• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Class A2 Direct MOSFET Coupled SE

[IMGDEAD]http://electrolyre.com/tubeamps/meteor-front.jpg[/IMGDEAD]

Well, I finally got power applied to the breadboard amp I've been
working on. The purpose is to experiment with driving power grid
tubes with the MOSFET coupling circuit alluded to in these discussions:

http://www.diyaudio.com/forums/showthread.php?postid=1828338#post1828338

http://www.diyaudio.com/forums/showthread.php?postid=1815088#post1815088

Here is the schematic as currently operating:

[IMGDEAD]http://electrolyre.com/tubeamps/meteor-stacksupply.png[/IMGDEAD]

The main idea is to provide a fixed reference voltage for the driver
anode and power tube grid (fixed bias) while coupling the driver
anode into a mu-follower to isolate the driver from the sudden onset
of grid currrent.

Q2 and R3 provide a DC reference with high AC impedance. I made R9
adjustable to allow dialing in the bias voltage. Good thing, too...

Q1 provides a high impedance AC load with fixed voltage to the
driver anode, and provides a low impedance mu-follower output
to drive the power grid.

Q1 is supplied by a separate stacked power supply (ultra simple)
which is returned to the cathode of the power tube such that the
grid current loop does not include the driver power supply.

As shown below, it works quite well, with the 5842 driving up to
300V P-P to the 4-65A grid (to +70V in this trace) while supplying 50mA
of grid current:

[IMGDEAD]http://electrolyre.com/tubeamps/grid-current50.jpg[/IMGDEAD]

The 4-65A triode load line I am working with needs about 260V P-P
drive, going about +40 maximum, so I have some headroom in this
drive circuit.

I'm also thinking about smaller screen grid resistors.

I am getting about 16W output, which the 10 Watt Transcendar
OPT seems to handle remarkably well. I was worried looking at the
waveforms on the scope but the low end is quite nice sounding.
I haven't done an A-B comparison with my 16W push-pull amp but
this one seems to play quite loud before the distortion gets
objectionable.

The distortion spectra is all 2f up to ~1 watt, then some 3f + 4f,
etc in a waterfall pattern increasing up to about 10 watts, when
the higher order harmonics start filling in. I just made some brief
measurements and have been listening to it on and off over the
weekend. It has a very neutral sound, flat frequency response, and
sounds great with some drum parts I usually find challenging for
tube amps.

Cheers,

Michael
 
lineup said:
Beautiful circuit.
Unfortunately I can not or will comment your intesresting circuit.
Because when comes to tubes I am still freshly beginner in knowledge.

At least you have there a very unusual hybrid type design.
Valve-> Mosfet-> Valve-> Output transformer.

..get you copy for reading/thinking: J.C. On Audio matters - collected

Thanks for that paper from John Curl! The stories of Matti Otala and
TIM distortion brought back some memories of my own.

In the late 1970s I worked in an audio store in Alaska and we sold
mostly Sansui. I was into DIY audio design back then and working
on my own versions of comp-symm output circuits, blowing up a lot
of transistors but I was tired of my day job as an audio tech consisting
of mainly replacing from 4 to a dozen transistors in an amp and
hoping you get them all so the whole thing doesn't smoke again...

Sansui back then was in the habit of sending their design engineers
out to collect customer feedback from reps. So one day who should
show up but Susumu Takahashi, wanting to talk about TIM and their
new DD/DC output circuit!

I was still trying to get my head around this new concept but to me
it explained a lot of what I and others knew, that harmonic and IM
distortion measurements were not predictors of sound quality. I
knew this because I was still listening to my old Fisher, HK, or
Scott tube receivers.

So I spent the afternoon talking with Susumu about Sansui's
approach to solving the TIM problem. As I recall it was a direct
coupled diffamp output stage with huge peak current capability
and local current feedback to boost the power bandwidth above
100 KHz. Sansui claimed a slew rate of 200V/uS for those amps.

But at the time I wasn't sure about the TIM claim; in the back of my
mind it had the ring of some snake oil claims. So I kept pestering
Susumu about it until I realized he must get that from a lot of
people. He finally explained quite well why you needed 100 KHz
power bandwidth for 20 KHz audio using the spectral mirror. Of more
precisely why the power bandwidth needs to equal the small signal
bandwidth.

Anyway, enough of that. I only read the first few pages and need
to turn my attantion elsewhere now.

Thanks again!

Michael
 
leadbelly said:
Wow! Nice amp-porn!

One question: why the 2N7000 to bias the 5842? Don't like LED's or small signal diodes?

Thanks! I like to turn the lights off and look at the glowing anodes.

I like all the above. The LEDs I've tried have dynamic resistance of
about 10 ohms at 12mA. The 2N7000 connected like this has a dynamic
resistance of about 7 ohms (1/gfs I guess) and it's cheap and easy.
It gets me close to the 3V needed, using a single device.

Michael
 
Can you be more specific, Wavebourn?
What is obvious to you, might need a bit of explaining to others, professor!
Currently Active Users: 404
There are currently 177 members and 227 guests on the boards.
Most users ever online was 1145 on 4th July 2006 at 13:05

I often think about to write my posts, so that guests from different mother languages.
And all different level in audio would be able to get something from my text.
This means adjusting to
basic english understanding & basic audio electronics understanding.

Demo - Krati .... by the people, for the people
and justice and equal rights to every woman and man regardless of gender, religion and status in society

/lineup Sweden May 2009
 
smoking-amp said:
Nice circuit!

I think it's cool too. Especially nice, since it's a novel approach that's been built.

BTW, I agree with JC on his 10 points. But I still like tubes. They have such a nice glow, and they don't go phfft, if you abuse them for a bit.

Don,

Drifting further OT, and speaking of tubes, and something they might do that sand can't, did you ever get a chance to explore the DHT seasoning factor?

Sheldon
 
OT, re: Sheldon

Everything (the parts) are sitting on the shelf for the DHT experiment, but I've been way too busy with some other major projects:
1) The toroid winder machine (what a pain trying to find all manner of little mechanical parts for it, I probably over-did it with spring suspension rollers for the toroid rotation supports), have to go get my lathe yet too.
2) I'm modding a Tek 576 curve tracer to do tube curves (increased the V step range to 250V to handle even screen grid drive), now I'm including Mu and gm curves via a HF grid signal injection and synchronous V or I detection at the plate.
3) One of the surplus venders has a switching supply card for sale that is cheap and easy to convert to HV (exposed toroid HF xfmr). So I'm tracing the circuitry out for that.
4) I just bought a used Jet-ski, Yamaha Waverunner. The trailer tongue is too short for my car (I have to back the car into the water to get it off), so another project there. B.O.A.T. = Break Out Another Thousand $

May be a while :xeye:

Don
 
smoking-amp said:
OT, re: Sheldon

Everything (the parts) are sitting on the shelf for the DHT experiment, but I've been way too busy with some other major projects:
1) The toroid winder machine (what a pain trying to find all manner of little mechanical parts for it, I probably over-did it with spring suspension rollers for the toroid rotation supports), have to go get my lathe yet too.
2) I'm modding a Tek 576 curve tracer to do tube curves (increased the V step range to 250V to handle even screen grid drive), now I'm including Mu and gm curves via a HF grid signal injection and synchronous V or I detection at the plate.
3) One of the surplus venders has a switching supply card for sale that is cheap and easy to convert to HV (exposed toroid HF xfmr). So I'm tracing the circuitry out for that.
4) I just bought a used Jet-ski, Yamaha Waverunner. The trailer tongue is too short for my car (I have to back the car into the water to get it off), so another project there. B.O.A.T. = Break Out Another Thousand $

May be a while :xeye:

Don

Ahh, just figger out a way to eliminate sleep from your schedule, and you can get it all done - no sweat. The curve tracer project does sound cool.

Sheldon
 
Michael Koster said:

..get you copy for reading/thinking: J.C. On Audio matters - collected

Thanks for that paper from John Curl! The stories of Matti Otala and
TIM distortion brought back some memories of my own.

In the late 1970s I worked in an audio store in Alaska and we sold
mostly Sansui. I was into DIY audio design back then and working
on my own versions of comp-symm output circuits, blowing up a lot
of transistors but I was tired of my day job as an audio tech consisting
of mainly replacing from 4 to a dozen transistors in an amp and
hoping you get them all so the whole thing doesn't smoke again...
----

Anyway, enough of that. I only read the first few pages and need
to turn my attantion elsewhere now.
Thanks again!
Michael

Glad you got your Curl paper copy.
The woek to collect wasnt done by me. a member called Dimitri did a heluva job!

Regarding symbiosis MOSFET + TUBES or even tubes + transistors
I miss a lot anybody doing such projects.
I am of this opinion, that sometimes tubes do a better job in a circuit .. with all that high voltage across and their different good parameters.
And sometimes transistors does it better, from some angle seen.

But too seldome do they meet - due to division into 2 camps:
- nothing but tubes
- nothing but transistors or IC:s

This two separated clans - where nothing but clensed and fundamental thoughts count
is very unfortunate :(

Because I am sure there are such good things to find in the Hybrid Thinking.
There are a few that are handy with BOTH.
We have Wavebourn
we have one other Sheldon (stokessd) at his home SDS Labs http://www.quadesl.com/
Profile:
http://www.diyaudio.com/forums/member.php?s=&action=getinfo&userid=3901
and there are some others.
SY, our moderator, I think knows BOTH worlds, too.


Myself have only elementary knowledge in tubes.
I could build a SET (single end triode). No problem.
But a little angst I have. ..... Angst(german) = Anxiety = fear.
====


I have advocated Transistor+Tubes hybrid designs at a number of times here in forum:
See for example:
http://www.diyaudio.com/forums/showthread.php?postid=1228902#post1228902
lineup said:

beau2317

I am of the opinion ( I have expressed this in other contexts at our board, too )
that
There are too few designs done to explore Combination of Different Semiconductors.
MOS JFET BJT VALVES

Let each type do what does best and take care of this, in Amplifier.

I have started quite a number of such topics, over the years at www.diyaudio.com
Why so relatively little interest?

I have tried to explain such:
diyAudio builders are divided into camps, you 'belong' to one sort of builders
- a gang for mutual encouragement and exchange of applauds
We have:
1. The MOSFET maffia. FETdaMOSalists.
2. Solid State BJT Super Group. They do not make much Noise in circuits, but ...
3. Chim Amps Gain Crown rulers.
4. The only, The real AudioPhile Understanders: VALVES Lovers!


And we have not so many doing HyBride explorations.
And if they do, they do not get attention.
They do not Belong to 'Da Group of COOL People' :cool:
...as seen from view of 'those gangs'
---

Regards
lineup

Thanks. Michael, for you friendly post.
It pleases me to know I am not the only gentle man around here.

your lineup :)
 
There are other hybrid projects here in tube land

I just finished a Mosfet/Aikidio hybrid (Moskito) there is a thread here somewhere. Its the only SS project I've ever got to work... I like tubes mosetly because they seem, to me at least, to be more forgiving. The picture at the beginning of the thread made me drool though. :drool:
 
Hey Michael,

Old thread I know, but was just curious about your project and A2 type tubes. Have you tried many other tubes in A2 or zero bias? Is the Meteor zero bias (sorry I have not checked myself). I am just curious if it's worth to try a tube like this versus another thoriated tungsten that is mostly in class A1 (211). It also makes sense to me to use a tube that will be in current production and perhaps Taylor would make this one. I don't think I'll ever do a 304TL because of how much heat it demands. Nice work though!! I am sure it's very snappy and scary good bass. :eek:

Regards,
-Michael Woods
 
Hey Michael,

Old thread I know, but was just curious about your project and A2 type tubes. Have you tried many other tubes in A2 or zero bias? Is the Meteor zero bias (sorry I have not checked myself). I am just curious if it's worth to try a tube like this versus another thoriated tungsten that is mostly in class A1 (211). It also makes sense to me to use a tube that will be in current production and perhaps Taylor would make this one. I don't think I'll ever do a 304TL because of how much heat it demands. Nice work though!! I am sure it's very snappy and scary good bass. :eek:

Regards,
-Michael Woods

The Meteor amp circuit ran with grid current for about 2/3 the positive half of the sine wave just before clipping in the driver. I think the bias was about -20V. The circuit would work great with 854 or 211 in partial-A2. I've even contemplated a 300B A2 using this circuit.

My current breadboard-in-progress (below) in this line of amps is using a high plate resistance triode (the 35tg) with local feedback to enable it to drive a readily available OPT. There is about +25V on the grid at idle.

The pentode driver is loaded by the current through the feedback resistor.

Michael
 

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The Meteor amp circuit ran with grid current for about 2/3 the positive half of the sine wave just before clipping in the driver. I think the bias was about -20V. The circuit would work great with 854 or 211 in partial-A2. I've even contemplated a 300B A2 using this circuit.

My current breadboard-in-progress (below) in this line of amps is using a high plate resistance triode (the 35tg) with local feedback to enable it to drive a readily available OPT. There is about +25V on the grid at idle.

The pentode driver is loaded by the current through the feedback resistor.

Michael

Yes that is very cool from an engineering perspective, but you completely missed my question! How do these things SOUND??? :D:D:D

Okay to be fair I must have been thinking (how do they sound) and should have asked more explicitly ;)
 
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