Scrap or save?

[Burnedfingers]

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]

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

Tim

 

[dhaen]

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

Tim

What, you mean black-on-black

 

[Burnedfingers]

I'll try this again

 

[Burnedfingers]

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

 

[dhaen]

That's better...

Hi,

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

Cheers,

 

[SY]

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]

Hi,

Third, scrap.

Cheers,

 

[Burnedfingers]

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

 

[Burnedfingers]

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

 

[fdegrove]

Hi Joe,

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

This is set to allow for DC coupling to the CFs.

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.

Any idea what's in there for the moment?

100nF would be to low on it's own but you could try replacing the output loading R's to 100K if you have a pair.

Cheers,

 

[Burnedfingers]

Frank

Thanks for the reply.

I took out the 1mfd coupling caps and replaced them with what I happened to have. I could only find some .1/600v. The original caps were letting some good old DC thru. Shame on them!
Quote:

This is set to allow for DC coupling to the CFs.

Ok, what is the correct procedure to set the B-?


Joe