Would it be possible to drive a 300b with new power jfets? - diyAudio
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Old 4th April 2011, 02:18 AM   #1
JZatopa is offline JZatopa  United States
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Default Would it be possible to drive a 300b with new power jfets?

If I understand this right these new Semisouth Jfets can handle some pretty high voltage. What do you guys think about driving a 300B, 2a3 or possible a 847 tube with one? Is there something I am missing with this idea?
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Old 4th April 2011, 03:27 AM   #2
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Old 4th April 2011, 06:28 AM   #3
JZatopa is offline JZatopa  United States
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Interesting read, something I may try at a later date but not quite what I am talking about. What I am talking about would be an amp with a jfet directly driving a tube.
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Old 4th April 2011, 07:42 AM   #4
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Im not knocking your idea, but isn't the virtue of tubes the linear voltage amplification and that of solid state devices the ability to deliver current? What is your motivation for doing it the other way round, apart from "because you can"
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Old 4th April 2011, 08:58 AM   #5
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Quote:
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Im not knocking your idea, but isn't the virtue of tubes the linear voltage amplification and that of solid state devices the ability to deliver current? What is your motivation for doing it the other way round, apart from "because you can"
Correct me if I'm wrong, but I thought that both valves and FETs are essentially voltage controlled current sources.
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Old 4th April 2011, 10:19 AM   #6
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Originally Posted by mastodon View Post
Correct me if I'm wrong, but I thought that both valves and FETs are essentially voltage controlled current sources.
yes thats my understanding too, but fets are capable of delivering more current.
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Old 4th April 2011, 03:53 PM   #7
Sheldon is online now Sheldon  United States
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Originally Posted by DJNUBZ View Post
If I understand this right these new Semisouth Jfets can handle some pretty high voltage. What do you guys think about driving a 300B, 2a3 or possible a 847 tube with one? Is there something I am missing with this idea?
They don't look very linear to me, although it's hard to tell, because the curves on their spec sheets are for much higher currents that you'd ever use in a driver for an output tube. Also, the input capacitance is about 10 times or more than would be typical for a driver tube, so your source would have to be capable of driving it.

Of course, you can get around those issues, with cascoding, feedback (degeneration), etc., but I'm not sure what the benefit would be.

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Old 4th April 2011, 04:22 PM   #8
JZatopa is offline JZatopa  United States
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This was more of a "could it be done" question. I have not seen an amp that uses only a single high power output tube and no other tubes. I wonder if it would be one of those things where the amp sounds better then what it's measurements would show.

A tube driving a power jfet would seem more obvious but similar hybrid amps already exist.
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Old 4th April 2011, 04:50 PM   #9
Sheldon is online now Sheldon  United States
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Sure, it can be done. But as mentioned previously in the thread, the high voltage FET's that are available are pretty high current devices. And in a tube amp, it doesn't make much sense to have your driver stage eating up much current, certainly not more that the output tube. That doesn't mean that you can't use these devices at lower current, but I think they will be much less linear than available tube choices for the same application. And, they have much higher input capacitances. You can deal with those issues, but what have you gained, as a practical matter?

I'm by no means an expert on any of this, take the following with that in mind. The FET's designed for audio are more linear, but also for the lower voltages typical of SS amps. And there seem to be small signal devices (linear, low voltage) and output devices (high current), with little in between. I would guess that audio FET's could be designed to substitute pretty well for a driver tube but there's no commercial reason to do so. So unless it accidentally fits well with some other application, it won't happen.

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Old 4th April 2011, 05:14 PM   #10
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In one of my designs 6P15P drive Semisouth JFETs. Incredible sound.
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