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Old 2nd September 2003, 11:34 PM   #1
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Default Scrap or save?

I purchased this preamp off ebay a few months ago and it had a shorted transformer(one side). I replaced the transformer and it has a nasty hum in it. It consists of (3) 5814 Phillips tubes a solid state regulator and an adjustable B- supply. I was wondering if it is worth the time to try to solve the hum problem.

I will try to attach a schematic in wonderfull handcad.
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Old 3rd September 2003, 01:33 AM   #2
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Doesn't help that you used the same color as background

Tim
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Old 3rd September 2003, 09:37 AM   #3
dhaen is offline dhaen  Europe
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Quote:
Originally posted by Sch3mat1c
Doesn't help that you used the same color as background

Tim
What, you mean black-on-black
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Old 3rd September 2003, 10:37 AM   #4
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I'll try this again
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Old 3rd September 2003, 11:25 AM   #5
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Ok, I finally got a picture to load but the quality is bad. You can however make out the channel on the right side of the page. It has a B+ of around 260 and a adjustable B- supply of about -65v to - 94volts. The bottom panel was missing from the unit and it looked like someone had tried to repair the unit. I am wondering if the unit might at one time had a Dc heater supply or am I just prone to playing with items exhibiting a higher than normal hum. All the grounds go to one point and the wires are twisted. I have tried moving the wires to different locations and have taken the pc boards and transformer out of the unit and set them on the bench as I tried to vary the hum level. Nothing helps. Any ideas? Should I junk it? The transformer I installed was a 250-0, 100-0, 6.3 volt.

Thanks
Joe
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Old 3rd September 2003, 11:25 AM   #6
dhaen is offline dhaen  Europe
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Default That's better...

Hi,

I think it's worth a try!
First of all, add 2 grid leak resistors to the inputs.

Cheers,
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Old 3rd September 2003, 12:14 PM   #7
SY is offline SY  United States
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Second, start looking for ground loops. This circuit looks like it has a large number of potential places for hum to rear its ugly head. And about twice as many valves as you need, but that's another story.
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Old 4th September 2003, 01:27 AM   #8
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Hi,

Third, scrap.

Cheers,
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Old 4th September 2003, 01:38 AM   #9
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Well, I added two grid resistors....hum still there. I looked for ground loops and checked all grounds and found no problems. Looks like it going to be headed for the scrap heap.

Joe
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Old 6th September 2003, 02:31 PM   #10
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Default Quiet now

On the way to the scrap box I decided to try one last thing. The heater winding didn't have a centertap so I added two 1% 90ohm resistors (didn't have any 100's)to ground and the hum left like a beaten stepchild. What was once a deafening hum is now gone. It is amazing how a 90plus db hum can be caused by this.

Was the preamp worth the bother? No, it sounds like something one might have purchased from Kmart at the bargain table.


The B- is adjustable by the way. Is this so you can zero the DC offset?

How can I make this sound better? I need to get a little more low end out of it. Any suggestions for coupling cap values? I have some .1/600 volt.

Joe
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