Help! 120Hz. hummmmmmmm

[Heater]

Hey All,
I'm just about finished debugging my Hybrid A build. So far, it sounds quite good and I'm very happy with the tone! But, I've got a 120hz hum all of the time. It seems to get worse with the "volume" (after CL1) at the first or last third of it's travel. The hum is least when the volume is in the middle third. If I pull V1, the hum is still there. If I pull V2, it's gone. So, it's somewhere in the preamp. Mid boost and tone stack bypass only make it worse. Do you think that I have bad filter caps in the PS on B+5? Lead dress is pretty good and heaters are elevated with 100r's to ground. I've managed to take care of the other little issues, but this one is eluding me.

Thanks!
Chris

[Bob-I]

Can you post some pics? especally of the offending sections and heater wiring.

Also, how did you wire the heaters? Centertap or false centertap using 100 ohm resistors?

[Heater]

Hey Bob,
The heaters are wired false center tap using 100r resistors. I will try to get some pics posted.

Thanks!
Chris

[Bob-I]

Hey Bob,
The heaters are wired false center tap using 100r resistors. I will try to get some pics posted.

Thanks!
Chris

I'll assume you've double checked your grounds, especally around the tone stack and volumes. Next I'd disconnect the heaters and connect a 6V lantern battery, don't forget to disconnect the 100 ohm resistors..


Wait... first put a meter on the 100 ohms. I screwed up once and put a 100k (damn colorblindness). It hummed like it didn't know the words.

[paddy]

Hey Chris
Have you tried swapping out the CL2 tube?
Cheers
Paddy

[Heater]

Hey Guys,
Thanks for all of the suggestions! I will try the lantern battery and double check grounds and R values. I have tried swapping tubes-no dice. I will get pics posted today.

Thanks!
Chris

[Tdale]

Probably not the case, but in my last build, I tried just about everything, and then discovered that the star ground point was not screwed tight enough to the chassis....

I poked around with a screwdriver... with plastic handle, and discovered it when I touched the grounding point..

Tommy

[Bob-I]

Oh, also post pics of the controls and grounds. I've seen bus bars soldered to the back of the pots cause ground loops, I prefer a bus bar that's not connected to the pot casing at all, and only grounded at the input side.

[Tdale]

Do most of you guys use a bus bar?

I've used one wire for each ground, all connected in one point at th - side of the filter caps. Sometimes a few grounds are wired together, like the cathode cap and resistor... Still, lots of wires!

Here are som pics
http://www.acplink.com/chassis/display.aspx?curpage=229

Tommy

[Bob-I]

Do most of you guys use a bus bar?

I've started using a bus wire above the pots. I solder it to the input jack which is grounded, and pass it above the pots, with a small non-insulated wire to the ground connections on the controls as needed. This wire is suspended over the pots by these wires. It's only grounded at the input jack.

I've used one wire for each ground, all connected in one point at th - side of the filter caps. Sometimes a few grounds are wired together, like the cathode cap and resistor... Still, lots of wires!

Yes, I've done that and there are way too many wires. I'm concerned about all the stray EMI floating around the chassis that way.


Here's a pic of a build I'm working on.

[img:640:303]http://www.patmedia.net/bob-ingram/DClone/PPic4.jpg[/img]

[Normster]

"Small" bus wire? Dang Bob, I get away with using 18AWG.

I'm with Bob on this one. A simple bus wire grounded near the input is clean looking and hardly ever causes ground loops in D-style amps.

[Tdale]

OK, so the bus bar is just for the grounding of the pots and components close to it, while the filter caps etc. are grounded at a separate point?

Or is everything connected to the bar, and grounded at the input jack, as the only grounding point?

[paddy]

Hi Chris
here is a link to some possible answers

http://www.geofex.com/ampdbug/hum.htm

Good luck

[Bob-I]

"Small" bus wire? Dang Bob, I get away with using 18AWG.

I never said small

That's 16AWG that I happened to have around. Works really great.

[Heater]

Hey Guys,
I just finished rebuilding the ground buss for the preamp. The pre ground buss is now grounded only at the point of the input. No other grounds exist for the preamp and all controls. That seemed to help the hum. We still have some noise though. I'm going to investigate the power supply and output section grounding next. I'll keep you all posted.

Thanks!
Chris