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power tubes and learning SET design

Sometimes the best way to learn is by doing. So, I want to design my own power amplifier.

A SET design is likely one of the most simplest circuits as components can be kept to a minimum.

Another good reason for me to do this is because, I don't have the funds to actually build anything right now, but may have a little bit in the not-too-distant future. I also like the way that Morgan Jones goes through circuit design and components choice in his Scrapbox Challenge amp.

Now, there are many different types of power tube out there. I have a very nice Sylvania 6AS7 which looks like NOS that I purchased at a local surplus store in Phoenix. It looks big and chunky in the great retro way. However, I have noticed that the amplification factor is low, so maybe it would be difficult to implement this into a simple SET design.

Anyway, I also priced-up 300B's (ouch!).

My plan is to design a couple of SET's, using a simple circuit so that I can learn how to use the datasheets, choose operating points, anode resisters, cathode bias, etc. and figure out how the input tubes and output tubes complement one another, and the O/P complement the OPT.

So, can anyone recommend any output tubes that I should consider? Ideally, they would be cheap, especially in my final budget limited project, although I will for the sake f learning design my own 300B. Also, I have quite a few spare 6SN7 which I may want to use (but am not limited to) for input tube.

Anyway, hopefully the exercise of designing a couple of SETs using different tubes combos will allow me to learn some of the important design principles and then actually give me a design that will sound good when it gets built.

An ideas or advise?????

Charlie
 

7N7

Member
2003-01-18 10:43 pm
England
Many here will have lots of ideas, but for output valves, some of the current Russian and Chinese production is really very good.

You could probably make a decent SE amplifier using something like a 6550 or KT88. These will dissipate around 40W triode-strapped, so you should be able to get comparable power to those expensive 300Bs.

If you can live with a little less power, then EL34 is an option and has the advantage of having lower anode resistance.

I am using Russian 6L6-type valves in my Quad IIs; they are fine and are available very cheaply - around $20 for a matched pair; at these prices you could go parallel SE, which would have a number of advantages.

There have been several threads about 6L6 type valves, and it seems that some of the new production can dissipate plenty of power without problems. "Tubelab" has written about this.

7N7
 
In triode mode the EL84 sounds great to my ears. It is also very easy to drive. If you can live with just under 2 watts it is a nice, cheap, good sounding tube. You can usually pick up matched pairs for $16 or less.

The KT88 also comes to mind. Pretty easy to drive, 5 watts should be pretty doable in triode, higher in UL. A bit more money than the EL84, maybe $25 each on the low end.

Of course there are tons more but I've built SETs with both of those tubes and enjoy the sound.
 
Well, it seems that I should also look at the KT88. I have no idea of what power I am going to need. I also plan on building the Rob Brines FB-16 single-driver. I expect that it is at least 90dB efficient.

I hope I will leran a lot from doing this. Morgan Jones's book is certainly very good, although I do find that he can go off at a tangent and the heading styles make for some confusion, and his figure legends (on occassion) can be totally un-explanatory. However, that is not to say that his books aren't good. I expect that they're the best out there.

Charlie
 
As Sherman said, the EL84 makes a decent triode. However, the 6V6 family is even more linear, when triode wired, and some members of that family are dirt cheap. For instance, RES shows 12AB5s as costing $4.00 each. ;) You could follow the DECWARE SE84 example using Hammond 125ESE O/P trafos, a 5965 for voltage gain, and 12BA5s to get a half way decent amp at low cost.

"Full wave" voltage double a Triad N-68X for the B+. Add a filament trafo and hardware to finish off a very low cost, but respectable, project.
 
cbutterworth said:
Well, it seems that I should also look at the KT88. I have no idea of what power I am going to need. I also plan on building the Rob Brines FB-16 single-driver. I expect that it is at least 90dB efficient.

I hope I will leran a lot from doing this. Morgan Jones's book is certainly very good, although I do find that he can go off at a tangent and the heading styles make for some confusion, and his figure legends (on occassion) can be totally un-explanatory. However, that is not to say that his books aren't good. I expect that they're the best out there.

Charlie


Charlie,

Paul Joppa's 102 dB. rule tells us that 90 dB. efficient speakers need at least 16 WPC driving them. You can't get that out of a SE triode wired KT88, sorry. Also, the big "iron" needed for "high" power SE amps is COSTLY. :(

If you want to use SET amps, you need efficient speakers. Otherwise, you get eaten out of house and home by the construction costs.
 
You can do a SET with your 6AS7G. Use self biased, and a pentode driver stage. It would only be two stages and can be powered off a single supply. That would be beginner friendly :)

Also, I'm currently building a 6as7g push-pull class-A amp. It only uses 1 output and one driver tube per channel and puts out 10Wrms. You could go that route too.

Regarding speaker efficiency, I'd totally agree with the above statements, if it will be used as your primary system. However, I've used 90dB efficient speaker with a 4Wrms amp in my room and have found it to be plenty.
 

cbutterworth

Member
2006-10-03 11:02 pm
When I say the FB-16s are 90dB, it is only a guess, the larger FB-20 (?) look like something in the region of 95dB from their curves. A look at the Fostex specs and suggested cabinets indicate just above 90dB.

I certainly am not intending floor-thudding playing and am doing these designs from more of a learning rationale.

UNLESS, anyone knows of any cheap high efficiency speaker projects that'll give me more volume with a lower powered SET.

I would ultimately want to build my own SET and/or PP amp. $5K of emergency surgery on one of our family dogs has nixed any building for quite some time. However, if I can build an amp for less than $300, I might be able to do so within the next six months - but right now, I am content to learn the design principles.


Charlie
 
Charlie,

Planet10, AKA Dave Dlugos, has some horn speakers that might get the "flea" power job done for you.

While its bass extension is non-existent, the Bottlehead "Climax" is also a possibility. For full bass, paired subwoofers will be needed.

You should be able to do the 12AB5 SET, with Hammond 125ESE O/P trafos, for under $300. Leave room on the chassis for better "iron", as the rest is more than decent.

Oh yeah, using a 5965 for voltage amplification assumes that the unit is driven directly by a CDP. No preamp is needed.
 
If it's design principles you want to know, then go get yourself:

Radiotron Designers Manual

Steve Bench's Site

John Brosky's Site

Technical Books Online (S. loads of tech material & books, from the basic to advanced)

Frank's (Tube spec sheets by the car-load.)

Unless you have some uber-efficient speeks, or just want a headphone amp, then about the only practical proposition for a SET would be something like a 845 or 805 based design. Then, again, that's not something a raw VT virgin should be attempting anyway.

You just might be better off using the 6AS7s for a push-pull design that'll get you enough power to be useful.
 
I would take a good look at TubeLab's SimpleSE.

It can be built easily within the budget you are working with, it's well documented on his website, the output stage can be easily wired UL, triode, or pentode, with or without feedback, and it can use virtually any output tube from 6V6 to KT88. You could use it like a little laboratory without the inconvenience of having to layout your own PCB.

And it sounds really good! Mine has no trouble driving 87dB ish speakers to very loud levels, even wired as SET.

Win W5JAG

edited to add: "even wired ...."
 
I have a SimpleSE running KT88's connected to my 87db studio monitor speakers. These are in a 10 X 11 foot room and are designed for "near field monitors". They are very loud when sitting in the intended listening area, but they would not do so well in a large room. The same amp connected to 96 db speakers with 15 inch woofers will shake the neighbors walls.

My SimpleSE was built for $281, but I got a few bargains. Most run $300 to $350.

http://www.tubelab.com/Industrial1.htm
 
I considered the TubeLab offerings a while ago, and will certainly not dismiss them when I come to build an amp.

My main reason for designing my own is to learn and to see exactly how cheap I can make something. For example, the 6V6HiFi on the Boozhound site can be built very cheaply. I really liked the simplicity of the circuit, which is what made me think about designing my own amp.

I may, or may not end-up building my design, although it would be fun to see what the sound would be. I expect that I'll end-up designing a couple of SET, then design a PP.

I'll post the designs when I've done them, and hopefully get some critique on them.

Charlie
 
If learning the How-To of SET is your prime objective, just drop the any high power tube for a start.

I strongly recommand 6C45pi or 5842 SPUD. I learnt a lot from these 2 tubes. They can pump some beautiful SE 1W output by themself, and good as driver for driving power tubes such as 2A3 and EL34, What I did with these tubes were:-.
1) "conventional" auto-bias with resistor cathode bias single stage amp,
2) battery cathode bias
3) fixed bias
4) plate choke parafeed
5) active load CCS parafeed
6) led cathode auto bias
7) various combi of the above,
8) drive my 45, 2A3,..etc

AND its extremely simple to implement!

Also, they are "low" voltage, an isolation tranny of 110Vac would be sufficient.

Just 2 cents worth
 
cbutterworth said:
...

My main reason for designing my own is to learn and to see exactly how cheap I can make something. For example, the 6V6HiFi on the Boozhound site can be built very cheaply. I really liked the simplicity of the circuit, which is what made me think about designing my own amp.

I may, or may not end-up building my design, although it would be fun to see what the sound would be. I expect that I'll end-up designing a couple of SET, then design a PP.
...
Charlie

Charlie,

I built Mikael's KT88 before I had the knowledge to design my own and it turns out to be a really nice amp. Mine make about 7.5 watts before clipping wired UL. B+ is about 424V and idle current is 82mA. For these (I made monoblocks) I used James 6123HS transformers and connected the tubes to the 5K tap.

I've since designed a couple different variants and breadboarded one channel of each. It is great to have a resource like Morgan Jones' book but as I learn more from doing putting together circuits like this and taking measurements sort of puts the stuff I read in sharper focus.

For my breadboard circuits I've used Edcor transformers which at about $20 each might not be the best sounding in the world but are certainly inexpensive enough to do testing with. Plus they have a UL tap so you can see the differences between triode, UL and pentode in the same circuit.
 
Looks like you'll need either one or two stages, so one issue is how you couple the stages up.

Have a look at the Edcor site at the http://www.edcorusa.com/classx/tube_output/xse/xse15-8-5K.htm first for SE OPTs and then have a look at their interstages, which are:

WSM series $9.75 – 0.5W
XSM series $12.32 – 2.5W

You should look at the 10k:10k ones and the newer 15K:15K ones. this would be a cheap and effective way of coupling as long as the plate resistances of the tubes you use are fairly low, like around 5K. this should be listed as Rp in the datasheets of the tubes. You would need to consider parafeed for the Edcors which would be another thing to learn.

Good luck!
 

WBS

Member
2007-01-03 12:13 am
Napa Valley
cbutterworth said:
[snip] Morgan Jones's book is certainly very good, although I do find that he can go off at a tangent and the heading styles make for some confusion, and his figure legends (on occassion) can be totally un-explanatory. However, that is not to say that his books aren't good. I expect that they're the best out there.

Charlie


I'll second that. I just wish he would add a glossary to define the British-isms that appear throughout Building Valve Amplifiers! :D

BTW, thanks Mr. Jones! I'm certain your books are an inspiration for others like myself who lack the skill but possess the desire to improve their audio experience.
 
At this point, since you're goal is to learn, I suggest you design many amps on paper. Go ahead and design a SE 300B. Who cares how much they cost? You don't need to buy any just to design around them. Figure out what it would take to get your 6AS7 working, even if it would give you only a couple of watts. KTxx, ELxx, 845, whatever... take a look at all of them.

By the time the money is available you will have a much better sense of what it takes to make any of them work. You'll understand other peoples designs better when you look at them. You'll see the shortcomings of your first designs and be glad you didn't build them.

Then comes building, which is even harder and more fun that designing on paper... Spending money is the easy part. :)