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In Reply to: RE: Negative bias supply problems posted by vetmedrobert@gmail.com on June 25, 2017 at 15:43:44
My apologies to all, the mystery has been solved. Simple really, I did not have the meter connected correctly. what I was measuring was the voltage drop across the IT winding! Thanks to all that responded, this method of biasing is new to me.
Follow Ups:
mt
Yes, easy once I figured out what I was doing wrong. Thanks for asking.
There shouldn't be a voltage drop across the IT winding. No DC current flows to the grid.
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Buy Chinese. Bury freedom.
He said: "Hooked it up to the secondary of the interstage transformer I intend to use and the voltage fell to -0.4 volts." So he was measuring the tiny drop across the IT secondary. This is what you would expect. He needed to measure the top of the secondary to the cathode of the OP Tube.
Edits: 06/26/17
That doesn't make sense. If he was measuring across the winding, how would he ever see the -32V he described? He didn't say he measured at a different location after hooking it up, but maybe that's what happened. Anyway, glad it was a simple fix. :)
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Buy Chinese. Bury freedom.
The transformer isn't even in a circuit. This thread (including the supposed corrective explanation below) is practically incoherent.
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Buy Chinese. Bury freedom.
Some so-called "experts' here would do better to learn how to read rather than shooting off their mouths...
Nothing inconsistent at all. In my original post I said -0.4 volts was measured and the winding was 400 ohms. 400 ohms/0.4 volts gives 1 ma of current, so I concluded what I was measuring was the voltage drop across the winding. As I said in subsequent posts once I figured out that I was measuring incorrectly I was able to measure the voltage I expected and wanted.
Is there a grid resistor connected to ground in addition to the IT connected to the bias supply?
I'm just trying to figure out why there would be 1ma of DC current flowing.
There shouldn't be any.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
No grid was harmed in this experiment. Output was connected to secondary of IT I intend to use in a yet to be constructed amp. What I did initially was connect the negative output to one end of the winding and the positive end of the supply to the other of the winding. I then attached the meter one lead to each end of the winding. This is what gave me -0.4 volts. After my first post I connected one end of the winding to the negative output and the other to the red lead of the meter and the black lead of the meter connected to the positive ground of the supply. I was able to adjust this to the -32 volts I wanted. Hope this clarifies things.
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