Thinkin' about laying out an F-2B with mosfet follower

Tube or solid-state, this section goes to eleven!

Thinkin' about laying out an F-2B with mosfet follower

Postby Timpanogos Slim » 27 Feb 2018, 07:53

I've seen various threads here and elsewhere about the F-2B bass preamp.

There's plenty of talk about a 2-tube version with the 2nd tube providing cathode followers. And some talk about just using an irf820 source follower to buffer the output for better compatibility with power amps that have lower-Z input.

Didn't see a schematic. I'm not super well versed in how this stuff works, but did find some sites discussing how to implement, in general terms, a mosfet source follower to a tube doing the same job as a cathode follower.

Drew up this. Does it look right to anybody?

F2B Plus.png
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Re: Thinkin' about laying out an F-2B with mosfet follower

Postby microbailey » 02 Mar 2018, 20:05

Hi Slim. I’m not great with tube design, and I don’t know this preamp, but the source follower circuit looks right. Just make sure that output cap has a suitable voltage rating as it will have most of the the HT supply across it. If this is for bass it’s worth checking the high-pass filter cutoff frequency of your C4 with R11 / R9. Take the parallel resistance of R11 with R9 and plug in to one of the online RC corner calculators with C4. By the way you won’t be able to buy a 0.5uF cap, closest is 0.47uF.
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Re: Thinkin' about laying out an F-2B with mosfet follower

Postby peterc » 03 Mar 2018, 11:02

I have done this a couple of times, works fine. As per microbailey's advice, check the voltage of the caps.

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