DHT for 300B problem

neskor · 13th May 2011, 06:02 PM

Hello
I just finished DHT for 300B, but when I start on output I have 8.75V for 10 sec. and than goes to desired 5V.
Can I leave like this or it will damage 300B?
Any one have idea how to solve this problem?

tomchr · 13th May 2011, 06:27 PM

That's about the opposite of what you want to do. Applying 8.75 V to a dead cold filament will cause a lot of current to run in the filament, thus, causing a thermal chock. That's not nice...

I wonder if it has to do with the fact that you have two control loops in your circuit. One that controls the voltage across the filament, the other controls the current. I would eliminate the voltage control loop and just program the current to 1.4 A. You could make it adjustable so that you can adjust for 5.0 V across a hot filament.

~Tom

astouffer · 13th May 2011, 06:51 PM

The schematic says a maximum of 7 volts input and you get 8.75 out? Perhaps you are overloading it?

Wavebourn · 13th May 2011, 06:53 PM

It looks very strange. You should have slowly increased voltage between + and - signs for filament connection, as drawn.

the_manta · 13th May 2011, 07:01 PM

Why not taking a dead simple regulator/pass transistor ? The PSU in the pic is over engineered in many ways. There are so many parts, I could build another amplifier from that

Wavebourn · 13th May 2011, 07:24 PM

Here is my example of more than adequate voltage regulator:

neskor · 13th May 2011, 09:09 PM

I found what was a problem.
Value of capacitor C8 determine time to reach desired voltage.
So I put 10uF and now is working like a charm

13th May 2011, 06:53 PM	#4
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	It looks very strange. You should have slowly increased voltage between + and - signs for filament connection, as drawn. __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

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13th May 2011, 06:27 PM	#2
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	That's about the opposite of what you want to do. Applying 8.75 V to a dead cold filament will cause a lot of current to run in the filament, thus, causing a thermal chock. That's not nice... I wonder if it has to do with the fact that you have two control loops in your circuit. One that controls the voltage across the filament, the other controls the current. I would eliminate the voltage control loop and just program the current to 1.4 A. You could make it adjustable so that you can adjust for 5.0 V across a hot filament. ~Tom

13th May 2011, 06:51 PM	#3
astouffer diyAudio Member Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland	The schematic says a maximum of 7 volts input and you get 8.75 out? Perhaps you are overloading it?

13th May 2011, 07:01 PM	#5
the_manta diyAudio Member Join Date: Nov 2006 Location: Munich, Bavaria	Why not taking a dead simple regulator/pass transistor ? The PSU in the pic is over engineered in many ways. There are so many parts, I could build another amplifier from that

13th May 2011, 09:09 PM	#7
neskor diyAudio Member Join Date: May 2008	I found what was a problem. Value of capacitor C8 determine time to reach desired voltage. So I put 10uF and now is working like a charm

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