white cathode follower : Rk question
« on: April 09, 2018, 05:27:21 PM »
calculated Rk value is giving me slightly more distortion then distortion optimized Rk...
i.e.: if i place a 1k pot at Rk, i can get better distortion figures.
0.34% (calculated) vs 0.13%  with pot

at 600 ohm load : 22dbu out

would this be due triodes are not perfectly matched ? or am i missing something.

(my fuses are dead on my DDMs, cant measure to confirm real life data - calculation formulas from tube cad journal)

after double checking my calculations, i picked 1.2k Ra and 570r for Rk,
but 312r Rk is less distortion
6SN7 WCF with 310 VDC supply

edit2: changed topic from while to white :)
« Last Edit: April 09, 2018, 05:30:52 PM by kambo »


Re: white cathode follower : Rk question
« Reply #1 on: April 09, 2018, 06:30:49 PM »
How old is the datasheet,  does it represent the actual brand tube you are using? Could you post what you are using?

I would tend to go with what you are actually measuring. The manufacturing spread of tubes I would guess is much wider than what we expect with modern transistors.


Re: white cathode follower : Rk question
« Reply #2 on: April 09, 2018, 06:58:37 PM »
just an old RCA data sheet, check couple other ones, cant see a difference!


Re: white cathode follower : Rk question
« Reply #3 on: April 16, 2018, 05:34:44 PM »
Kambo, a few notes.

Tube Cad journal's formula was later revised after some adverse comments.

Unless you plan to elevate the heater supply, 310V is too high making the mid point 155V, check the spec of the tubes for maximum heater-cathode voltage.

Best way to optimise a WCF (with any cathode resistor) is to use a pot for RA and adjust for maximum output voltage.  Distortion occurs when the sine wave can no longer reach it's peak and starts to clip and output falls.

Run whatever tube you choose at about half its maximum current for longevity, this is set by Rk.

Twin triode tubes rarely match exactly, vari-mu types are even worse. 

The WCF in the LA-2 is a good place to start.

Hope this helps.

Soundcloud: Delayed Action.


Re: white cathode follower : Rk question
« Reply #4 on: April 16, 2018, 06:39:26 PM »
Ahhhhhh that makes sense... thanks so much DaveP :)


Re: white cathode follower : Rk question
« Reply #5 on: April 16, 2018, 09:55:14 PM »
Rk sets total current. The current must be large to drive 600r with a puny 6SN7 (rp=6K).

Rp sets go-around gain. The optimum is different for lo-Z or hi-Z loading. This can be predicted. It would clearly be easier for you to just trim it.

The WCF, properly balanced, can get THD below clipping in the zero-point-zero area.


Re: white cathode follower : Rk question
« Reply #6 on: August 13, 2019, 04:59:45 PM »
so, i am getting  0.012% THD to 10K load
same 3.5mA current at 600 ohm load is terrible  ;D
i realized changing Rk on preamp section changing standing current of WCF  ???
so with a  switch on preamp Rk
i am flipping standing current from 3.5mA to 9mA. on WCF, so at 600ohm load
distortion dropping to 0.34 % with only a db or 2 loss!
and my WCF s balance is staying the same,  and keeping the WCF tube running cool when 600 ohm not needed! i have no 600 ohm equipment, so it will stay in 10k load... but if i want to drive i would switch to
9mA  :)

edit: could pull more current for less distortion at 600ohm load, but this is good the way its!


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