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Old 25th August 2012, 12:23 AM   #1
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Default mosfet source follower output stage.

Was told this was an idea with no merit, actually forgot about it for a while. Source followers are used to give greater current capacity to a tube driving a tone stack or even an output tube. I was wondering if you took a preamp pentode and added a mosfet to it could you drive an output transformer instead of using a power tube? I was told the result would not sound like either a triode or pentode output amp because there was no interaction between the tube and the transformer. I am not sure I agree with the conclusion, thought I would ask you guys.
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Old 25th August 2012, 01:15 AM   #2
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I saw a comment once by Patrick Turner on his website about a "tube" amp he built using Mosfets for outputs. As I recall, it was deemed very "tubelike" sounding. No doubt the transformer output used also contributes to some of the traditional "tube" sound. A grounded source configuration would be the usual choice to mimic a pentode. Using a source follower will likely just give the sound of the driver/gain stage instead. The power law transfer function of Mosfets changes from square law thru 3/2 power law toward linear law as the current thru them increases. So by moderate idle current biasing one can get them centered on the 3/2 power law section to mimic tubes quite well.

Another trick you can try is to put some TV damper rectifier tubes below the source terminal of the Mosfets (like cathode bias resistors). This gives curves on the curve tracer that look very much like pentode tubes. To get triode results, also include some local "Schade" feedbacks (resistive loop from "plate" back to "grid") for each device.
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Old 25th August 2012, 03:46 AM   #3
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by Printer2 View Post
Was told this was an idea with no merit, actually forgot about it for a while. Source followers are used to give greater current capacity to a tube driving a tone stack or even an output tube. I was wondering if you took a preamp pentode and added a mosfet to it could you drive an output transformer instead of using a power tube? I was told the result would not sound like either a triode or pentode output amp because there was no interaction between the tube and the transformer. I am not sure I agree with the conclusion, thought I would ask you guys.
If you are going to use a power mosfet, why bother with an output transformer. The mosfet can drive the speakers. If you did use a transformers you'd need a MUCH lower impedance one because the mosfet would be operating at lower voltage than the tubes (I assume you don't use 400 volt rails with your mosfet)

There are plenty of these amps around. we call them hybrid amps. Yu really do get a tube sound out of a tube preamp and you can save much $$$ using the power mosfet output stage because you don't need the expensive output transformers.

Some curent production Bass guitar amps have tube preamps and SS power sections because bass players need LOTS of power and kilowatt (or even 500W) tube amps are expensive. So they compromise. The same concept would work for stereo HiFi.

That said some early solid state amps did use output transformers.
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Old 25th August 2012, 03:52 AM   #4
AJT is offline AJT  Philippines
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have a look here: New amp: 13DE7 SE hybrid
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Old 25th August 2012, 03:53 AM   #5
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Originally Posted by ChrisA View Post
If you are going to use a power mosfet, why bother with an output transformer.
Just curious.

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The mosfet can drive the speakers. If you did use a transformers you'd need a MUCH lower impedance one because the mosfet would be operating at lower voltage than the tubes (I assume you don't use 400 volt rails with your mosfet)
About 300V I would guess.

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There are plenty of these amps around. we call them hybrid amps. Yu really do get a tube sound out of a tube preamp and you can save much $$$ using the power mosfet output stage because you don't need the expensive output transformers.
I have transformers already.

Quote:
Some curent production Bass guitar amps have tube preamps and SS power sections because bass players need LOTS of power and kilowatt (or even 500W) tube amps are expensive. So they compromise. The same concept would work for stereo HiFi.
Don't need much power, thinking around 5-10W.

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That said some early solid state amps did use output transformers.
I have one.
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Old 25th August 2012, 04:02 AM   #6
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Originally Posted by Tony View Post
have a look here: New amp: 13DE7 SE hybrid
See, I knew it was doable. Thanks.
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Old 25th August 2012, 04:04 AM   #7
AJT is offline AJT  Philippines
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of course....if you can imagine it.....it can be done......

you can use 6EW7 also or 10EW7 if you have those.....
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Old 25th August 2012, 05:56 AM   #8
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I've been interested in a Mosfet output stage for a while, but I really only understand tubes and how to bias them. Never read up on solid state. I don't like the idea of a big electrolytic cap in the output - surely that degrades the sound? And transformers are expensive. So I'd be interested in a design with no transformers. Trouble here is I don't know how to bias the tubes properly - almost all the schematics for solid state have input devices which I wouldn't want. I just want a complete self-contained output stage. The circlotron is interesting.

Andy
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Old 25th August 2012, 08:48 AM   #9
AJT is offline AJT  Philippines
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Quote:
Originally Posted by andyjevans View Post
I've been interested in a Mosfet output stage for a while, but I really only understand tubes and how to bias them. Never read up on solid state. I don't like the idea of a big electrolytic cap in the output - surely that degrades the sound? And transformers are expensive. So I'd be interested in a design with no transformers. Trouble here is I don't know how to bias the tubes properly - almost all the schematics for solid state have input devices which I wouldn't want. I just want a complete self-contained output stage. The circlotron is interesting.

Andy
film types can be used instead for the coupling cap, and as far as the OPT, with the lowish impedances used, such as 400ohms to 8 ohms makes design of opts very easy.....interleaving can be dispensed with so much so that power transformers can be used with good results.....

i am building one myself.....

wrt the capacitance issues with mosfets, the solution there would be to operate the driver tube with higher that ussual plate currents imho....
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Old 25th August 2012, 03:05 PM   #10
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Originally Posted by Tony View Post
film types can be used instead for the coupling cap, and as far as the OPT, with the lowish impedances used, such as 400ohms to 8 ohms makes design of opts very easy.....interleaving can be dispensed with so much so that power transformers can be used with good results.....

i am building one myself.....

wrt the capacitance issues with mosfets, the solution there would be to operate the driver tube with higher that ussual plate currents imho....
Keep us posted on your progress.
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