Hi,

I noticed a channel had no sound, investigated and found a blown 500mA fuse on the right side, designated F2 in the assembly instructions. I went against better judgment, swapped the 500mA fuse, and fired her up , this time blowing F2 AND F3.  So then I wised up and checked voltages against what is shown in the drawings attached to the end of the instructions.  

I also ohmed the speaker which was attached to the load at 5.0Ohm, no issue there.

I measured the output at 2.83 volts, and this did not change hardly at all with bias adjustment.  Could not zero it.   Stayed in the same range.

Following voltages were measured:

-375 in instruction was measured at -407
-315 measured @-352
+315 measured @+352

one thing I noticed is that on V6 and V7 I measurd 350 on BOTH sides of the 4.7 ohm resistors! NO idea what in the world is causing that unless 300Bs are bad?  Hope not, that would mean 2 of them.

I have a lot of measurements but these were the important ones on the output, which is where the trouble seems to lie. 

No loose soldered connections, shorted diodes, or open resistors.  


I would MUCH appreciate any advice...
I changed the fuses again but I hope its the last time.

THANK YOU!!!
Michael
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Anonymous
When there is no voltage drop across a resistor, that means no current is flowing.  You have an open circuit. That is why the bias control has no effect because the tubes are not conducting.
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Carson
when you measured voltage on both sides of the 4.7R resistor (no current flow)...I assume you pulled the tubes out as they were blowing fuses.

Have you tried swapping the suspect tubes into the working amp?

The voltages you measured sound fine for an amp with no tubes installed.
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Deleting Attachment...