Killing 60Hz Hum

[thejaf]

If you're inputs are grounded to the chassis, then I beleive you're creating a loop by running separate grounds that go back to the star point at the PT.

[The New Steve H]

This is what I was afraid of.

Right now I'm putting a 220uF cap in the place where I currently have a 250pF. If that doesn't get me anywhere, I'll snip the ground wire on the inputs and see what happens.

[The New Steve H]

I didn't have a 250uF cap, so I stuck a 220 in there. Still hum. Going to change the input ground deal.

[The New Steve H]

Cutting the grounds seems to make the hum worse.

[Firestorm]

You have to deliberately put the preamp grounds together: input, preamp cathodes, tone stack ground, 8uF PS cap ground. Make a little ground buss.

[The New Steve H]

Is it sufficient to put them together on their way into one of the star grounds?

[Firestorm]

You just have to ground them at the same point and away from the power amp grounds. Always works for me.

[NickC]

Is it sufficient to put them together on their way into one of the star grounds?

Good reading and diagrams on grounding schemes at these links:

http://www.aikenamps.com/StarGround.html

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

[The New Steve H]

It sounds as if it might be wise to make a common grounding point for all the small signal stuff at the input end of the amp and get it away from the PT.

[Structo]

Yeah, there are many methods and everybody seems to have a favorite.

For example, Dumble uses 5 points to ground the amp.

But all the preamp grounds go to a lug near the input jack, that's because that is the quietest place in the amp.
So all the preamp cathodes, pot grounds (ground buss), input jack, preamp power filter go to a lug near the input jack on the floor of the chassis.

The original Fenders had a brass plate behind the pots that things grounded to.
But that is not very practical these days and you can achieve the same thing by using a ground buss wire.

Here is a D'Lite amp.
See the ground buss wire above the pots?
There are short jumpers from the pot ground lugs, then it travels to the right and down to the chassis floor to a ground lug.

[The New Steve H]

I noticed that I had virtually all the preamp stuff grounded to a PT bolt, along with the bias circuit, which has a big rectifying diode in it. I figured that had to be bad.

I stuck the bias circuit ground on the same lug as the filter cap ground (PT bolt I just referred to). Then I put all the preamp grounds from the circuit board on their own lug and attached it to one of the mounting standoffs on the input end of the board, by the 820 ohm resistor. I figured this would be a good test point before drilling a hole and putting a new bolt through the chassis. I also ran the bias ground wire behind the output hots so there would not be a closed loop of ground wire around them.

The pots are all grounded on the other side of the amp. I can move them to the same point as the preamp stuff, or to the filter cap ground, but I am reluctant to run new ground wires that far. The old ones are long, but they lie nice and flat on the chassis, away from most of the crap, and that makes me happy.

I have been told that grounding the pots to themselves is not good, and that's why I used wires.

The amp is humming a lot more quietly now, but still annoying, so I will continue making changes.

I have the power light and the AC ground on the same lug. Maybe that's stupid. I can move one or both to the main star ground, but I was told that you want the AC to have its own ground.

How important are tube shields on the remaining preamp tubes? I hate to cover them all up. They're so pretty.

I have to say, this amp has a very nice sound, apart from the hum. Super detailed and clean, at the microscopic volume level at which I am currently able to play it.

I will either fix this hum or I will make the amp so sensitive to noise that radio astronomers start bidding on it.

[The New Steve H]

SUCCESS!

Here is the story.

I removed the star ground from the back of the amp (pots) and turned it around so it could be attached to the same ground as the one from the front (preamp). This is on the input end of the amp. I rewired the pots so they turn in the right direction. I moved the pilot light ground off the AC receptacle ground and put it on the filter cap ground (PT bolt) with the bias ground.

I also replaced the cheesy 88nF ceramic capacitor on the presence control with a 0.1uF Orange Drop, and I ran it to its own ground wire.

I haven't shortened the input leads or substituted shielded wire yet.

I fired up the amp and still heard some hum, so I turned on my Blues Jr. and discovered that the Bassman is actually quieter. That's good enough for me at this moment, although I am going to keep modifying it, using the suggestions everyone gave me.

I love this amp. It sounds good at practice levels, and for a geezer like me, that means not loud at all. The sound is hot, sweet, detailed, and punchy. And that's on 3. I won't know what it sounds like cranked until I get my new speaker put into a cabinet.

It's considerably nice than the Blues Jr., which I have always enjoyed. It seems less muddy, more dynamic, and sweeter. It's just about exactly what I like in a guitar amp. Everybody pooped on me when I bought my $45 Weber speaker, but sounds great, so I can't complain.

I appreciate everyone's patience. I hope I can become a useful member of the forum and repay you to some degree.

[diagrammatiks]

did you drill that capacitor board yourself?

that idea actually looks pretty good in execution.

[The New Steve H]

Everything is 100% from scratch, which is why I had so many delays! This is how it is when you know almost nothing but you think your own ideas are better than Leo Fender's.

Here is the sad thing about the capacitors. I made the board maybe 3/8" narrower than it should be, so the leads touched the cases. That's what killed my first two or three fuses. I know this sounds stupid, but it never occurred to me that the cases would be electrically connected to the guts.

I didn't do a fantastic job of locating the holes on the circuit board, so I had to open them up a little, and I have to work to get the screws in.

I'm amazed at how good this amp sounds, and how quietly it can play. I never expected to be able to use it. I was just building it in order to learn. I may retire the Firefly and practice with this.

[briane]

glad you got it!

youll find (in time) just about everything can contribute to additional humm. That said, a great build should not humm at all with no input, or a guitar that is built correctly (I'll just leave it at that for now)...

so.
-grounding. Separate one for pre and power.
-short wires. less is better
-layout
-orientation of OT to PT & distance is a big one. Ussually its best if the OT is also 45 degrees off the PT

You have to be really ***lly retentive to build an amp thats perfectly quiet!