to the chassis at that point via the mechanical connection. Tried jumpering the gnd from there to diff points with no effect.

Heater hum shows up at double the mains frequency, ie 120Hz, not 60Hz.
Most likely your grounding scheme on the preamp stages is not ideal. Without actually seeing it, it's impossible to diagnose properly.
For what it's worth, I don't use the grounded switchcraft jacks anymore and use the physical jack to chassis as a ground connection.
I use the the cliff type for everything now, isolation is easier and it also makes compliance with various export markets easier because people can't poke something right through a cliff jack in to the chassis and get zapped like they can with an open frame switchcraft.

[rutledj]

Well, if it helps, my preamp gnd scheme is very simple. All pot gnds and input jacks are grounded via a buss wire running across top of pots and grounded to chassis at input jack. All cathode grounds (except output tubes) are also grounded at this point. The filter cap for the B+ to v1-PI is grounded at this point.

The Power amp tube grounds, Filter caps for b+1,2, PT centertap, bias ckt gnd all go to a chassis gnd near the PT.

Again, I have a Dumble with the same wiring scheme and it is dead quiet.

[drz400]

Run a DC heater supply like this
$30 and no hum, no transformers either
237-1295-ND Digikey

How do you find room to fit a 5" x 3" x 1.65" open frame P/S in a chassis?

I have put them in the side wall of Marshalls (slightly smaller different brand), plenty of room. Bogner uses a similar unit in the Shivas.
You dont need that much current if you are not doing the power tubes and can certainly find smaller units if that is a problem. You can also of course do a simple 12V regulated supply for the preamp tubes but no need to do the phase inverter. Downside is adding a transformer but you could have the same supply drive switgching. No substitute for good grounding and layout but DC heaters sure do make tube selection and really the only way to make a gainy amp totally hum free.

[rutledj]

If you use a 12vdc power supply do they just hook across pins 4 & 5 of the 12xx7 tubes and pin 9 is NA?

[mdroberts1243]

Put my scope on the output of v1b cap. Got a 60hz signal there so I guess it is definitely heater hum.

Heater hum shows up at double the mains frequency, ie 120Hz, not 60Hz.

Hi Alan,
I think you got the 60/120Hz flipped around... 120Hz hum is always related to the rectified DC supply (B+ ripple is double the mains frequency due to full-wave rectification).

Hi Rut,
The 60Hz could be electromagnetic induction from heater wires or other AC wires (primary/secondary) due to poorly twisted pairs, etc. or ground loops (poor grounding technique)... not just the heaters themselves.
Alan's ideas about grounding and ground loops through the chassis are bang on and the most likely culprit IMHO. The Hoffman article is a good guide to distributed grounding.
I wouldn't think you'd have to go to regulated DC heaters.
Here's another article with some thoughts on hum reduction: http://www.lynx.bc.ca/~jc/hum.html