Killing 60Hz Hum

[The New Steve H]

I got the best advice I could on transformer placement, and someone still says I got that wrong! You do what you can.

The great thing about getting to this stage is that now I have a wonderful amp to play and unlimited time to make little improvements in it. I'm hoping I can kill the remaining noise soon.

[diagrammatiks]

naw your transformer orientation is fine.

the laydown mounting style has the laminations perpendicular to the chassis already.

the choke will screen the output transformer from the power transformer anyway.

if anything I'd move the choke is that it sat right in center between the end bells.

transformers are so big that distance makes the least difference. you'd have to increase a lot more then the size of the chassis to get -3db of hum reduction.

[David Root]

Star grounds are designed to keep high current grounds away from low current grounds. By grounding all your star ground circuit nodes at power transformer mounting bolts you totally defeat the principle of star grounds. I assume your star grounds are at dropping resistor capacitor nodes, if not they should be.

Star grounds work if properly implemented. It would probably be easier for you to look at some Dumble circuit ground layouts, which are not galactic star grounds but work the same way in practice. In general strict galactic single star ground system doesn't work too well in anything much over 50W in my experience. Don't know why.

Basic principle: PT CT grounds and plate and screen node capacitor grounds to the same point, in the power transformer area.
All others ground near the input jack.

If you have global NFB from the OT secondary to the PI input, ground the PI capacitor node to the same spot as the OT secondary common. This assumes push pull fixed bias, if cathode bias the cathode ground goes to the plate/screen ground.

[The New Steve H]

I looked over Merlin Blencowe's ground loop chapter, and it seems like he confirms what I believed. Physical loops are a problem. I'm not an EE, so I don't know who's right, but he seems to know what he's doing.

He says the way to do shielding is to use shielded cable and ground only one end of the shield. Ground two ends, and you get a loop.

Honestly, I'm tempted to grab some desoldering braid, snip some off, and slip it over my input wires. That would be a whole lot easier than replacing everything with new wire. The braid is sitting around in my garage, and I never use it. I'm wearing out the solder sucker, though.

[diagrammatiks]

you can make a twisted pair first to see if it makes a difference.

if it does you can switch to a shielded cable later if you don't have any shielded cable handy.

[The New Steve H]

I went to the only decent local electronics store today. Shielded cable: $80 for 100 feet. I went and grabbed a 6' audio connector for $3. Shielded wire on the cheap.

Audioguru has a good price on the real thing, so I ordered 100'.

[The New Steve H]

Merlin Blencowe's second book arrived today. I knew I was in for trouble when I saw the subtitle: "How Not to Design a Power Supply Like Steve Does."

He says the way I opened my heater wires up to go around the sockets is no good. Man, I think I'd rather put a DC heater circuit in than redo the socket wiring.

[Tone Lover]

yea it sucks when you learn new things after you lay it out and put it together I just got done going through an amp I built 2 years ago rebuilt alot of stuff. my lead dress was awfull sad thing is as well as it is now its still awfull compared to some of you guys. (LOL)
Bill

[The New Steve H]

I'm steeling my nerves in anticipation of rolling it over and getting to work. I'm hoping there is enough slack in the wires to correct the problem without removing anything. IT COULD HAPPEN!

[The New Steve H]

Here's a good question for everyone's amusement. I'm taking the amp somewhere so a friend can try it in public. I'm sticking a wood base on it and putting wood over the ends to keep him from electrocuting himself. Is there any point in sloshing conductive paint on the wood as a temporary noise reduction measure? I know it won't do anything about noise coming from the amp itself.

[Firestorm]

You could try heavy-duty aluminum foil on the inside surface of the wood (just make sure there's nothing that could accidentally contact it.)

[The New Steve H]

Okay, Merlin Blencowe is the MAN. I read his stuff on "humdinger" circuits. I found a crappy old trim pot and stuck it on my heaters, and the hum is WAY down. Very nice.

Then I sat on my spare 12ax7. I hope it's okay.

[tubeswell]

Okay, Merlin Blencowe is the MAN.

That's what I've always said.

FWIW R.G. Keen also has a helpful memo on grounding here. Same principle as Merlin's Chapter.

[The New Steve H]

Took the amp to my church and let the guitarists try it out through a 10" Ragin' Cajun speaker. Sounds magnificent. The speaker sounds bright at home, but in a bigger room, it's fine. Very exciting to see my first big amp in action in a large space. Tomorrow one of the guys will be using it in front of a thousand people or so.

[Structo]

Yes, Dan Electro among other amp companies used wooden end blocks on their U shaped chassis.

They had foil glued on them then wood screws to hold them on.