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"Estudiante" Hybrid (7370 + MOSFET)
"Estudiante" Hybrid (7370 + MOSFET)
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Old 7th February 2018, 02:08 PM   #31
Antoinel is offline Antoinel  United States
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"Estudiante" Hybrid (7370 + MOSFET)
Originally Posted by xrk971 View Post
Nice work JP. What DC-DC step up are you using? Note that you need to use the LM317HV for the CCS of the SE Class A output stage. I tried regular LM317 and it did not work. Very finished design.
Hello xrk971. A constant current source [CCS] which uses LM317 can oscillate unexpectedly. Consider adding a Zobel [10 Ohm resistor in series with 0.1uF] like that found in power amps between the output of LM317 and ground to remedy!

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Old 10th February 2018, 10:55 PM   #32
xrk971 is offline xrk971  United States
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"Estudiante" Hybrid (7370 + MOSFET)
I think the issue was the 48v supply - LM317 is not rated that high and it probably latched into a safe mode.
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Old 14th February 2018, 04:26 PM   #33
Obe1 is offline Obe1  United States
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Default Cathode Resistor Clarification

Thank you SodaMod for presenting this project and all your nice tutorials on the wtfamps.com web site.

I'm a tube noob, so please help me understand this: Since the target quiescent operating point is -1 volt DC on the grid, to created 2.5mA of plate current, it seems like the cathode resistor should be something closer to 400 Ohms instead of the 750 Ohms shown on the schematic? Please set me straight.
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Old 14th February 2018, 05:02 PM   #34
Obe1 is offline Obe1  United States
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My apologizes to Sodacose for misspelling you name in my earlier post.
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Old 14th February 2018, 05:24 PM   #35
Sodacose is offline Sodacose  United States
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Your observation is not flawed, Obe1! The plate curve calc suggests that 400 ohms would put us where we want to be (1V/2.5ma).

Our goal is to get the anode to sit around half the supply voltage so that the output stage can swing up and down with signal. In practice, I found that a 750 ohm resistor put my anode at 24V. I didn't explain this anywhere on the page though. I'll revisit my write up and maybe add some additional voltage check measurements (probably helpful for others, too).

Here is a detail of the different cathode resistors:

Click the image to open in full size.

You can see that although the difference in value appears to be large, the anode voltage (found on the green dashed line) varies less than you might expect (between dashed red and dashed blue).
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Old 14th February 2018, 10:24 PM   #36
Obe1 is offline Obe1  United States
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Thank you Sodacos for your prompt and comprehensive responce to my inquiry.

It all makes much more sense now.

My lesson of the day: Creating a design using vacuum tubes is like working with any other component. When you wish to push it close to it's specified boundaries, science & logic must accept some assistance from a strong dose of thaumaturgy, a pinch of good luck, and possibly a bigger hammer.

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