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6S41S WCF OTL Design
Mar 6, 2020 at 6:13 PM Thread Starter Post #1 of 38
Type26

Type26

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6S41S WCF OTL Headphone Amp
I am trying to design an OTL that will drive ANY impedance cans. My main phones are Audeze LCD-2 CB which are 70 ohms. I also have 32 ohm ohm HE-4xx and 300 ohm HD-650s. I have built four HP amps using Aikido topology, which sound fantastic with 300-600 ohm HP, but not enough oomph for low impedance. I do have 600 ohm OPT but the secondary is only 8 ohms (I was saving them for a 6C33C project). The schematic is attached (preliminary), PS schematic to follow.

Please note the drawing error; V1 output goes from plate to V3 grid, not grid to grid
 

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Mar 7, 2020 at 12:04 PM Post #2 of 38
You should also make sure that your output and coupling capacitors is rated to or above your B+ voltage. That way if something shorts, your caps wont blow up.

This project might also be a non start simply because you will need to have pretty good matching between 4 6s41s tubes which may not be possible without buying a large lot of them and hand matching them.
 
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Mar 7, 2020 at 4:12 PM Post #3 of 38
I will also point out that this is not a WCF or White Cathode Follower amplifier. This is just a basic cathode follower with your second 6S41S configured as a constant current sink. You are missing the negative feedback loop around your output stage to make it a white cathode follower.
 
Mar 8, 2020 at 12:26 PM Post #4 of 38
Tjj226 Angel said:
I will also point out that this is not a WCF or White Cathode Follower amplifier. This is just a basic cathode follower with your second 6S41S configured as a constant current sink. You are missing the negative feedback loop around your output stage to make it a white cathode follower.
Click to expand...
You are correct, but too late to change the title of the thread lol
 
Mar 8, 2020 at 1:44 PM Post #6 of 38
Type26 said:
You are correct, but too late to change the title of the thread lol
Click to expand...

Lol. Fair enough.

I would probably add the negative feedback back in.

A cathode follower's output impedance is defined by Plate resistance / the amplification factor of the tube. If I am reading this right, the plate resistance will be 10 ohms and the amplification factor will be about 2ish (maybe 3 if you are lucky).

That means that your output impedance will be 50 ohms at best and 75 ohms at the worst.

A WCF amplifier will have much lower impedance and twice the current swing. Win win.
 
Mar 14, 2020 at 6:16 PM Post #7 of 38
Going dual mono with each side having a hybrid bridge using 6D22S. All toroids and the 6N6P plate choke will be under the chassis. Going to paint it with "Double Blue Vein" powder coat from Prismatic Powders. The chokes and caps will be painted with textured black paint. Going to have dual Alps 100K attenuators for the volume. The output caps are friggin huge! Going to run the 6D22S heaters on series connected 12v toroid (dual secondaries) when all is said and done, will have less than $500 in. I fixed the schematic and changed the output tube current to 100ma and input tube to 6N6P, I also fixed so that it is a real WCF. This amp will be a monster and will weigh around 60-80 lbs! The output caps are paralleled 200uf (400uf each side)250v. polypropylene
 

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Mar 14, 2020 at 6:24 PM Post #8 of 38
I'm thinking this amp will be able to drive just about ANY pair of cans! I had thought about going nuts and making the bottom tube 6C33C, but not sure how well dissimilar tubes would work in this situation.
 
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Mar 14, 2020 at 7:03 PM Post #9 of 38
Type26 said:
Going dual mono with each side having a hybrid bridge using 6D22S. All toroids and the 6N6P plate choke will be under the chassis. Going to paint it with "Double Blue Vein" powder coat from Prismatic Powders. The chokes and caps will be painted with textured black paint. Going to have dual Alps 100K attenuators for the volume. The output caps are friggin huge! Going to run the 6D22S heaters on series connected 12v toroid (dual secondaries) when all is said and done, will have less than $500 in. I fixed the schematic and changed the output tube current to 100ma and input tube to 6N6P, I also fixed so that it is a real WCF. This amp will be a monster and will weigh around 60-80 lbs! The output caps are paralleled 200uf (400uf each side)250v. polypropylene
Click to expand...


Jesus.

Someone likes film capacitors :beerchug:
 
Mar 14, 2020 at 7:05 PM Post #10 of 38
Type26 said:
I'm thinking this amp will be able to drive just about ANY pair of cans! I had thought about going nuts and making the bottom tube 6C33C, but not sure how well dissimilar tubes would work in this situation.
Click to expand...

If you are using a real wcf, then you do not want to do this. A WCF is a push pull amp, so the bottom tube will actually be contributing to the sound.
 
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