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Aikido CF cathode resistor?

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Sodacose

Sodacose

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Joined 2006
I'm trying to work out an Aikido CF circuit from Tubecad's articles on the subject, but I'm having some trouble. Does anyone know how to calculate the cathode resistors?

I'm referring to the Rk's in graphic on this page comparing a traditional cathode follower and the Aikido CF.

It doesn't seem to be as straight forward as checking some data sheets and drawing load lines. Anyone with an ACF care to give me some hints on their setup? I'm thinking about trying ECC88's and will end up with a B+ of 250ish (1/2 B+ to each tube).
 
SY

SY

Ex-Moderator
Joined 2002
SYclotron.com
Sodacose said:
I'm trying to work out an Aikido CF circuit from Tubecad's articles on the subject, but I'm having some trouble. Does anyone know how to calculate the cathode resistors?
Click to expand...

Sure. First, decide on the voltage drop you want across each tube and the current. Let's say you want 125V across each tube and 10mA. Now, compared to 125V, the voltage drop across the cathode resistors is negligible. So... go to the plate characteristic curves for the tube, draw a line at 10mA (that will be horizontal). Now draw a line at 125V (that will be vertical). They intercept very near the Vg = -3V curve. So, to first order, you want a 3V drop across each cathode resistor. Use Ohm's Law, R = V/I = 3V /10mA = 0.3k or 300R.
 
Gost22

Gost22

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Joined 2012

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Gost22

Gost22

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Joined 2012
kevinkr said:
Gost,
Using R= E/I you calculate the cathode resistor. E in this case is the grid bias of -0.5V and I is your 1.5mA of plate current so R = 0.5V/0.0015A or 330 ohms.

I would mention that a grid bias of 0.5V is too low with the 12AX7, you will have excessive grid current.
Click to expand...

What do you suggest for R in this case?:(:confused:
Can we take R = 560R or 680...1k?
Thank you!
 
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kevinkr

kevinkr

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It is more than just cathode R, you will need to pick a better operating point with a higher plate voltage so that Vg is higher, anything lower than 1V results in grid current, I have gotten acceptable results at 0.8V..

I also think the 6DJ8 would be a better choice for this application.
 
Gost22

Gost22

Banned
Joined 2012
kevinkr said:
It is more than just cathode R, you will need to pick a better operating point with a higher plate voltage so that Vg is higher, anything lower than 1V results in grid current, I have gotten acceptable results at 0.8V..

I also think the 6DJ8 would be a better choice for this application.
Click to expand...
R=0.8/0.0012=666R!?:confused:
R=680ohms
P.S.
I have two pieces JJ ecc83;)
 

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