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Noise Problem, Help Please

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Troy Madden

Member
Joined 2004
If it was EM from the choke, would we not be seeing 120Hz noise?

grovergardner said:
It could be the input choke. They can generate a good amount of magnetic radiation. The hams used to tune them. I see on the GEC schematic that there's a tuning network across the input choke. You could try a .47 cap in front of it and see if that helps. You have to watch the resonance, though or the voltage will shoot up. Here's an old thread from Audiokarma:

choke input PS throws noise like crazy

Just a thought if all else fails.
Click to expand...

I wonder about that too, have read that thread, Thanks! But, I would think it would be 120hz noise if from the choke.
 
Nico Ras

Nico Ras

Member
Joined 2005
If it is 60Hs sine then a tube(s) are picking up induced magnetic field from the transformer or from the mains leads in the chassis. The choke is not the problem unless there is a big 60Hz component on the dc supply. Take the mains transformer away if you can or change the orientation.
 
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primalsea

Member
Joined 2003
Is there a good electrical connection between the chassis and the transformer and coke housings and shields?

You may need to scratch away some of the paint and use star washers to really bite into the chassis and the transformer housings.

I cant quite make out the grounding scheme. Have you tried a star earth point that is not bolted to the chassis as a direct electrical connection and then having a flying lead from the star earth point to the chassis. The mains earth connection is also connected direct to the star.
 
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DF96

R.I.P.
Joined 2007
myweb.tiscali.co.uk/g8hqp/
Troy Madden said:
Star grounding
Click to expand...
I assume the PSU caps are not directly connected to the star?

All the PS components use a star ground with substantial conductors. Bridge rectifier neg. to first cap ground is very short 12 gauge solid copper.
Click to expand...
You may be optimising the wrong thing. No need for thick or short conductors. Just make sure things are connected in the right order, with small current loops. However, if your hum is 60Hz then the PSU is unlikely to be the cause. Poor heater-cathode insulation is possible, combined with no DC reference.
 
T

Troy Madden

Member
Joined 2004
Nico Ras said:
If it is 60Hs sine then a tube(s) are picking up induced magnetic field from the transformer or from the mains leads in the chassis. The choke is not the problem unless there is a big 60Hz component on the dc supply. Take the mains transformer away if you can or change the orientation.
Click to expand...
I have another power supply on a breadboard, I can power this amp with that power supply and see if it's an induced problem.

primalsea said:
Is there a good electrical connection between the chassis and the transformer and coke housings and shields?

You may need to scratch away some of the paint and use star washers to really bite into the chassis and the transformer housings.

I cant quite make out the grounding scheme. Have you tried a star earth point that is not bolted to the chassis as a direct electrical connection and then having a flying lead from the star earth point to the chassis. The mains earth connection is also connected direct to the star.
Click to expand...

I have made sure the choke and trans are grounded to the chassis. The powder coat on the power trans required some serious scaping, good stuff! I have not tried a flying ground to the chassis, its grounded right at the star point, along with the mains earth.

DF96 said:
I assume the PSU caps are not directly connected to the star?


You may be optimising the wrong thing. No need for thick or short conductors. Just make sure things are connected in the right order, with small current loops. However, if your hum is 60Hz then the PSU is unlikely to be the cause. Poor heater-cathode insulation is possible, combined with no DC reference.
Click to expand...

The first PSU caps are connected to bridge rectifier negative with a very short solid 12 gauge wire. Then I am using the neg of the first PSU caps as my star point. Everything else is connected there. I have tried elevating the heaters to 100 volts, with a .1uf bypass to ground. You can see my voltage divider that's not currently connected.

grovergardner said:
Well, not necessarily. I would think that magnetic radiation from a transformer would be 60Hz AC. At least that's my experience with transformer orientation or shielding issues.
Click to expand...

Yes, 120 hz from the choke, 60 hz from the main transformer.

OK, getting somewhere. I powered the filaments of the first voltage amp/phase splitter tube with a lantern battery instead of the 6.3 volt AC filament supply in the amp. At first the noise was way worse, then I grounded one side of the battery to chassis ground. Noise is reduced by two thirds , down to 8mv.
 
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Troy Madden

Member
Joined 2004
Yes, the majority of the noise was induced by the filament supply. I just connected a separate filament transformer. That didn't bring the noise down any further, but if I hold a grounded copper sheet near the bottom side of the amp, I can get noise down to 4.5 mV. So I may line the wooden case I made with copper sheet.
 
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