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optimizing 6N6P WCF

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ChuckT

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Joined 2002
#1
I have a 6n6p white cathode follower headphone amp. After measuring various voltages I believe the circuit can be further optimized for 300 ohm load and exactly 247V supply.

The plate resistor is 220R, cathode resistor is 100R. I have 244V at plate, 89V at the middle, -1.4V on the lower grid.

Based on John Broskie's optimized Ra formula

Ra = (Rp +2Rl) /mu = (2000 +2x300) / 22 = 118ohm

Is this correct? And what of the cathode resistor?
Thanks for help.
 

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#5
ChuckT said:
...white cathode follower
....I believe the circuit can be further optimized
...I have 244V at plate, 89V at the middle
Click to expand...

The WCF's 2nd harmonic is minimized when the two tubes have =equal= plate-cathode voltages. You have a nearly 1.75X difference?! You might try trimming the driver to set the WCF grid for more equal distribution.

The bottom resistor essentially sets the idle current. This must be more than half the peak signal current. Unlike simpler amplifiers, setting the current much higher does little good, just runs hotter.
 
#6
I recently bought one hundred of 8AL9 tubes for preamps that can drive even 32 Ohm headphones, $1 each on esrc1. They are triodes-pentodes, with a high gm frame grid pentode capable to dissipate 10W of power. The triode will be loaded on a Gyrator, the pentode would work as a cathode follower loaded on a CCS. Rectifying 6.3V by Shottky I am getting clean 8.6V for their filaments.
 

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#7
The 8AL9 looks like a very interesting tube. I didn't know a tube can have both a triode and pentode. I will keep this tube in mind for future project. But for now, I am still learning about tubes and I'll stick with kits and modification of it.
I also just ordered a 60's Foton 6n6p square getter from ebay. I'll see how that sounds.
 
#8
PRR said:
The WCF's 2nd harmonic is minimized when the two tubes have =equal= plate-cathode voltages. You have a nearly 1.75X difference?! You might try trimming the driver to set the WCF grid for more equal distribution.

The bottom resistor essentially sets the idle current. This must be more than half the peak signal current. Unlike simpler amplifiers, setting the current much higher does little good, just runs hotter.
Click to expand...

Yeah, the unbalance voltage is the reason I start this post. So what does it mean by "trimming the driver to set the WCF grid".

Also, I should increase the 6n6p cathode resistor? What would be a good value?
 
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