3C24 in a SE output stage - output transformer impedance recommendation !!! - Page 2 - diyAudio
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Old 5th September 2008, 11:40 PM   #11
aldovan is offline aldovan  Brazil
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Default 3C24

In the Eimac datasheet is recommended 9200ohms in class AB for two tubes with 750V in plate. 4600 ohms in each tube.
I will use with 300V,one tube.
http://www.mif.pg.gda.pl/homepages/f...090/3/3C24.pdf
For me, 8K4 is very high and the power is small with this impedance.
I will try with 1K, 3K5, 5K and 10K.

Aldovan
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Old 5th September 2008, 11:53 PM   #12
nhuwar is offline nhuwar  United States
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Thats class AB not class A. I once made the same mistake. The rp in class a is higher then ab1 or 2. Also it's class B on the data sheet.


Nick
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Old 5th September 2008, 11:56 PM   #13
nhuwar is offline nhuwar  United States
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On a lighter note. That little tube's a tank. I had one cooking at 150 watts dissipation in the garage for about 15 minutes and it didn't even phase it.

The darn tube just took it and smiled back at me, the thing works fine. And honestly I was trying to destroy it.
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Old 6th September 2008, 12:09 AM   #14
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3,5k is probably a fine value regarding power, should yield some 7watts with adequate drive. The problem as I see it is that you will need quite som feedback to achieve an output impedance that will let you drive a real world speaker.

Eimac data sheet states a mu of 23 and a transconductance of 2.5mA/V (@ 100V, 25mA) that gives a plate resistance of approx 9.2K. Will probably be somewhat lower @ 300V, 75mA. No feedback and 3.5k OPT will give a damping factor of approx 0.4
You'll want to better that.

/Olof
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Old 6th September 2008, 06:23 AM   #15
kmaier is offline kmaier  United States
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Cooking a 3C24 for 15 minutes is just bad but probably not lethal... for continuous service you need to provide some heatsinking on both the plate and grid leads or the seals will fail. It's not about torturing the little guy, it's about getting good service life and performance.

As for loading, I would suggest a 3.5K load and 400 volts plate supply. Bias it up so you optimize the power output... around 60ma and probably good for 5-6 watts. As mentioned, you'll most likely need some NFB to clean things up a bit.

Lastly, a DC filament will be required to get the hum level down. Make sure you use properly rated parts here as the current requirement is 3 amps. Do NOT run the filament voltage low... this is a thoriated-tungsten filament. Also note that the filament alone generates 20 watts of heat plus another 20-25 watts of plate dissipation so consider it a 50-watt heater and ensure adequate ventilation.

Regards, KM
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Old 6th September 2008, 10:18 AM   #16
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I took the curves for the 3C24 and plotted the operation point: 300V on plate, 40V on grid, resulting in ~75mA of anode current (blue point at intersection of the red lines).

According to MJ Valve amplifiers, 3rd edition, p74, the equation to calculate plate resistance is:

ra = delta Va / delta Ia

Therefore I draw the two green lines at 100V and 500V, representing a Vpp of 400V. The resulting current at 100V is about 55mA, while at 500V it is 105mA. Entering those values in the above mentioned equation yealds:

ra = 400V / 0.05A = 8000R

Does this sound right?

Erik

Due to file size constraint I had to cut all of the class B operating points, which are of no interest for the SE demons anyway
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