Is this a ground loop?

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Bieworm
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Is this a ground loop?

Post by Bieworm »

Dangggg!!! I'm having trouble fixing a buzz/hum in my last build. I tried about everything, but I can't get it out.
It has a LarMar PPIMV and the buzz/hum increases with the MV. Volume or gain pots have no influence, nor do tone pots.
I checked every wire, every solder joint, filter cap, changed grounding wire connections, tube rolling, diodes, omitted the relays system while disconnecting its power supply, played with heater wiring,.. you name it!!
I have a scope but IDK where to begin...
The buzz stops when I ground the PI grids. Without preamp tubes V1 to 4 (preamp + reverb) it still buzzes

Listen:
https://www.dropbox.com/s/rrhz72jb0s85p ... d.m4a?dl=0
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Re: Is this a ground loop?

Post by JMPGuitars »

Have you tried with the LarMar PPIMV removed from the circuit?
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Re: Is this a ground loop?

Post by Bieworm »

JMPGuitars wrote:
Tue 03/22/22 2:34 pm
Have you tried with the LarMar PPIMV removed from the circuit?
Yes. Replaced it with 2x220k to ground.. no change at all.
Just disconnected all wires from the relays circuit and disconnected its PS again.. no change whatsoever...
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Re: Is this a ground loop?

Post by JMPGuitars »

Transformer bolts tight? Have you tried a different outlet?
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Re: Is this a ground loop?

Post by Bieworm »

JMPGuitars wrote:
Tue 03/22/22 4:55 pm
Transformer bolts tight? Have you tried a different outlet?
Yes they're tight. I even loosened the transformer and put 1mm rubber washers between it and the chassis to prevent vibrations. I did that because when the power transfomer rested on my workbench I had a rather loud vibration. Maybe a clue this hammond is flakey?
Same problem on every outlet..

But, I might be onto something. Yesterday I disconnected the 50V winding to the tremolo bias thing and the hum reduced dramatically. Instead I got a lot of heavy hiss. This hiss is probably due to the 50V wire hanging in the air :) (I disconnected it at the diode connection)
Tonight I will go deeper down that route. I'm going to disconnect the tremolo circuit everywhere it's connected and see where that will lead to.
On the layout there is a blue jumper wire from the diode to point "G". When I first started the amp up I got about -70V there so I lowered it to 50V by replacing that jumper by a 100k 2W metal oxide resistor. Maybe I have to reverse that, because at the time of initial start up I had extremely much AC from the outlet (245V .. that's normally 235V)

NOTE: the first time I built this amp the PT didn't have a dedicated 50V tap on the HT, so the neg bias was tapped at the rectifier. I did put the diode and 1M dropping resistor right next to the rectifier socket. So on that amp the 50V wire crossing all my power stage wires was DC loaded. On the current build it's AC right onto the board where it connects to the diode. Maybe I better copy that principle on the current build. The 50V wire runs under the board , so it's hard to see if it has a chance to couple to a DC loaded wire somewhere (or affect OT primaries)

Sorry if I'm guessing too much, but I'm on the end of my rope here...

Here's a link to all pictures:

https://www.dropbox.com/sh/36qip83onofg ... jNy5a?dl=0
schematic complete.png
layout execution 20220215.png
20220316_211829.jpg
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Re: Is this a ground loop?

Post by Bieworm »

I'm starting to think the config of neg bias for the trem might be the culprit...
Things I did wrong IMHO:
- grounded the cap C35 for the bias at the preamp ground bus. Should be at the power amp star ground I think.
- running that 50VAC wire across the 6V6, OT and PI wires is bad practice, even if it crosses straight angled
- the grounding of the tremolo intensity pot lug 3 is joined with the grounding of the speed pot lug 3 and ran to the preamp ground bus. While it worked perfectly for the former build I think I'd better follow the circuit of my 6G3, being, speed pot to preamp bus and intensity to the neg side of the bias cap at the conjunction of the cap and diode. The 6G3 system works perfectly.
20220323_111427.jpg
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Re: Is this a ground loop?

Post by Bieworm »

Ok! It's solved. Moved all the ground connections of the tremolo circuit to 1 small buss, and that buss to power amp star ground . Also moved the diode to the transformer area and put a shielded wire from there to the board.
Now I have to address some pretty loud hiss when volume and MV is turned up. But that responds to the volume and reverb. Cathode bias is set at 82% dissipation, so that's just fine.
One other odd thing it still has is the volume drop when trem is engaged. Have to look into that too...
At least that darn hum is gone😏
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