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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

3C24 in a SE output stage - output transformer impedance recommendation !!!

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I would agree 10 to 15k if not higher . If it's used as a driver it like a choke load of atleast 500h. So I would assume the primary ot impedance would need to be pretty high.


I also have wanted to try this tube but have yet to do it.
Try the bottlehead audioassylum, thats where I got my info for it.



Hope this helps
Nick
 
Re: 3C24 output transformer

aldovan said:
10-15K is wrong.
I will use this tube with 300V in the plate, only.
In the schematic the author use 2k5 with 290V, correct for me.

Aldovan


I'm just going by what I have read about the tube which always stat's that it wants a ton of inductance on the anode.



And you ask for the help don't you think what you said was a tad rude. I very well could be wrong it's just when you ask people for help it's not the nicest thing to turn around and say they are wrong.
I will step off the soapbox now.

Nick
 
Re: 3C24 output transformer

aldovan said:
10-15K is wrong.
I will use this tube with 300V in the plate, only.
In the schematic the author use 2k5 with 290V, correct for me.

Aldovan



Are you going to be ok with the 2 watts or so that amp schematic produces? Running the plate a bit higher and going to a higher impedance transformer will cut the distortion down a bit and perhaps make the amp more useful, although that schematic is all finished and ready to go for you. It might be easier to run this guy in parallel feed and really concentrate on a very high inductance plate choke. You might talk to Mikey at Magnequest and see what he can wind for a plate choke/OT.


It might also be nice to throw a current source on top of the first stage to squeeze what you can out of the triode half of the 6BM8 and have some flexibility about using more feedback.
-Paul
 
All,

The 3C24 dates way back... you can find specs in a 50 year old ARRL handbook for 112 watts class B P-P modulator service, but this is at both (very) high voltage and high P-P impedance. If you want class A2 SE service at reasonable voltages, you need to reduce the load impedance and bias it well into class A2, no magic. Increasing load impedance without a significant plate voltage increase will result is less power. You can probably push it to 5-6 watts, but no real point. The plate has a 25 watt CCS rating, but considering the envelope size you'll need some creative cooling to keep it from high temps which will compromise the grid and plate lead seals.

The 3C24 is nice tube, and if you run it on the light side it should yield significant life... more so than most DHTs... but again, class A2 is a different animal and you may well find the driver setup does not yield the same sonics as a transformer drive. It's not about right or wrong... it's trying to use a transmitting triode for a purpose which was not intended.

Regards, KM
 
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.
 
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
 
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
 
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 :D
 

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