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Runaway Bias

6/5/2000 1:52 AM
Casey4s
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Runaway Bias
A few minutes ago I finished putting together my newest DIY. It is an Ampeg SB12 clone/variation.I am useing a Stancor PT 375-0-375 with a pair of GE 6550's, and a GZ34. I am getting 535v on the plates with no tubes, which is well within limits for these tubes. The first thing I tried to do was bias the amp. With a swamp probe, I am reading 180+ ma! I put a 25k bias pot in the circuit, and lowered the 56k bias resistor to 50k when I put this together. The pot dosn't seem to have any affect at all. Pin 4 on the power tubes reads 534v and pin 5 reads -40.5 volts. That's with no tubes into an 8 ohm 160 watt resistive dummie load.  
 
I am getting ready to start trouble shooting this thing, so does anybody have any thoughts?  
 
Thanks ...  
 
 
Casey4s

 
Replies:
R.G. (1) did you get the pinout right? -- 6/5/2000 3:26 AM