Author Topic: Improving ground scheme - star ground? Partial star ground?  (Read 458 times)

Offline Jezza

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Improving ground scheme - star ground? Partial star ground?
« on: August 07, 2018, 08:43:07 PM »
The photo below is my Wurlitzer 112 amp. It works fine, but it's not up to modern studio noise floor standards. Can any circuit tweakers guess how much benefit I would get from a partial star ground system?

The arrows point to 14 different points where the circuit touches ground, and I assume this is the cause of my 60hz ground loop hum. (All caps are new, 6.3V heater supplies are shielded, 120V wires are twisted, AC wires are in corners when possible, reed bar is shielded, etc.)

I will not rebuild this amp again to achieve a true star ground scheme, but do you think it would be worth creating a little local star system? I'm considering it for the input, output, and small tubes. Essentially the bottom third of this photo would be isolated from the chassis and meet it at a single point.

It would be a bit tedious, so I don't want to attempt it unless some of you enthusiastically tell me it will help. :-)
- Jezza

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Offline cinnanon

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #1 on: August 07, 2018, 10:32:19 PM »
Is the amp quiet without the reedbar RCA plugged in? I had a nasty hum on a 140B once. The amp was fine until I plugged in the reedbar. I also have a 112 so this interests me.
« Last Edit: August 07, 2018, 10:35:16 PM by cinnanon »

Offline Jezza

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #2 on: August 07, 2018, 10:34:07 PM »
This is clearly amp noise, evident when nothing is plugged in. Some of it changes with the volume pot and some is ever-present.
« Last Edit: August 07, 2018, 10:40:13 PM by Jezza »
- Jezza

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Offline cinnanon

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #3 on: August 07, 2018, 10:36:56 PM »
Are the heater wires twisted also? UncleDoug on youtube has some great stuff on tube amps. He explains stuff so well my wife could understand it.

I remember breaking down the symptoms piece by piece.  In my 140B, instead of using the volume pot, I just wired in a jumper across Hot-Wiper and a resistor equivalent to the pot across Wiper-Ground to eliminate the Pot causing noise.
« Last Edit: August 07, 2018, 10:38:45 PM by cinnanon »

Offline cinnanon

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #4 on: August 07, 2018, 10:40:56 PM »
If I remember from UncleDougs videos, the hum that varies with volume is a different issue than the hum that is constant with volume change.

Offline cinnanon

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #5 on: August 07, 2018, 10:43:10 PM »
Uncledoug talking about hum...

https://www.youtube.com/watch?v=GrVtX0QGNls

Offline Jezza

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #6 on: August 07, 2018, 10:59:55 PM »
This amp uses only one leg of what's presumably a 12.6V winding to create the 6.3V for the heaters. The other leg is grounded, so there's nothing to twist the heater wires with. I have shielded them with foil tape and pressed them into corners as best as I can.

Your mod to your 140 volume pot is interesting. I just tried shorting Hot-Wiper and got nothing but 60Hz hum. Perhaps I should swap the pot before I do anything else more serious.

Thanks for the Uncle Doug video. I remember it from years ago and I actually watched it earlier today too. As far as I can tell, it confirms that I do not have tube noise.

I'm confident it's a ground loop, and I'm hoping if I can protect the preamp section from loops that perhaps I won't have to mess with the rest!
- Jezza

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Offline cinnanon

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #7 on: August 08, 2018, 09:00:38 AM »
Did you wire a three-prong cable or is it still two-prong? If it's two-prong, did you try reversing the leads? I'm sure you tried this but i'm not sure.

Offline Jezza

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #8 on: August 08, 2018, 11:24:07 AM »
It's not a polarity problem. This is somehow a design or layout problem. I took it to an amp shop in Hollywood and the guy told me all my recap work was good, but it was a noisy design. Since I don't have a scope it's hard for me to figure out where the noise is coming from, but since the power supply caps are new I suppose it's most likely the preamp section picking up something.

\_(ツ)_/¯
- Jezza

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Offline Fred

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #9 on: August 08, 2018, 07:17:43 PM »
Place your mic a few inches further away from the speaker than you might normally be inclined to do. Problem solved. ; )
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Offline The Real MC

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #10 on: August 08, 2018, 10:43:43 PM »
Passive pickups can also contribute noise.  This is a common problem with the pickups in Rhodes pianos.  What really makes a difference is the input impedance of the amp or DI.

Offline pnoboy

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #11 on: August 16, 2018, 11:47:53 AM »
This amp uses only one leg of what's presumably a 12.6V winding to create the 6.3V for the heaters. The other leg is grounded, so there's nothing to twist the heater wires with. I have shielded them with foil tape and pressed them into corners as best as I can.

Your mod to your 140 volume pot is interesting. I just tried shorting Hot-Wiper and got nothing but 60Hz hum. Perhaps I should swap the pot before I do anything else more serious.

Thanks for the Uncle Doug video. I remember it from years ago and I actually watched it earlier today too. As far as I can tell, it confirms that I do not have tube noise.

I'm confident it's a ground loop, and I'm hoping if I can protect the preamp section from loops that perhaps I won't have to mess with the rest!

There are so many potential causes for hum, no one could possibly predict with confidence what would ameliorate your issue unless they had solved the identical problem with the same model piano.  One thing you mentioned that is almost ALWAYS a source of hum is heaters powered by a grounded AC power source.  The usual way to power tube heaters is through an isolated transformer winding that is configured to have equal and opposite voltages at the connections to the tubes.  There are typically 2 ways of achieving this result.  One is to use a center-tapped winding, whose center tap is grounded.  The other is to connect the isolated heater voltages to ground through equal resistors, or to use a pot whose center tap is grounded.  I have a tube amp I sometimes use with my Rhodes piano that has a so-called hum balance control, which is, in fact, a pot connected across the heater windings.  When set just right, amp hum almost completely disappears.  Setting the pot to one end of the other results in plenty of hum.  One reader mentioned Uncle Doug's dissertation on hum.  One point is germane to your problem.  Hum from a full-wave rectified power supply is at 120 Hz.  Hum from an unbalanced heater connection is at 60 Hz.  Good luck!

Offline Jezza

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Re: Improving ground scheme - star ground? Partial star ground?
« Reply #12 on: August 16, 2018, 03:16:04 PM »
Thanks for this.

There is a bit of 120Hz hum, but it's dominantly 60Hz. The heaters are a more likely the main culprit than the power supply.

The image below is how the heaters in this amp are wired. The idea of using both sides of the transformer wiring and adding a hum balance pot is a good one. It would be a little nuisance to implement, but maybe I'll try it. Many thanks for that suggestion.

Swapping the tubes helped a little bit, and changing the power supply caps to slightly nicer ones helped a bit also. The amp is in pretty good shape, so it's a matter of finding little degrees of improvement wherever I can.

Your hum balance pot suggestion probably addresses the problem more directly than creating a star ground scheme for the whole amp in hopes of fixing something unknown.
- Jezza

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