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Negative bias supply problems

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Posted on June 25, 2017 at 15:43:44
Posts: 453
Location: Ontario
Joined: September 20, 2007
I found a circuit on the web that uses a 24 volt transformer and a voltage doubler to provide the -32 volts I need. Assembled everything and was able after a few adjustments to get the necessary voltage. Hooked it up to the secondary of the interstage transformer I intend to use and the voltage fell to -0.4 volts. The secondary only measures 400 ohms so what else might account for this drastic drop in voltage? Thanks for any help, this one really puzzles me!

 

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RE: Negative bias supply problems, posted on June 25, 2017 at 17:57:06
Paul Joppa
Industry Professional

Posts: 7294
Location: Seattle, WA
Joined: April 23, 2001
Did you ground the other end of the secondary (accidentally of course!)?

If it's not that simple, then it would be a great help if you could post a schematic of the hookup and of the bias supply.

 

RE: Negative bias supply problems, posted on June 25, 2017 at 18:24:21
What kinda of filtering are you using on the bias supply voltage? How are the caps wired into the circuit?

 

RE: Negative bias supply problems, posted on June 25, 2017 at 21:32:01
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
Like Paul said, a schematic would be helpful. Other than that, it's worth mentioning that a 24V transformer driving a full-wave bridge will produce the 32 volts you're after. Attaching a doubler to the transformer will produce about 65 volts. Sounds like you might have wired something incorrectly.



 

RE: Negative bias supply problems, posted on June 26, 2017 at 06:29:09
Posts: 453
Location: Ontario
Joined: September 20, 2007



Here is the schematic. The IT is not in a circuit, the amp I want to use it in has yet to be assembled.

 

Voltage doubler seems correct, posted on June 26, 2017 at 07:06:15

Wonder why "R?" Maybe, check the resistance. I don't think you need this. Just compensate by adjusting R1 and R2 values, downstream.

You sure the polarity of the diodes and 'lytics are correct, in the circuit? Must be (+) to ground.

 

RE: Voltage doubler seems correct, posted on June 26, 2017 at 08:20:18
Posts: 453
Location: Ontario
Joined: September 20, 2007
R? worked out to be around 9K. The transformer is a Hammond that is identical in all respects to the Allied part. I used UF diodes, C3 is 100 uf because that was what I had, and the pot is 20K. It does seem to take a long time for the voltage to come up to my desired -32 volts, might this have anything to do with the value of C3?

 

RE: Negative bias supply problems, posted on June 26, 2017 at 08:32:39
TomWh
Audiophile

Posts: 764
Location: Tucson Az
Joined: August 7, 2003
If my understanding of fixed bias is correct there is no real Current going thru it. You said you do not have the interstage in the circuit did you ground the output side? That could pull some current and drop some real voltage.

All my hookups with interstages the secondary is tied to the grid of the tube. That stops any real Current from flowing through the circuit.

If you are getting the right voltage range from the fixed bias circuit I can not see how connecting a dead end street could change that. If I am barking up the wrong tree there are plenty of circuit guys on this forum who will chime in.

 

RE: Negative bias supply problems, posted on June 26, 2017 at 09:03:05
Posts: 453
Location: Ontario
Joined: September 20, 2007
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.

 

Calculation, posted on June 26, 2017 at 09:09:35
PakProtector
Audiophile

Posts: 12356
Joined: May 14, 2002
It looks like you're minimum voltage occurs when the 25k pot is effectively shorted. I would rather not vary the current draw on the negative rail depending on its output setting. Also, 15k and 47k looks to have a minimum output slightly more negative than 40V...so I am not quite sure how you get this pup down to 32V. It could be that the 15k/10 uF RC is knocking down a lot of ripple, and that there exists plenty of it at the 47 uF doubler stack.

I tend to want more capacitance than you have shown...like 10x, and at a minimum 5x...:)
cheers,
Douglas
Friend, I would not hurt thee for the world...but thou art standing where I am about to shoot.

 

RE: Negative bias supply problems, posted on June 26, 2017 at 11:31:51
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
There shouldn't be a voltage drop across the IT winding. No DC current flows to the grid.

 

Yep, In his original post, posted on June 26, 2017 at 12:40:16
Chip647
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Posts: 2633
Location: The South
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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.

 

RE: Yep, In his original post, posted on June 26, 2017 at 13:07:18
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
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. :)





 

That is why no one could figure it out. Inconsistent. (nt), posted on June 26, 2017 at 16:20:40
Chip647
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Posts: 2633
Location: The South
Joined: December 24, 2012

 

So you are getting proper negative voltage on the grids? (n/t), posted on June 27, 2017 at 09:13:17
mt

 

RE: So you are getting proper negative voltage on the grids? (n/t), posted on June 27, 2017 at 09:31:29
Posts: 453
Location: Ontario
Joined: September 20, 2007
Yes, easy once I figured out what I was doing wrong. Thanks for asking.

 

RE: That is why no one could figure it out. Inconsistent. (nt), posted on June 27, 2017 at 09:38:10
Posts: 453
Location: Ontario
Joined: September 20, 2007
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.

 

RE: Calculation, posted on June 27, 2017 at 09:42:36
Posts: 453
Location: Ontario
Joined: September 20, 2007
I get the voltage I wanted and in the desired flavor. I appreciate what you are saying about filter capacitance but is not ripple that is at most a fraction of the DC current when that current is only one ma not something to worry about?

 

400 ohms/0.4 volts gives 1 ma of current, posted on June 27, 2017 at 11:16:18
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
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"

 

RE: 400 ohms/0.4 volts gives 1 ma of current, posted on June 27, 2017 at 12:50:57
Posts: 453
Location: Ontario
Joined: September 20, 2007
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.

 

No kidding, posted on June 27, 2017 at 13:20:20
Triode_Kingdom
Audiophile

Posts: 10012
Location: Central Texas
Joined: September 24, 2006
The transformer isn't even in a circuit. This thread (including the supposed corrective explanation below) is practically incoherent.

 

RE: No kidding, posted on June 27, 2017 at 14:14:29
Posts: 453
Location: Ontario
Joined: September 20, 2007
Some so-called "experts' here would do better to learn how to read rather than shooting off their mouths...

 

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