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Runaway Bias

6/5/2000 1:52 AM
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 ...  
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6/5/2000 3:26 AM

(1) did you get the pinout right?  
(2) do you read the bias voltage on the socket pin?  
(3) do the sockets really make contact? Stick an octal header in there and see if the bias voltage appears on the grid pin  
(4) Does twiddling the bias pot actually change the voltage that appears at the grid pin?  
(5) Have the tubes worked correctly in other amps?  
(6) Is a screen voltage problem possible in your wiring?
6/5/2000 4:21 AM

Thanks for the speedy reply.  
(1) Yes, I double checked all the pinouts.  
(2) Yes, I can read neg voltage on the "tube side" of the sockets.  
(3) I read the voltages,thru my bias probe so the sockets are good all the way thru.  
(4) Yes, I can get from -38.5 to -41.6 by turning the 25k pot.  
(5) The tubes are good, NOS, TV-7 tested, but I put a known good used pair of 6L6's in ther just to be sure, they biased at around 160 ma :o(  
(6) Not sure here...The screens have 534v and the plates have 535v. The original drawing calls for 475v on the plates and 470v on the screens, BUT, that was for a pair of 6L6GC's. The screens could be a "little" high but not really that much. I should still be able to get the bias in the ball park I would think.  
This has almost got to be some stupid oversite someplace.  
Let me make sure I have my head on strait here... if I INCREASE the neg voltage to the grids, this should drive the bias reading (ma) down right? or is my head up my @$$? A simple resistor change and I can get the base neg voltage up or down. Somthin' just ain't kosher.  
Thanks again RG,  
Any more suggestions?  
6/5/2000 5:47 AM
Bryan James

are you grounding out your bias somewhere??  
my one DIY i put a MV and when i jumper the two connectors on the daul pot the jumper wire was touching the chassis ov the pot. when i turn it on the bias was shorting to ground at that point and i was ready about 100+ma i shut the amp off didn;t want to see how high it would get. simple mistake took a day and ripping the amp apart to find
6/5/2000 7:36 AM

Beam forming tubes don't seem to need much signal to go full bore so make sure you aren't trying to "bias out" a simple oscillation.  
If there is nothing wired or dressed wrong, maybe  
-42v is not negative enough to hold back a pair of 6550s at 535vdc screen and plate.  
I'd start by checking for parasitics though and then redo the bias supply and make sure you can swing it down to at least -60vdc.  
I'd decrease the screen voltage to about 450vdc too.  
6/5/2000 11:42 AM
Peter S

I think you need to look at your bias resistors. I havent used 6550's in a while, but it seems that at 535Volts the -40 isnt negative enough. I know for sure that it isnt for almost any 6L6GC tube. I had a pair of Sveltana 6L6GC's yesterday running at 450Volts and I had to put  
-56vdc on pin 5 to get them to run at around 35ma.  
Peter S  
Kimerik Amplifiers
6/5/2000 2:11 PM
I agree.. around -55 to -60 should be closer,look at the old sunn schematics, they run about the same voltage to 550 560..  

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