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Old 31st August 2009, 02:29 AM   #1
Bigun is offline Bigun  Canada
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Default SET amp using Triode-FET complementary feedback pair ?

Please forgive me if this is old news, it's my first post to the Tubes forum and I'm unfamiliar with Tubes and their history. I've tried a search for "Tube FET CFP" and got nothing.

I'm interested in the idea of a SE Triode amplifier. I've never made one (or heard one) but there is a legend around the SET that intrigues me.

Trouble is, I'd like more output power and perhaps the option of a lower damping factor. I'm willing to make some compromises on the sound to achieve this - for example, by going OTL.

I thought perhaps I could use a MOSFET as a 'slave' to the Triode in a complimentary feedback pair arrangement (CFP).

This arrangement keeps a fairly constant current through the Triode and allows the Triode to offload the dirty job of flowing lots of current to a MOSFET.

I used Nelson Pass zen v4 amplifier as the basic building block for this purpose.

Note that my knowledge of tubes is limited, the one I show in the schematic was chosen simply because my friends Chines hybrid amp uses it and I had an old simulation file already that had a custom spice model in place already.

Question - has this been tried before (if yes and it worked, can you point me to where I can get further information) ?
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Last edited by Bigun; 31st August 2009 at 02:32 AM.
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Old 31st August 2009, 03:08 AM   #2
Gordy is offline Gordy  United Kingdom
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I do not have the experience to comment in-depth on your hybrid endeavours, however I think you should Google "mu-follower" and also search this forum for the same.
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Old 31st August 2009, 03:17 AM   #3
Gordy is offline Gordy  United Kingdom
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What I meant was that your circuit reminds me of a mu-follower, even though those that I have seen have been for low current driver applications:
http://www7.taosnet.com/f10/mustage.html
http://www.tubecad.com/june2000/page10.html

I think Gary Pimm proposed an output-capable mu-follower on his interesting site. Have a search around... http://www.pimmlabs.com/
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Old 31st August 2009, 03:21 AM   #4
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Hello Bigun,

Yes, somewhat similar ideas have been around on the tube forum. The complementary current driver on top is approx. similar to a version of what has been called anti-triode. The Sziklai combo on the bottom has often been mentioned loosely as a Darlington variant.

Usually the current multiplication function has been discussed using a Bipolar part with near constant Beta rather than a Mosfet. The Mosfet non-linear V to I transfer function will dilute the triode sound. A bipolar transistor however has DC/thermal stability problems of course when loaded by an xformer (low DC resistance) or low Z speaker.

Not sure if any real amplifiers other than exploratory tests have been executed. I would start by searching on anti-triode and Darlington, also Sziklai (but often miss-spelled) in the tubes forum. I have also heard of a relatively recent patent for a tube/SS Sziklai combo. (yes, quite annoying to see old patents re-patented) Also another patent was mentioned for a SS current multiplying current mirror load for tubes.

These are some of the posters with ideas related to this, just off the top off my head, no doubt there are others:

smoking-amp
kenpeter
michael koster
tubelab
wavebourn
revintage

Don
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Last edited by smoking-amp; 31st August 2009 at 03:34 AM.
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Old 31st August 2009, 03:24 AM   #5
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Hi Bigun;

I've been following this path for a while; looking at your schematic I can say that what you have to think about, is how to prevent thermal run-away. I've made one running amp such a way, but it was not an easy task: I burned lot of high power resistors and MOSFETs until figured out how to protect them from making harakiri.

Also, making a triode with such huge transconductance think of an equally huge Miller capacitance.
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Old 31st August 2009, 04:00 AM   #6
iko is offline iko  Canada
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I recently looked into a similar thing, but using a dht and with headphones in mind. I built a one channel prototype of something like this:

Click the image to open in full size.

and got this type of output

Click the image to open in full size.
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Old 31st August 2009, 04:25 AM   #7
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You might want to check this out: http://www.tubelab.com/SuperTubeSE.htm as well
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Old 31st August 2009, 05:39 AM   #8
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Quote:
Originally Posted by Bigun View Post
Please forgive me if this is old news, it's my first post to the Tubes forum and I'm unfamiliar with Tubes and their history. I've tried a search for "Tube FET CFP" and got nothing.

I'm interested in the idea of a SE Triode amplifier. I've never made one (or heard one) but there is a legend around the SET that intrigues me.

Trouble is, I'd like more output power and perhaps the option of a lower damping factor. I'm willing to make some compromises on the sound to achieve this - for example, by going OTL.

I thought perhaps I could use a MOSFET as a 'slave' to the Triode in a complimentary feedback pair arrangement (CFP).

This arrangement keeps a fairly constant current through the Triode and allows the Triode to offload the dirty job of flowing lots of current to a MOSFET.

I used Nelson Pass zen v4 amplifier as the basic building block for this purpose.

Note that my knowledge of tubes is limited, the one I show in the schematic was chosen simply because my friends Chines hybrid amp uses it and I had an old simulation file already that had a custom spice model in place already.

Question - has this been tried before (if yes and it worked, can you point me to where I can get further information) ?
I have built and tested a few complementary current push-pull amplifiers and
continue to pursue the design.

Here is a series version like yours but without the current multiplier on the tube:
Spud-Assist: Totem-Pole Current-Mirror PP Hybrid

and here is a parallel version using an op-amp driving a MOSFET:
Another kind of hybrid

Nice use of the BJT in your mirror circuit to increase open-loop gain; good for low
impedance use. For high impedance, using an OPT, a simple cascode seems
to work well.

Cheers,

Michael
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Old 31st August 2009, 03:42 PM   #9
Bigun is offline Bigun  Canada
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Wow, thanks everyone, the Tubes Forum was clearly the right place to start with ! - I'm busy reading and following-up on the links you've posted for me.

Gordy - thanks for the advice. The term 'mu-follower' is (was) unfamiliar to me. Those were handy links and there's quite a bit for me left to read. The first one is very encouraging about the approach I've taken with a MOSFET CCS on top of the Triode. If I understand correctly, 'mu' is different from transconductance and transistor beta, but defines for an active device the voltage gain achieved at a fairly constant current ?

Smoking-amp - I have never used MOSFETs before, only BJTs so it would have been my first choice as you suggested. The reason I didn't use BJTs for this proposed design is because with a standing idle current of 2A required there would be a considerable bass current of 20mA. This base current would translate into a higher current through the Triode. I assumed that this is too much to expect of an amplifying tube. Since this is a single stage amplifier (aka Zen) I don't have the 'luxury' of using separate amplifying and power stage tubes.
The MOSFET has negligible dc gate current requirements. It's disadvantage is the rising load presented by it's gate capacitance, which will result in an early high-frequency roll-off I have to live with. I'm not too concerned, my only and first amplifier project was an all BJT ClassAB amplifier and there you need good high frequency performance so that the negative feedback can take care of cross-over distortion. I don't expect to need that for the Triode-FET amp. Does this make sense ?

Ikoflex - thanks for sharing your design, it certainly helps to see some good results presented; square wave looks great, what frequency was it tested at?. In your design you have an output transformer. This is something I decided not to use - mostly because it's another piece of technology I'm unfamiliar with, but also size, weight and cost and deciding on what specifications are needed. I noticed that if you could use a BJT somewhere you'd have every kind of active device represented in your amp !

Miles - that's a great link, looks to me like a SS transistor in Complimentary Feedback Pair arrangement with a Tube. Very encouraging. It has taken the path of being two-stage. Interesting that the author makes the statement "Mosfets do not have the SOA limitation, but so far I have obtained better sound with bipolar devices." - not sure I can do much about this since I don't believe I can use a Bipolar device unless I go the next step and make it a Darlington to reduce the standing bass current draw from the Triode ? Unfortunately Super Tube has stopped work on that project.

Michael - The Spud-Assist looks great! OK, so it's got different goals than my proposal in that you employ an output tube but the simplicity of your circuit is what I'm seeking. But I am seeking to eliminate the output transformer and achieve higher output powers - essentially limited only by my appetite for heatsinks. Both of the links you posted mention 'push-pull'. It's true that my proposed design uses a modulated CCS at the top, but the intent was not to achieve push-pull operation. In fact, the design should be functional without any modulation of the CCS. This CCS design originates from Nelson Plass Zen amp (actually Aleph, but I digress) as a means to reduce distortion. In reality the value of this for me is that I can adjust the amount of CCS modulation as a % of the output signal and change the distortion profile of the amplifier to 'suit my ears'. Maybe it's all the same thing in the end ? So I did a search for 'anti-triode' and now I see the connection is well known.


So what next ? - I think I'm going to build an all SS version into an old power supply box first off to gain some experience with this simple topology. In parallel I want to refine the Triode version. I have been generously offered a source of Triodes today.

The 'big question' remains, how much of the Triode's character will survive the vice-like grip of the CFP Mosfet ?
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Old 31st August 2009, 04:58 PM   #10
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"The 'big question' remains, how much of the Triode's character will survive the vice-like grip of the CFP Mosfet ? "

You could try a technique I use (and probably used by a few others too) that takes the Mosfet transfer characteristic out of the equation: Replace the U1 triode with a pentode, replace R3 with a current source set at the idle tube current, and use a zener to drop the voltage from the M2 source down sufficiently to power the g2 screen grid of the pentode.

The g2 grid acts like the feedback terminal for a normal triode, so it servos the Mosfet output to get its Mu (g2/g1 Mu) satisfied on g2. This also operates the g2/g1 triode at constant current for low distortion. Since g2 and plate track each other, there is near linear variation in the screen current as it intercepts a constant fraction of cathode/plate current.

The downside is that output voltage swing will be constrained to the operating range of the g2 grid. But for a direct output setup (no output xfmr) as you have, this should not be a problem.

Another variant would use an additional Mosfet above the existing triode in cascode to emulate the pentode connection described above. (feedback then to the cascode gate = g2 above) This might be more esthetically pleasing for some to have an actual triode in the unit, and allows for tube rolling to try different triode sounds.

Don
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