Killing 60Hz Hum

[The New Steve H]

My new Bassman 5f6a clone is up and running. My only issue now is 60Hz hum. Seems like I'm getting more of it than I should. It doesn't dominate the signal, but I would rather not hear it at all.

I used 3 star grounds. I could have reduced it to 1, but I felt like I had enough wire in the amp already. Each star is attached to a power transformer mounting bolt under the chassis.

I tried to fix it so there were no places where any wires went through loops created by the grounds. I put the ground wires as close to the metal of the chassis as I could.

There is a lot of loopy-looking ground wire on the inputs. I made the ground in such a way that it's one continuous wire running through all inputs and then to a star, though, so it should not form a loop. I guess there are unavoidable loops between the chassis and the wires and lugs, though.

The tubes are all above the chassis. I'll put up a photo. I stuck a shield on the first preamp tube, figuring it was the easiest place for noise to get in. I have shields for the other two preamp tubes, but I haven't installed them yet.

The capacitors have no metal pan.

The input jacks are the shorting type. The tubes are 12ay7, 12axx7, 12ax7, and 6L6, in that order, from input to output. The rectifier is a Chinese GZ34 backed up by 4 1n4007 diodes. The power transformer is a Classictone 18029 with 2, 4, and 8 ohm outputs. The output transformer is a Classictone 18000.

The wires attaching various components could be shortened a fair amount. I didn't want to make the assembly impossible, so I left some slack.

Right now I'm playing the amp through a 25-watt 12" Weber Signature speaker. The volume is on about the 2 setting, and it's about all I can take. It sounds a little dark with single coils, but it really sings with the humbuckers on my old Burny Les Paul clone. The sound is very detailed in spite of the hum; you can hear every noise I make when I touch the strings. Seems like the amp is very dynamic; the difference between picking a string lightly and picking it hard is very big. Of course, I'm used to practicing on a Firefly, so I guess I should expect a difference.

Check out the sexy legend plates.

[Zippy]

My new Bassman 5f6a clone is up and running. My only issue now is 60Hz hum. Seems like I'm getting more of it than I should. It doesn't dominate the signal, but I would rather not hear it at all.

I used 3 star grounds. I could have reduced it to 1, but I felt like I had enough wire in the amp already. Each star is attached to a power transformer mounting bolt under the chassis.

I tried to fix it so there were no places where any wires went through loops created by the grounds. I put the ground wires as close to the metal of the chassis as I could.

There is a lot of loopy-looking ground wire on the inputs. I made the ground in such a way that it's one continuous wire running through all inputs and then to a star, though, so it should not form a loop. I guess there are unavoidable loops between the chassis and the wires and lugs, though.

If you'd like help sorting out the noise issue, a gut shot would be more instructive than a shot of the outside.

[surfsup]

I made the ground in such a way that it's one continuous wire running through all inputs and then to a star, though, so it should not form a loop.

Just b.c its one wire doesnt mean there cant be loops. Do you have a gutshot?

[The New Steve H]

It's like spaghetti in there but I'll see what I can do.

[Gibsonman63]

You might double-check your input jacks to make sure they are really shorting when no cable is plugged in. Is it humming with no guitar and cable plugged in at all? If not, then it's not the amp.

[The New Steve H]

Okay, be gentle. I am not what you would call a skilled soldering technician. I burned a few things and created some serious globs, and I was pretty generous with the wire.

[The New Steve H]

I'll see if I can make life easier.

The black and white twisted wires go to the heaters. I tried to stick with red, white, orange, and yellow for stuff that was relatively positive, and I generally used black, green, and various blues for grounds and wires on the negative sides of things.

The star ground wires are generally green. I think I may have run some of the heater wires between grounds and chassis.

The black and white wires go from a 6L6 to another 6L6, and then they go phase inverter, second preamp tube, first preamp tube.

The giant ceramic cap on the pot is there because I misplaced the expensive 100 pF I intended to use. I believe it's an 88 pF. I paralleled a couple of mica caps on another pot for the same reason.

There are a couple of vacant eyelets. The big empty place at the end of the board was left there in case I needed it for anything.

[ChrisM]

To start I would have rotated the PT 90 degrees.

[Gibsonman63]

Do you have a cover for the chassis? I would try putting it on and see if the shielding helps. I have a flourescent light on my workbench. I have a habit of forgetting to turn it off when I am testing and it generates quite a bit of hum. You might try turning the amp on and poking around with a wooden chopstick and see if find the wires that are giving your grief and move them around a bit.

[The New Steve H]

I haven't made a cover yet. I was thinking I might make something out of metal to cover the bottom and ends and then put some kind of roll cage over the top. You make a good point. The amp was noisier when it sat next to my soldering iron.

I am wondering if it was a mistake to ground the inputs the way I did. It seems like I basically built an antenna to help hum get to the inputs. The jacks are self-grounding, so I initially thought I didn't need wires. I added wires because I thought an input wiring problem might be killing the sound.

I could also move one or more stars away from the PT.

I don't know if I need more shields or not. My guess was that once the signal had been through one tube, it would be big enough to overwhelm noise.

[Structo]

Generally you want your power supply caps to ground down near the power transformer.
It's not a good idea to use the transformer bolts but many people do.

I'm not sure you understand what a ground loop is.
It is not a physical loop of wire but rather a invisible return path of current traveling between ground points, it's the resistance between the points that causes a voltage drop and resulting hum.

Then most people utilize a ground buss wire that runs either along the top of the pots or along the edge of the circuit board.
Then you use small jumper wires from the pots to the buss.

I think your main problem is lead dress resulting from a very cramped chassis.
I think you would have been better off having all the tubes in a row at the bottom of the chassis, then have your board so it is not over the tube sockets.

Ideally you want to keep the heater wires away from any signal carrying wires.

You should probably use a shielded wire from the input jack to the first tube.
Ground the shield at the input jack only.

I have a similar cramped chassis made from an old organ amp.
This amp is actually pretty quiet so I know it can be done.

[Tone Lover]

Grounding is very important go to valve wizards site and look at his idea,s, It will help you understand ground loops he has nice big pictures that helped me really understand it, when I thought I already did.
use the search here. lead dress is very important also.
Dont forget to check your tubes v1 if its a noise tube can give you problems. recently I built a marshal poweramp with an slo pre amp had hum drove me crazy I couldnt find it
I rewired everything it was a bad cap finally found it. bad orange drop right out of the box never had one before.
Bill

[Firestorm]

In order to make the star ground thing work, it's usually necessary to isolate the jacks from the chassis so you don't create a second path. That said, you may not have to go that far: Fender used random grounds forever with not too many ill effects. I think it's a mistake to run your preamp grounds (input jacks, preamp cathode resistor and caps, preamp PS cap etc.) back to the main filter grounds. That just takes the noisiest ground in the amp and connects it to the input. I'd lift that connection to the PT bolt and move it to the input jack ground. Tom's suggestion of making a ground buss there is a good idea.

Your grid wires are longer than they should be (made even longer because you've got the circuit board flipped over) so shorten them up and maybe use shielded wire if you have to.

What is the bypass cap on the first 820R cathode resistor? I am sure that can't be the stock 250uF. Looks more like 250pF (but I can't read the printing). If so, there's another potential hum source. If the bypass cap is relatively small (think of the .68uF on Marshall bright channels) any filament hum in the preamp (even though they're wired "humbucking")doesn't get bypassed to ground and is preserved and amplified.

[Phil_S]

Do you have a cover for the chassis? I would try putting it on and see if the shielding helps. I have a flourescent light on my workbench. I have a habit of forgetting to turn it off when I am testing and it generates quite a bit of hum.

+1 on this. All sorts of interference come from outside the amp.

Cut cardboard from a corrugated box a little larger than the amp. Smear with glue. Apply aluminum foil, the kind you find in the kitchen. I like the heavy duty stuff, but it doesn't matter. Place this over bottom and see what happens. I got a real surprise from doing this on more than one amp.

[The New Steve H]

I admit, I am lazy about studying. Here is how I understood ground loops. You unintentionally create a circuit, involving ground connections, which is susceptible to induction. This results in a flux of fields through the circuit. This causes unwanted movement of electrons in stuff connected to the ground, which makes noise. This vague notion, correct or incorrect, is why I tried to keep anything carrying varying current out from between ground wires and from between ground wires and the chassis.

Back to Merlin Blencowe's book, I guess.

Isolating these jacks would be a real pain. I would have to put nonconductive washers on each end of each jack, and then I would have to open up the holes and insert some kind of nonconductive sleeve. I hope that won't be necessary.

The 820-ohm-paralleled capacitor...you caught me in a major brainfart on that one. You are absolutely right. It's 250pF. I'm very absent-minded, so I make this kind of mistake a lot. After the debacle involving using the wrong resistors, I went through the circuit and checked a whole bunch of stuff, but this one got by me. I'll fix it. Thanks.

I like the idea of shielded grid wire. Can't hurt. I was thinking of doing that anyway.