Can you use a MOSFET as a cathodyne

aardvarkash10 · 14th February 2009, 07:39 AM

splitter? Looking to deliver into a -50V biased grid on an EL36 pp - I'm a bit worried about having enough drive from a 6J7 voltage amp to 6J5 cathodyne splitter. As an option, I could go 6J7 to 6J5 two stage amplification, but I don't have space another tube to split the phases. I do have space for sand... Yeah, got MOSFET follies, and I can see the 'follower option is all good - since I'm not looking for amplification, can I take a grid from each of the Drain AND the Source of one MOSFET just like anode and cathode of a tube?

fishandchips · 14th February 2009, 10:55 AM

So, as far as I can understand your post, you want to add a Mosfet to split the signal after a 6J7 gain stage?

What kind of Mosfet? Input resistance? Output resistance? Supply voltage?

And how many EL36 do you have to drive?

Or did I misread what you like to do with the Mosfet

How about using two BF459 transistors for full drive and split?

SY · 14th February 2009, 11:58 AM

If you've set the DC conditions correctly, yes. High frequency balance will not be as good as a tube because of the gate-drain capacitance.

Eli Duttman · 14th February 2009, 01:09 PM

SY's point about capacitances is on target. A "concertina" phase splitter is a tricked out voltage follower. THE crucial parameter for a FET in voltage follower service is a low and stable reverse transfer capacitance. I recommend the ZVN0545A, for this job. While the IRFBC20 is much more robust, I'm afraid of HF roll off, given the driving 6J7.

The source sets up a few Volts below the gate. Assuming the source is at the same potential as the preceeding anode is a fair approximation. Pick a drain current (say 3 mA.) and use Ohm's Law to calculate the source to ground resistance. Symmetry requires that the same resistance and potential difference be used on the drain side. Make 1 of the loads variable, as that permits "perfection" of the 2 out of phase signals.

Johan Potgieter · 14th February 2009, 05:40 PM

Quote:

Originally posted by Eli Duttman
.... is a tricked out voltage follower.
[/B]

Yeh, right

Bandersnatch · 14th February 2009, 06:42 PM

hey-Hey!!!,
Use a cascode MOSFET. The input capacitance is then quite close to static, instead of varying about with drain-source voltage.
cheers,
Douglas

aardvarkash10 · 14th February 2009, 11:11 PM

yeehaa! Time to break out a power supply and do some testing in situ. THanks for your advice!

14th February 2009, 07:39 AM	#1
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	Can you use a MOSFET as a cathodyne splitter? Looking to deliver into a -50V biased grid on an EL36 pp - I'm a bit worried about having enough drive from a 6J7 voltage amp to 6J5 cathodyne splitter. As an option, I could go 6J7 to 6J5 two stage amplification, but I don't have space another tube to split the phases. I do have space for sand... Yeah, got MOSFET follies, and I can see the 'follower option is all good - since I'm not looking for amplification, can I take a grid from each of the Drain AND the Source of one MOSFET just like anode and cathode of a tube? __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012

14th February 2009, 11:58 AM	#3
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	If you've set the DC conditions correctly, yes. High frequency balance will not be as good as a tube because of the gate-drain capacitance. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

14th February 2009, 01:09 PM	#4
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	SY's point about capacitances is on target. A "concertina" phase splitter is a tricked out voltage follower. THE crucial parameter for a FET in voltage follower service is a low and stable reverse transfer capacitance. I recommend the ZVN0545A, for this job. While the IRFBC20 is much more robust, I'm afraid of HF roll off, given the driving 6J7. The source sets up a few Volts below the gate. Assuming the source is at the same potential as the preceeding anode is a fair approximation. Pick a drain current (say 3 mA.) and use Ohm's Law to calculate the source to ground resistance. Symmetry requires that the same resistance and potential difference be used on the drain side. Make 1 of the loads variable, as that permits "perfection" of the 2 out of phase signals. __________________ Eli D.

14th February 2009, 06:42 PM	#6
Bandersnatch diyAudio Member Join Date: Jul 2003 Location: Ann Arbor, MI	hey-Hey!!!, Use a cascode MOSFET. The input capacitance is then quite close to static, instead of varying about with drain-source voltage. cheers, Douglas __________________ the Tnuctipun will return

14th February 2009, 11:11 PM	#7
aardvarkash10 diyAudio Member Join Date: Mar 2007 Location: Auckland, NZ	thanks guys! yeehaa! Time to break out a power supply and do some testing in situ. THanks for your advice! __________________ Yes, conservatism thrives on low intelligence and poor information. But the liberals in politics... continue to back off, yielding to the supremacy of the stupid. It's turkeys all the way down. - George Monbiot, guardian.co.uk, 6 Feb 2012

14th February 2009, 10:55 AM	#2
fishandchips diyAudio Member Join Date: Feb 2009	So, as far as I can understand your post, you want to add a Mosfet to split the signal after a 6J7 gain stage? What kind of Mosfet? Input resistance? Output resistance? Supply voltage? And how many EL36 do you have to drive? Or did I misread what you like to do with the Mosfet How about using two BF459 transistors for full drive and split?

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