Single Channel 6g3 120hz hum issue

[TurboGuitarMelton]

Hi,


I built a 6g3 in single channel form and am having issues with 120hz hum on the phase inverter. I've attached a schematic. Will also post pics of amp.


Changes to original -
Layout (ptp) but mostly tried to follow general layout of original.
Omitted tremolo
3.3k first cathode resistor
cathode biased 6v6's
5y3 rect




Unsoldering the 100k from V1b 12ax7 to the phase inverter input makes no change. Removing V2 does stop the hum.
Grounding either grid of V2 makes the hum very quiet (how i wish it would be normally)
The phase inverter ground is connected to same ground as the OT ground




I am about to try replacing the 1M resistors and the 0.01 cap and 0.1 cap to see if it could be a bad cap or a bad resistor

[TurboGuitarMelton]

Here is a pic of the insides/layout


the 100k on v1 is still desoldered FYI

[sluckey]

Disconnect the .01 cap going into the PI top grid. Still hum?

Disconnect the 56K NFB resistor. Still hum?

Get a 22µF filter cap and temporarily bridge across each of the four filter caps.

[TurboGuitarMelton]

Disconnect the .01 cap going into the PI top grid. Still hum?

Hum stopped!!! Let me try replacing that cap

[sluckey]

Disconnect the .01 cap going into the PI top grid. Still hum?

Hum stopped!!! Let me try replacing that cap

I don't suspect that .01 cap. I really suspect that 8µF filter cap that supplies V1.

[TurboGuitarMelton]

Hum returned when new cap was installed... 

Disconnect the 56K NFB resistor. Still hum?

Hum increases a bit when I disconnect NFB

[sluckey]

I don't suspect that .01 cap. I really suspect that 8µF filter cap that supplies V1.

[TurboGuitarMelton]

i only have a 100uf 160v on hand would that be okay to use to go across the 8uf cap instead of the 22uf?


The filter caps are a brand new CE 16 16 16 8 cap can hope its not faulty! it was expensive!

[sluckey]

i only have a 100uf 160v on hand would that be okay to use to go across the 8uf cap instead of the 22uf?

no

[TurboGuitarMelton]

ill order a few caps then


Anything else I can try while I wait for them? is there another test I can do to verify the 8uf is bad?


I did end up replacing the 1m resistors and the 0.1uf cap no change there either. At least that's ruled out

[sluckey]

Disconnect ONLY the .01 cap lead that's connected to the junction of the 100K and 15K. Now connect the dangling end of that .01 cap to ground. If the hum is gone that means the PI and output circuit is OK. This would strongly suggest the 8µF is bad and was feeding hum into the PI through the 15K.

[TurboGuitarMelton]

Did that test and the hum was still there. That would mean my 8uf cap is good right?


So... Removing the 0.01 side that connects to the 1m and grid of V2 stops the hum but disconnecting and grounding the other side doesnt? What? 

[sluckey]

Did that test and the hum was still there. That would mean my 8uf cap is good right?

It seems that the 8µF is not the source of the hum, but keep it on the suspect list for now.

So... Removing the 0.01 side that connects to the 1m and grid of V2 stops the hum but disconnecting and grounding the other side doesnt? What? 

This is a bit confusing. Try this...

Disconnect ONLY the .01 cap lead that's connected to the junction of the 100K and 15K. But don't ground the dangling lead this time. I'm beginning to suspect a grounding issue. Post another hi-rez pic that shows all of the guts. I want to examine all your chassis ground points.

[TurboGuitarMelton]

The plot thickens...


I removed the 100k/15k junction end of the 0.01 cap without grounding and there was a slight hum but not as bad as when its hooked up normally.


I reattached a wire to ground that end and it hummed again. I moved my clipped in grounding point to a spot closer to the input jacks and it was silent.


Another thing to note is that when I had it grounded near the back of the chassis ( near output jack) my hand getting close to the clipped ground lead would make the hum stop.


This chassis is a powder coated black chassis. I sanded all ground contact points with a dremel.


I will post some more hi-res pics

[TurboGuitarMelton]

Here is one pic


Grounds are here:


1.5k PI ground is at the top


V1 cathodes are at the right


220k 6v6 grids are under the  twin 0.1 caps

[TurboGuitarMelton]

The other:


6.3 CT/ HV CT/ Cap can ground are on the left PWR transformer lug


cathodes for 6v6s are on right lug

[sluckey]

You clearly have grounding issues. Here's my suggestion...

[TurboGuitarMelton]

Since the 8uf preamp cap is a cap can - I can't separate those grounds with the power amp caps and the preamp cap. Ill get all those others in line with your suggestions and report back.


Is there any way to separate the cap can grounds?


Edit: just read the text in your diagram. Ill get a 8uf cap!

[TurboGuitarMelton]

I redid the grounds - hum is still there. I am ordering the cap and will report back once I've changed that around.


On a side note- I had a very similar issue with a 5E3 build I did a few years back. Once I separated the preamp power cap and the power amp caps it got rid of the hum.


Why is it that this only affects some builds? I built a Princeton reverb with all power cap grounds connected in a cap can and there wasn't the same issue.

[sluckey]

Show us some more pics of your revised grounds.

[shooter]

Why is it that this only affects some builds?

unless they are identical clones, with the same wire length, same.....  you will have varying results.  Human error contributes;  guessing instead of measuring, assuming instead of knowing....
Once you do get a clean build, replicate EVERYTHING as close as possible, you'll be amazed at how "consistent" the builds become.

[TurboGuitarMelton]

Here is the preamp (also ran wire from input jack to same ground)


I chose the terminal strip far right lug. Its relatively close to the input jacks and V1

[TurboGuitarMelton]

here is power amp


(the 220k grid resistors are a attached via a wire and the 1.5 PI ground is ran along the top from the circuit to the right)

[TurboGuitarMelton]

Thank you so much Sluckey! Turned out it was the ground for the preamp/phase inverter filter cap.


I replaced it with a 22uf. The 22uf was cheaper and I figure it would only help with noise to have great filtering for the preamp. I attached the ground to the preamp grounds as they were in your diagram and the amp is silent!