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Old 1st July 2012, 07:01 AM   #31
zoi is offline zoi
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Join Date: May 2012
After a full rebuild on a metal chassis (pic attached), with the star grounding mostly as described above, here are the developments...

The hum is reduced but still there. HOWEVER, if I disconnect the heaters the hum instantly stops! This tells me all I need is a clean heater supply...

But now there is another problem.

After the rebuild, there is severe blocking distortion and the resistor bypassing the cathode bypass cap on the power stage (labeled 25uf on the schematic) is getting really hot! As the tubes warm up, a voltage appears across the cap (around 50vdc or so).

I have checked and double checked and everything is the same as it was. Did I damage a tube? Or a capacitor?
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Old 1st July 2012, 02:34 PM   #32
Sheldon is offline Sheldon  United States
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Location: San Diego
Quote:
Originally Posted by zoi View Post
After a full rebuild on a metal chassis (pic attached), with the star grounding mostly as described above, here are the developments...

The hum is reduced but still there. HOWEVER, if I disconnect the heaters the hum instantly stops! This tells me all I need is a clean heater supply...

But now there is another problem.

After the rebuild, there is severe blocking distortion and the resistor bypassing the cathode bypass cap on the power stage (labeled 25uf on the schematic) is getting really hot! As the tubes warm up, a voltage appears across the cap (around 50vdc or so).

I have checked and double checked and everything is the same as it was. Did I damage a tube? Or a capacitor?
The cap is dead, or unreliable at best. It should not conduct any current. For now, remove the cap. Measure the resistance of the cathode resistor. If that checks out, power up and measure the voltages at the plate, grid and cathode of the output tube. That will point to the problem.

Your noise may be due to a failing output tube - maybe not surprising if you check the maximum ratings for that tube.

Sheldon

Last edited by Sheldon; 1st July 2012 at 02:43 PM.
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Old 1st July 2012, 03:41 PM   #33
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Hope I can help first what is the tube compliment ? on the board . the 5902 is a sub minny with a max of 165 volts on the plate . I am at odds that the picture does not match the schematic . On the schematic replacing the r23 r24 100 ohms with say a 250 ohm center taped pot and take the tap to ground adjusting from side to side to get the quietest point . http://www.mif.pg.gda.pl/homepages/f...141/5/5902.pdf Regards
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