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Burnedfingers 3rd September 2003 12:34 AM

Scrap or save?
2 Attachment(s)
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.

Sch3mat1c 3rd September 2003 02:33 AM

Doesn't help that you used the same color as background :p


dhaen 3rd September 2003 10:37 AM


Originally posted by Sch3mat1c
Doesn't help that you used the same color as background :p


What, you mean black-on-black:D

Burnedfingers 3rd September 2003 11:37 AM

2 Attachment(s)
I'll try this again

Burnedfingers 3rd September 2003 12:25 PM

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.


dhaen 3rd September 2003 12:25 PM

That's better...

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


SY 3rd September 2003 01:14 PM

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.

fdegrove 4th September 2003 02:27 AM


Third, scrap.


Burnedfingers 4th September 2003 02:38 AM

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.


Burnedfingers 6th September 2003 03:31 PM

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.


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