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PostPosted: 03 Jun 2016, 02:20 
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MOSFET Source followers work very well.

There's plenty of examples online and I use them in one of my more successful tube builds :D

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PostPosted: 11 Jun 2016, 20:54 
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Location: Ribeirão Preto, SP - Brazil
Geek wrote:
MOSFET Source followers work very well.

There's plenty of examples online and I use them in one of my more successful tube builds :D


From what I've seen, you'd need both positive and negative high-voltage supplies to get to class AB2, which would complicate the circuit a bit too much to my tastes.

However, it got me wondering if I couldn't go for a pair of 6J6's in the output, solid-state preamp, and cathode-drive via transistors, like in the hybrid MusicMan amps. I think that would be an interesting thing to try, I just can't find anything on how to design such a setup.


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PostPosted: 12 Jun 2016, 04:29 
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blackcorvo wrote:
From what I've seen, you'd need both positive and negative high-voltage supplies to get to class AB2, which would complicate the circuit a bit too much to my tastes.


Not at all, actually! Especially if you use a typical HV transformer with centre tap.

The MOSFET will be a class-A load with predictable current draw. You can calculate Rdrop pretty easy.

Cheers!


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* Ratings are for transistors - tubes have guidelines*
Home: GeeK ZonE
Work: Classic Valve Design


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PostPosted: 12 Jun 2016, 09:12 
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Geek wrote:
Not at all, actually! Especially if you use a typical HV transformer with centre tap.

The MOSFET will be a class-A load with predictable current draw. You can calculate Rdrop pretty easy.

Cheers!



I'm actually trying to use a SMPS to power the circuit, not a transformer.

High-voltage P.T.s are rare, and kinda expensive when you do find them where I live.
When I do use P.T.s, I improvise with a common transformer that has a dual primary (110 + 110). I use half of it to power everything from the mains (it's all 110v in my house anyways), and the other half to give me my isolated B+, and the low-voltage from the secondary to power the filaments.

I also know nothing about MOSFET circuit designs. Actually, I know very little about circuit design in general. My technician course basically only taught how to fix stuff, since it's the only thing a technician really does in this country.

My design skills can be resumed to: Ohm's Law and a lot of guessing. Nothing else.
THD, Frequency Response, Linearity, all that is gibberish speach to me. I use my ears + trial-and-error more than math for my builds. Math is just a tool I use to make sure nothing goes "BOOM".


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PostPosted: 14 Jun 2016, 12:52 
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I did a bit of research on other ways to achieve Class AB2, and I stumbled upon this topic:

http://music-electronics-forum.com/t255/

Where the following example is presented:

Image

Which is used here as a way to reduce "buzz"/"fizz" (aka crossover distortion) :

http://www.paulamps.com/18watterbuzz.html

Do you think this could be a valid way of getting a bit more juice from the 6J6s?


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PostPosted: 21 Nov 2017, 20:06 
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Joined: 18 Nov 2017, 09:40
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Location: Michigan
Some really interesting stuff in here!

I suspect you are only reading Merlin Blencowe's online pages. His Designing Tube Preamps For Guitar and Bass book has a lot of general coverage of circuit techniques that commonly appear in guitar amps, and a fair amount that do not but he thinks have merit. It's not overly technical, but enough so that a person who has some knowledge of the subject and how to stay alive while working with the circuits can learn a lot more.

He discusses the 'catch diode' overload recovery technique and has some examples of MOSFET followers. It's a good book.

There is only a small amount on power amps.

The first book I got on tube amplifiers, mainly because it was 'collectible' many years ago, before people were widely scanning books and making them available as CD or PDF, was Valley & Wallman MIT Radiation Lab Series Vol. 18, Vacuum Tube Amplifiers. It is pretty tough reading and being from 1948, every amplifier on the planet was tube/valve-based. It covers more non-audio applications and concepts than audio.

Merlin's book is apparently intended to be practical, but probably still aimed at someone with a technical background or a LOT of motivation and interest.

I recently got a used copy of Kuehnel's engineering analysis book "The Fender Bassman 5F6-A", which is described as being aimed at engineers and manufacturers. It's in between the technical level of Blencowe and V&W (books mentioned above). It's readable and usable if one already has enough technical education and wants to see a guitar-centric analysis of a classic amp, something you will not find in most other guitar tube amp books aimed at technically-oriented DIY'ers who don't necessarily have education in the field (but intense interest and experience), and not in any college textbook that taught 'proper' amplifier design in the tube era.

Check out ampbooks.com for a mix of practical and technical stuff. There is a lot on the website that is practical, including complete circuits and sub-circuits and online calculators, so one needn't buy the $50 books the first day. I set a goal of acquiring Kuehnel's preamp and power amp books over an extended time, but between Blencowe and the 5F6-A book, I have way more info than I need.

This is a pretty cool online triode & pentode loadline simulator...from a guy in Poland. http://www.trioda.com/tools/triode.html .There is, or should be some learning curve because it produces so much information and you run run wild creating variations you have no idea (speaking for myself) the significance of (javascript sliders).

Lastly, the motherlode of more tube and MOSFET projects any person could complete in one lifetime, John Broskie's Tube CAD Journal. I can't make any decisions on which to build...there are too many choices.

He has some cool tube input, MOSFET output hybrids, but some require more power supply voltages than tubes.

http://www.tubecad.com/


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PostPosted: 21 Nov 2017, 20:13 
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Broskie article on MOSFET-tube hybrids.

http://www.tubecad.com/articles_2002/Po ... index.html


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PostPosted: 22 Nov 2017, 12:40 
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Joined: 03 Apr 2010, 18:23
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I have actually given up on this project long ago. 6J6 are too microphonic for Guitar use. Plus, they use the same filament power as a 6AQ5, it's more efficient to build an amp around that tube instead.
Either way, I've been experimenting with subminiature tubes (5902, 6N21B, 6N17B, 5840, etc), and getting pretty good results.


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