Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Archives of favorite topics => Topic started by: tubenit on April 22, 2017, 01:02:42 pm

Title: Mosfet cathode follower
Post by: tubenit on April 22, 2017, 01:02:42 pm

I thought this might be helpful to someone.  I've used a mosfet in a number of amps in the cathode follower position, and I can't tell any difference between it and a tube triode in that position.  It has been very successful for me.

I find the mosfet to be as easy to wire up as a tube triode.  I've not had any problems or failures using this so far.   Hoffman carries all that one would need to wire this up in his parts store.

Note the photo matches the layout drawing. 

So, here are some examples & info of what I am referring to:

http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm (http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm)

http://www.valvewizard.co.uk/accf.html (http://www.valvewizard.co.uk/accf.html)

with respect, Tubenit

Title: Re: Mosfet cathode follower
Post by: purpletele on April 22, 2017, 08:33:32 pm

That is great science/art Mr. Tubenit!

Title: Re: Mosfet cathode follower
Post by: smackoj on April 30, 2017, 10:00:32 pm

What specifications did you consider when deciding on which mosfet to try at this positions? Have you tried other mosfets as a cathode follower?

thanks, Jack D

Title: Re: Mosfet cathode follower
Post by: MakerDP on May 02, 2017, 11:45:07 am

I have seen these MOSFETS used as cathode followers:

IRF820
IRF830
2N50

I've used the 820 with great results. You can look at the specs of these, see what is common to all of them, and find something else suitable as well but the IRF820 is pretty widely available for less than $1 each...

http://www.mouser.com/ProductDetail/Vishay-Semiconductors/IRF820PBF/?qs=sGAEpiMZZMshyDBzk1%2fWi1F3z9PgzPBnOBw7d1X7X7g%3d (http://www.mouser.com/ProductDetail/Vishay-Semiconductors/IRF820PBF/?qs=sGAEpiMZZMshyDBzk1%2fWi1F3z9PgzPBnOBw7d1X7X7g%3d)

Title: Re: Mosfet cathode follower
Post by: Stockdale on September 15, 2017, 12:24:01 am

Quote from: smeackoj on April 30, 2017, 10:00:32 pm

What noteworthy bingo bonuses (https://bingoviews.com/best-bingo-bonuses) did you consider when deciding on the which mosfet to try at this positions? Have you tried other mosfets as a cathode follower?

thanks, Jack D

That's very helpful. Thanks tubenit!

Title: Re: Mosfet cathode follower
Post by: Williamblake on May 02, 2019, 04:08:51 am

Thanks for another great info. Could you replace the cf after the oscillator in this circuit in the same fashion? Or would you need a resistor to ground and a capacitor in series to connect to the preamp cathode?
https://el34world.com/charts/Schematics/files/Vox/Vox_ac4.pdf

Title: Re: Mosfet cathode follower
Post by: tubenit on May 02, 2019, 07:24:23 am

I don't know the answer to that question.  Sluckey might?  Perhaps consider sending him a PM.

Tubenit

Title: Re: Mosfet cathode follower
Post by: sluckey on May 02, 2019, 07:02:27 pm

I believe you can use a MOSFET in that circuit.