3C24 in a SE output stage - output transformer impedance recommendation !!! - diyAudio
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Old 5th September 2008, 12:51 PM   #1
aldovan is offline aldovan  Brazil
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Talking 3C24 in a SE output stage - output transformer impedance recommendation !!!

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I want use the 3C24 in a SE amplifier output.
I think in apply 300V and 75mA in the plate.
3500 ohms is a fine impedance for the output transformer ?

Thanks,

Aldovan
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Old 5th September 2008, 01:03 PM   #2
Gluca is offline Gluca  Italy
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I never dealt with that tube... but its Rp is in the 10-15k range at least so you want to use feedback.
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Old 5th September 2008, 03:09 PM   #3
nhuwar is offline nhuwar  United States
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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
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Old 5th September 2008, 03:33 PM   #4
kmaier is offline kmaier  United States
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Suggest you take look here before doing anything else:

http://www2u.biglobe.ne.jp/~tossie/KY-3C24-J.html

Note that the 3C24 needs to be run in class A2 mode.

Regards, KM
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Old 5th September 2008, 05:37 PM   #5
aldovan is offline aldovan  Brazil
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Default 3C24 output transformer

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
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Old 5th September 2008, 06:00 PM   #6
nhuwar is offline nhuwar  United States
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Default Re: 3C24 output transformer

Quote:
Originally posted by aldovan
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
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Old 5th September 2008, 06:56 PM   #7
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Default Re: 3C24 output transformer

Quote:
Originally posted by aldovan
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
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Old 5th September 2008, 09:25 PM   #8
kmaier is offline kmaier  United States
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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
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Old 5th September 2008, 11:17 PM   #9
aldovan is offline aldovan  Brazil
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Default 3C24

You are correct !
I trace the load line and with 300V x 75mA, the value for impedance is next to 4200 ohms.
15Kohms is for high voltages 3c24 drivers !!!

Aldovan
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Old 5th September 2008, 11:23 PM   #10
nhuwar is offline nhuwar  United States
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Ya 4200 ohms so you would need a 8.4k primary impedance.


Thats pretty close to 10k, me personally I prefer to use a more then 2x ratio of rp to priZ I would like 3x but thats 12.6k which is too high. Ya it's less power but less distortion.



Nick
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