A guide to building the Pass F4 amplifier

[Dennis Hui]

What is the voltage range you're getting from the NCP431 when you adjust P1
through its range?

Any chance you just need more voltage out of the NCP431? Are you using 22.1K
for R8? A later revision of the schematics has R8=27.4K.

 

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I think you may have nailed it. I have 22.1K in the non working amp and 27.4K in the working amp.

Thank you!

 

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I just checked my order history, and was shocked to find i ordered 22.1R! Bonehead.

 

[2 picoDumbs]

Bloody bone head.

Don't worry there are plenty of bone heads around here.

Hahahaha

 

[Zen Mod]

maybe I mixed it with some other thread, but didn't you tell that amp was in working state?

with wrong resistors it couldn't

 

[2 picoDumbs]

This reminds me of the time I had an open frame headphone amp where a moth flew into it and shorted it out.

Hahahaha

 

[2 picoDumbs]

with wrong resistors it couldn't

Class B-

Hahahaha

 

[01010010 01010011]

No, the first one worked, but I built a second one for monoblocks. I ordered from the same Mouser BOM I used for the first one. I have no idea how this happened.

 

[Zen Mod]

 

[Dennis Hui]

Glad the mystery is solved.

 

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Thank you!

 

[cal3713]

Glad you found the issue. "The wrong resistors!"

 

[ggetzoff]

Okay, some more amp porn. Just finished another tube amp to drive the F4. This time an even simpler design: a 6CY7, which is a dual triode (driver and output) in a single envelope. I chose again to use the fabulous Type 80 rectifier and 4 ohm secondaries on the OTs.
If you guys haven’t heard the F4 driven by a flea amp, you should . The distortion products of a tube amp I think go beyond descending levels of H2, H3....so on and so forth. Meaning you can’t just add H2 and H3 (as I have tried) to get exactly the same experience of having and an actual tube. Credit goes to Matt for his great and very affordable design.

https://www.cascadetubes.com/wp/wp-content/uploads/2020/09/6CY7-v2-schematic.jpg

 

[cal3713]

Looks great. Would love to hear them...

 

[2 picoDumbs]

Nice one.

 

[Eric]

I've been bitten by the "wrong value resistor" problem as well. I've even had several orders filled incorrectly - I ordered the correct value, but the wrong part was put into the bag. After this, I do not put *ANYTHING* into a PCB without first measuring it.

ggetzoff: is that tube amp closest your F4 a stereo chassis? Looks like a nice little amp!

 

[ggetzoff]

Eric,

Yup, it's stereo

 

[Eric]

What's the output power? Looks like an inexpensive build (though I'm probably wrong)

 

[ggetzoff]

A whopping 1.6W. Very inexpensive. I just use the Edcor 10W OPTs. They very low inductance, way lower then "big iron" and measure better then many more expensive transformers out there. Edcor claims extension to 40Hz but these perform well into the 30's. You looking at $18 worth of tubes (I get tubes locally for low dollars). But even on Ebay, the ST bottle Type 80s are cheap and so are the the 6CY7s.

 

[Eric]

Cool - thanks for the details! Always looking for another project that I don't have time to build or a place to put it when I'm done