MC60 bias problem.

djef

Active Member
I have an MC60 with some baffling problems. It is completely recapped, new diode, metal foil resistors.

The -bias is extremely low. Right after the diode, it is about -140. Then after the first 56k resistor, on both sides, it's about -77. Then finally at the tube, it's only about -7.

Also, when I first turn it on it makes a buzzy oscillating whine. This whine dissipates after 20-30 seconds.

Anyone have any idea what this could be?
 
First you will need a schematic for the MC60 then you check the voltages and see if they do not match the schematic. Replace the defective parts until the actual voltages match the schematic. Then you will have proper sound.

Thanks,
Ron-C
 
I noticed something that might have something to do with this amp's problems. I was looking over pictures I took from before starting the rebuild and noticed it looks like the cathode of the diode is connected to pin 5 of the rectifier tube and goes to ground via 3 resistors in parallel. (the two big 10000ohm resistors and a smaller carbon comp to the left) I can't see from any of my pics where the wire from the transformer is. Has anyone ever seen this? I can't find any schematics that show those resistors. Also, there was an additional 10uf cap going to ground at the negative bias.

picture.php
 
Here is a late build schematic, should be the one...
 

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Thanks Terry. However, I think this amp is even different than that build. There's no 5.1k before the diode. Also, I've been looking at it more closely and I now see that the diode is not connected to it's own wire from the transformer. Rather, one of those 10k resistors is connected to pin4 of the rectifier. That's where the diode gets it's AC voltage. Weird, huh? I don't even see that wire, the one that normally goes right to the diode, coming out of the transformer.
 
OK, is early version where the bias does not have a separate winding.
 

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I've changed the point on the transformer from which the bias is derived, but to no avail. As soon as the tubes warm up, the neg bias drops down to zero. I disconnected the bias from each 6l6 and measured them; they each are around -120. So, the bias source is good, it's just something down the line that's no good. I've tried 3 different sets of tubes, so I know that's not the problem.
Could it be the output transformer? This amp was sent off to be rechromed and refinished. The output transformer that was sent back is not the same. It is an mc60 output transformer, but not the one I sent out; I had put markings on the original. Could there be something wrong with this one that is sinking the bias and causing this horrible noise?
 
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