Star grounding with a cap can

[dude]

Awful 60 cycle hum in the Ampeg I just fixed. I changed nothing except replaced the cap can, 70, 40, 40 with tree single filters all grounded at the same point as the old cap can. I replaced the OT too. Amps plays great except 60 cycle hum. I read-up on Aiken's Star grounding methods, everything.

I know the amp's grounding method is the same before I open the amp up, according to Randall I need to separate the grounding stages, power from the preamp but this mid seventies PCB bd, has  it's own ground scheme and it's not Aikens. I have all three filters grounded at one point, power supply and preamp, not good. I have hum. In these older PCB bds how does one separate the different stages..? I don't want to start scraping the traces out...?

The old cap can had all three filters grounded at the same point, same as three singles I have now. It didn't hum before I replaced the OT but if anyone remembers that original OT had one sec winding with two black and two green wires tied to the same winding, the replacement didn't. So I wired it like any Fender with neg feedback. Hum. I isolated the speaker jack running the common ground to the PI ground where the feedback loop ended. Still hum. Grounded the "chassis grounded inputs" to the first preamp stage too. Hum

If this was a PTP amp, no problem but what's one do with an older Ampeg PCB with grounds running all over the place...? Maybe this amp always had a hum but at high volume it's annoying, with reverb and tremolo even worst. 

Anyone shed any light on the subject?

[eleventeen]

I think I would start by removing tubes backwards from the output stage, one tube at a time, in an effort to isolate the hum to one particular stage. I would examine whether the PT has a center tapped filament winding, and if not, I would create one with a pair of 100 ohm resistors.

Is the hum sensitive or proportional to the rotation of the volume control?

Got a scope? Trace.

From your description, I'm finding it had to understand why there should suddenly be hum if all you've done is to replace a can cap with three singles. Did you remove something that was grounded to the case of the old triple-cap?

[HotBluePlates]

Awful 60 cycle hum in the Ampeg I just fixed. I changed nothing except replaced the cap can, 70, 40, 40 with tree single filters all grounded at the same point as the old cap can. ... Amps plays great except 60 cycle hum. I read-up on Aiken's Star grounding methods ...

Forget star grounding. The amp worked at one time with relatively low hum, exactly as it's built now. That proves grounding method isn't the culprit.

60 cycle hum? You sure? If so, it has nothing to do with the filter caps or high-voltage supply. 60Hz could only be coming from tube heaters, because the high voltage ripple after the rectifier is 120Hz.

But let's say it might be 120Hz hum... you said, "replaced the cap can, ... filters all grounded at the same point as the old cap can." How did you accomplish the grounding? Solder the cap "-" ends to a ring terminal and bolt to the chassis? If you tried to solder them to the chassis, you need at minimum a 100-150w solder gun, not a regular solder iron.

If you used a solder iron, the joint is probably cold. Take a flat-blade screwdriver and push at that joint like you were using a chisel. Dig in hard. If you had sufficient heat, nothing you do will dislodge that joint. If it was cold, it will probably pop off quite easily. I found out all this last bit the hard way a long time ago when I tried to move/resolder some grounds in a Fender amp with grounds soldered to the chassis.

[dude]

OK, maybe it's not 60 cycle hum, the heaters are lifted with a CT to the power R and cathode +, I thinks it 14v. as the schematic shows

The hum is there with no load, no guitar plugged in, when I turn the neg feeback on, the hum is a little less. But If I turn the volume, reverb or tremolo up the hum or feedback gets worst. It's playable but makes loud humming when idling with volume half way up.

The old cap can had two of the 40ufs grounded with the bridge rectifier ground point to the bd, no CT on PT. The other 80uf ground was tied to the OT common but all those grounds are grounded at the can's tabs, nothing was bolted or soldered to the chassis at that point so the ground for the filters were somewhere on the board....? I think probably through a standoff holding the bd. down...? I can't see any bolted or soldered ground point any where on the chassis except maybe a standoff holding the bd.

Another ground ran down the from the can to somewhere on the bd to the the preamp section. All pots and inputs were grounded by contact to the chassis and cases, I made sure they whee tight.

When I first replaced the three filters I had no hum but that weak OT problem, only thing was the old OT had one 8 ohm winding with two blk and two green sec. tied at the same point under the insulation, the new one didn't. I wired the new OT green to tip, ground to sleeve on isolated speaker jack mounted on the classic. I tapped off speaker jack's tip to a switch then to the neg feedback right before the 270K. The common I ran to the PI's cathode ground rih tafter the 8.2K R (end of feedback loop)

I used a long stand-off over the cap can hole and tied the three new filters wit ha couple of tie down, it's secure. I hooked up all the neg legs to one point o nthe three filters and soldered them to the stand-off point that was bolted to ground near t he PT. 

But like, with the old OT no hum, new OT hum. It's something to do with the OT installation. I have the OT's plate wires twisted and away from the preamp section and PI, also moved the neg feedback wire clear of any other wires.

Maybe I need to check the tightness of the bolt but I doubt it's lose.

This is on of those things you need to see, as the schematic tells me nothing about how the grounding method.

Should I isolate the speaker jack, or use a switchcraft jack and also run a common to the cathode after the feedback loop too?

I can post pic's. What I did different was tied in all the grounds on the tab on the standoff holding the three filters, bolted to chassis, this wasn't done before, one of the old filters was grounded with the common only of the OT...? and the speaker jack was hanging so I guess it got grounded through the OT somehow.

al

[dude]

Awful 60 cycle hum in the Ampeg I just fixed. I changed nothing except replaced the cap can, 70, 40, 40 with tree single filters all grounded at the same point as the old cap can. ... Amps plays great except 60 cycle hum. I read-up on Aiken's Star grounding methods ...

Forget star grounding. The amp worked at one time with relatively low hum, exactly as it's built now. That proves grounding method isn't the culprit.

Thanks, again HPB, you got me thinking the amp played OK till I changed out the filters, so what did I do differently...?   I changed the grounding scheme, the old can was grounded on the board "only" at the bridge rectifier. I kept that ground but I also grounded all the filters to the chassis using one of the stand-offs bolts. The old can didn't have this ground, I snipped it and voila, the amp has almost zero hum.

I guess I had two different places that all the power filters were grounded, not sure what kind of hum I had but it's fixed. I think I may have to start taking pictures of stuff before I start replacing wires.

It's usually something you last did that's the problem if the amp played ok before. This case I added a star ground that wasn't there.

Thanks, for getting thinking.

al