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Old 29th December 2006, 08:48 PM   #1
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Question Tube power supply questions

Hi !!

First of all, I searched around the forum, and of course I learned a lot, but I couldn't find the following:


This is about biasing.

For example: A 12AX7 with 150V on the plate.

The saturation point is 0V, and cut off is - 2.2v, on the grid voltage with respect to the cathode. Okay, I understood this perfectly, and then...

Self Biasing !

Some resistor is added to the cathode lifting the ground voltage to 1.1v, the effect is that we can connect the grid to the ground (through a resistor) and it's bias will be -1.1v in respect to the cathode, achieving our desired linear response biasing. Now the circuit is suitable to receive audio signals till 1v without clip. Perfect !


BUT what I didn't understand is:

I saw an example, with a 12AX7 with the plate connected to +233v through a 100K resistor, and the cathode connected to the ground through a 1.5K resistor.

With this 100K resistor, the voltage drop is 74v, so the plate is with 159v. The cathode resistor (biasing) was calculated ragarding the drop value of that plate resistor.

So, how do I determine the value of the plate resistor?


This is the document that I've read:

http://195.178.227.103/ax84/media/ax84_m35.pdf

It explains every thing very nicely, but not how to calculate the plate resistor. I'm sure it have a direct relation with the tube datasheet, but I confess that I don't know how to extract this needed information.



Sorry for this long post !

I just wanted to show you clearly what I know and what is my doubt !



Happy Xmas and a great new year !!



Best Regards
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Old 29th December 2006, 08:52 PM   #2
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Use constant current source instead of anode resistor, it is easier to calculate.
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Old 29th December 2006, 08:53 PM   #3
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Quote:
Originally posted by Wavebourn
Use constant current source instead of anode resistor, it is easier to calculate.

I didn't understand anything that you said

Can you explain it using the "newbie" language?
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Old 29th December 2006, 09:31 PM   #4
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A page to explain the plate resistor a bit:
http://www.diyparadise.com/tubeloadl...loadlines.html

and a couple to explain the CCS plate load, and a bit more on the resistor:
http://www.ecp.cc/meha.html
http://www.ecp.cc/c4s.html
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Old 29th December 2006, 10:45 PM   #5
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If you are going to get into tubes, bookmark this site. It's got plenty of advanced stuff, but lots of basic stuff too. Look down the page for the section on Loadlines. That's the information you need to set operating current, plate voltage, plate resistor and B+.

http://members.aol.com/sbench101/


Sheldon

edit: Note that the "load" of the loadline is the plate resistor in your application. So a 100k loadline would be obtained with a 100k resistor on the plate. With an output transformer on a tube, it would be the reflected AC load from secondary to primary.

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Old 29th December 2006, 11:17 PM   #6
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Quote:
Originally posted by -_nando-_



I didn't understand anything that you said

Can you explain it using the "newbie" language?
It is such thing that provides constant current, so you just need to select a resistor in cathode for needed voltage drop on that current. Except simplicity of calculations the CCS gives you one more significant advantage: it's dynamic resistance is huge, so the triode generates much less distortions.
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Old 30th December 2006, 02:58 AM   #7
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A "rule of thumb" for triode plate load resistance is that it should at least twice the tube's internal plate resistance, preferably higher.

You can get useful information in the technical articles at Aiken Amps website. It explains the basis for designing amplifier stages, among other things.
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Old 1st January 2007, 10:17 PM   #8
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I've got reading


As soon as I finish to read all these sites I'll post a feedback.

I thought that as it's a very basic question, you won't post so many nice answers.

Thank you all, these documents are all I need to read.





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