Help me find the hum...please?

[Ripthorn]

So I just finished this build and the tone is great. The only thing that bugs me is that there is a strong hum, even on the clean channel. If I touch the PT or one of the switches, it lessens, but is still prominent. If I roll the guitar volume all the way down, it goes away almost entirely, which suggests to me that it may not be a power supply issue (though I could be wrong, that is why I am asking). But if it isn't a power supply issue, I am not sure what else it could be short of interference coming through the guitar pickups (which I would hope not, for how expensive it is).

Anyway, I am looking for suggestions. As for a little more info on the amp, it is a submini amp, two channels with a switchable boost. Cathodyne PI, push/pull 5902 tubes. The power supply is my own design using Duncan's PSUD program. It goes 47u cap - 10H choke - 220u cap - 1k resistor - 100u cap - 1k resistor - 47u cap - preamp plates. The heaters are all DC. All of the hum comes from the preamp at this point, if that helps any. I am not sure if it is 120Hz or not.

So, any ideas? Seeing as how this is my first build, I am willing to consider anything.

[Ripthorn]

Some additional findings:

When the volume is all the way rolled off on the guitar, the hum is very low level and is 120 Hz. When the guitar volume is up, there seems to be some higher overtones, a kind of buzziness to it almost.

If I touch one of the panel mounted switches or the PT, the hum is greatly reduced, moreso for touching the switch.

Anyway, if you have any thoughts, I would appreciate it.

[Kregg]

Generally, when you touch metal components and a hum goes away it's a ground issue. Check the schematic with your amp. Make sure it's properly grounded to the chassis. If the chassis is painted scrape away the paint beneath the ground bolt. Is the 3-prong plug wired correctly? Whip out the chopsticks and see if moving ground wires (just a little bit) improves or stops the hum. Does it hum before the guitar is plugged in?

[Firestorm]

If it's 120Hz, it's something on the post-rectification side of a full-wave rectifier. How did you generate the DC for the filaments? You might want to measure the AC component on the DC filaments to see if you're injecting the hum via the heaters.

[Ripthorn]

When the guitar is unplugged, the clean channel is dead silent and the distortion channel has some 120Hz stuff that I will investigate, but the main hum is when the guitar is plugged in and its volume is up. Even on the clean channel, the hum is very prominent. I imagine there is some issue with the power supply due to the 120Hz noticeable on the dirty channel, but this other hum is much more annoying and much more mysterious to me. I will look into your suggestions when I get home this evening and report back to see if we can get this fixed. Thanks for the help so far.

[Andy Le Blanc]

A good thing to do with a grounding scheme is to give the rectifier and first
and second filter grounds each their own ground wire to your ground point
and try like hell to keep grounds along the signal path off the chassis as a
ground plane, the input of the first gain stage at the very least. The last hum issue
I resolved was all about that. I used to hate plastic input jacks, but they get
you off the chassis and avoid the issue.

[Structo]

That was going to be my suggestion, to take a look at your input grounding and preamp grounding.
If you are using a ground buss, it should be grounded to the chassis down by the input jack which should be grounded there as well.
Sure there are other schemes but that has worked well for me.

[paulster]

Have you got your DC heaters ground-referenced? They'll hum like crazy if the heaters aren't referenced to ground somewhere along the line, be it via the centre-tap of the heater winding, a pair of 100R resistors on the AC side, or the DC 0V side being tied to ground (note: only one of the three or you'll be at risk of shorting out your rectifier).

Also, if it's practical, try running your preamp heaters off a 6V lantern battery to see if you're getting noise through your DC filtering for the heaters. The buzz sounds like it could well be the triangle wave you get from insufficient filtering.

If the lantern battery isn't practical then see if you can wire in a switch so you can kill the heaters temporarily once they're up to full temperature and the amp is running. They'll retain enough heat for a second to tell you if this is the source of the noise before you switch them back on or kill the HT.

[Ripthorn]

The DC heaters are ground referenced. The grounding scheme is a quasi star ground. The HT power supply and power amp are grounded to the chassis close to the IEC socket, the DC heater supply is grounded to the chassis near the front (about 4" away from the ground point for the HT supply). The preamp is done on pcb (these are submini tubes) and the whole preamp shares the same ground and is grounded about mid way along the chassis length wise. The input jack is grounded via a buss all the way over at the DC heater ground point (it is grounded to the relay switching board which is grounded at the DC heater ground point). All grounds seem okay (referencing them to the center pin of the IEC socket).

I will try grounding the input. I am wondering if there is such a thing as too much HT supply filtering (see my post above). Thanks all for the suggestions, I will see if I can get this straightened out soon.

[Ripthorn]

Well, I just looked into it a little more. Grounding the input closer the jack on the chassis didn't affect anything. Also, I wasn't able to measure the AC voltage on the DC heaters (do I need a blocking cap or something?). The one thing that I found quite interesting is that I decided to change guitars just for the heck of it from my PRS to a guitar that I built with active emg pickups. The hum was reduced by a very good deal so that the clean channel is very silent, but the dirty channel still has a good deal of hum to it. If it would help, I could probably post a sound clip.

[Structo]

This may or may not be applicable to your amp, but the other day I did some re-wiring in my D'Lite amp to better the lead dressing.
I also piggy backed two 100uf caps on top of the existing 100uf filter caps for better filtering.

I buttoned it back up and put the chassis back into the head cab.

So I turn the amp on and plug a dual humbucker guitar in.

I have a home made pedal board with a Dyna/Ross Comp (this is a homemade compressor), Dunlop Crybaby into the front of the amp and a Boss RV-5 and EHX Stereo Memory Man in the FX loop.

I start playing and step on the OD foot switch and it starts humming like a kazoo. When I stepped on the boost button it really became buzzy/hummy.

I think, man what did I do!
So I start messing around with the pedals and find it is the Dyna/Ross Compressor that is introducing the hum. Never did this before so I'm not sure what happened but when I took that out of the chain, all became quiet again.
So I'll take the Comp pedal apart and see if something changed in the grounding inside.

All this to say, sometimes coincidences happen that lead you in the wrong direction for whatever reason.
Reduce everything down to a known setup and go from there.

[Ripthorn]

Thanks for the tip, Structo. Unfortunately, I don't have any pedals going right now. When the preamp was on the breadboard, there was some noise (though I am not sure that it was this much), but I attributed it to the circuit being on the breadboard. Maybe there is something going on that is a flaw with the design. That would be lousy, but possible. I will keep working on it and see what I can do. I would hate to have to scrap the project seeing as how it has taken me four or five months to do.

[Kregg]

How about posting some gut shots? It couldn't hurt.

[Ripthorn]

I will see if I can get some gut shots, but you might just shoot me. I think rat's nest would be an understatement. Though I did find some hum on the heater supply for one of the tubes, so I got that reduced by a good amount. I think the majority of what is left is actually coming from the guitar now, seeing as how with the guitar unplugged, both channels are acceptably quiet.

[Ripthorn]

Well, I think I got it. Thanks to all of you who have helped, the ideas and suggestions really got me to question myself regarding each part of the amp. So I was messing around in there some more and realized that if I touched a ground buss wire to the casing of the guitar cable, all signal went away. This really puzzled me because I knew it should be ground. Turns out the input jack was wired wrong. I have wired a bazillion jacks and this is the first one I have ever wired backwards and stuck in a project. So even the dirty channel is quiet as a mouse. Dirty+boost has a little something on it, but that is not much of an issue (I expect it some with five gain stages).

There is still some hum or noise that makes itself manifest on the dirty channel after playing for a while, but I think that is due to the fact that my voltage regulators for the DC heaters aren't getting rid of heat fast enough (and thus reducing heater current gradually over time, creating noise and degraded tone). I have heatsinks on them, but I think I am going to get some thermal epoxy and heatsink them to the chassis.

Again, thanks to everyone, you are very helpful and patient with the guys like me who might otherwise be the subject of ridicule.