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Old 20th May 2006, 07:24 AM   #1
eforer is offline eforer  United States
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Default Noob question about plate load

Hi guys, I read through the links on the sticky regarding the intro to tubes and tube amplification. It was very enlightening however I have a few questions that I can't quite seem to answer with the resources provided.


Firstly, I've been looking at designs using an ecl82 with the triode as the driver and the pentode as the output tube allowing me to do a low cost 1 tube per channel amp. The Data Sheet for this tube shows the anode current as being 3.5ma and the anode voltage as being 100v. In the operating characteristics section of the manual, it lists the B+ for 200v and 170v and the anode current at .52 ma and .43 ma respectively. It suggests a anode resistor of 220k. At the anode currents listed in this section, the 220k resistor drops the voltage close to the reccomended plate voltage from the beginning of the manual. Why is there a discrepancy between the anode current at the beginning of the manual and the current listed in the operating characteristics section? Is the current at the beginning of the manual the peak output current and the operating characteristics section the quiescient (sp?) current?

Also, in many designs I've seen, there is a resistor on B+ in series with a capacitor with the plate resistor paralleled in between. Here is an example ECL82 design . This seems like it would increase the voltage drop. Is this just an extra stage of filtering?
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Old 20th May 2006, 10:33 AM   #2
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Hi

The first operating conditions (with 3.5mA current) is just typical characteristics of the tube, it describes a set of values for the tube that is valid as a whole but it doesn't always describe the best operating conditions.

The other set of values is described as operating charactersitics and is a recomended set of values for using the tube as AF amplifier

There is no conflict between the 2 set of values but it indicates that it is more normal perhaps to use the tube according to the operating characteristics. The typical characteristics an be used to see if the tube is OK or not a healthy tube should measure close to these values.

You are correct in your assumption that the resistor capacitor combination is for filtering, it is very common to see this arrangement

Regards Hans
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Old 20th May 2006, 06:07 PM   #3
eforer is offline eforer  United States
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The additional stage of filter will also reduce the B+, right?
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Old 20th May 2006, 06:26 PM   #4
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You will notice that at the operating point you describe (ie 100V 3.5mA current), the bias point is 0V. This is not an operating point you will use in audio because it will introduce grid current when the grid swings positive.
In audio you will use a bias point of -1V at an anode voltage of 100V, for a anode current of 1mA.

Hope that helps.

Shoog
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Old 20th May 2006, 06:30 PM   #5
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Yes but not very much, in your example there is a current of 0.52A and a resistor of 27kohm so the voltage drop is only 11.5V which has practically no effect on the performance of the circuit but it will have good effect on the hum level.

Regards Hans
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Old 20th May 2006, 11:07 PM   #6
eforer is offline eforer  United States
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Quote:
Originally posted by Shoog
You will notice that at the operating point you describe (ie 100V 3.5mA current), the bias point is 0V. This is not an operating point you will use in audio because it will introduce grid current when the grid swings positive.
In audio you will use a bias point of -1V at an anode voltage of 100V, for a anode current of 1mA.

Hope that helps.

Shoog

I noticed that the bias point of the tube, along with some other triodes was 0v, and when I calculated the voltage drop at that current based on the cathode resistor it was about 1.17v, setting the grid bias point at about -1.17v. Is this general practice for a tube that specs bias voltage at 0?

Just a few more annoying questions:

Obviously, the voltage drop across the plate resistor is dependant on the current through the tube. So when the tube manual reccomends 200v in the operating charactersitics section of the manual, is that factoring in the voltage drop across the plate resistor at or around the bias current? IE, is the tube manual reccomended resistor size based on consideration of bias current and an ultimately different desired voltage at the plate?


Along the same lines, if I wanted to use a higer impedance output transformer, could I simply increase the B+ to offset the higher voltage drop across the transformer so in the end the plate was at the same voltage at the bias point as it would be with the reccomended lower impedance transformer?
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Old 21st May 2006, 04:54 AM   #7
eforer is offline eforer  United States
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Okay, finally started drawing my own load lines on graphs from the tube manuals and things started to click! Answered many of the above questions. I was really misinterperting some things!

From what I have learned, it seems that as long as I adjust my variables so that the load line is at no point crossing (tangential seems okay) the max dissapation curve and that the load line is suitably long, I can use different loads for output transformers or plate caps.

I noticed that after doing my dissipation curve and load line on the output tube,the quiescent point is tangential or close to being tangential with the power dissapation curve. On the driver tube the quiescent point seems much much lower than the corresponding X value of the power dissapation curve. Is this simply to lower distortion? What is a good guideline for output power at the quescient point?

Thanks for putting up with my learning process. The community this forum represents is made up of some terrific people!
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Old 21st May 2006, 06:47 AM   #8
lndm is offline lndm  Australia
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I have read that a load line can cross the dissipation limit curve momentarily as long as the average dissipation remains below the rated, and the quiescent point is below the line. Still, I chose not to do this, it's just good to know I could.
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Old 21st May 2006, 08:04 AM   #9
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In order to get a bias point of 0V there is no cathode resistor, otherwise you couldn't have a bias point of 0V. If you look at the datasheet it makes no mention of a specified cathode resistor at 0V bias. This is a special case with valves and I am not certain what zero bias point valves are generally used for.

Shoog
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Old 21st May 2006, 08:29 AM   #10
lndm is offline lndm  Australia
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Quote:
Originally posted by eforer
On the driver tube the quiescent point seems much much lower than the corresponding X value of the power dissapation curve. Is this simply to lower distortion? What is a good guideline for output power at the quescient point?
Plate curves are generally kinked toward the bottom of the chart. Crossing this region could cause non-linearity. High currents can lift your load line out of this region. High voltages will do this too, as well as to allow greater swings.

I think that the manufacturer recommended operating characteristics should be studied, even if you don't take their advice.
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