power tubes and learning SET design

[cbutterworth]

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]

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

[Sherman]

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.

[cbutterworth]

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

[Eli Duttman]

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.

[Eli Duttman]

Originally posted by cbutterworth
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.

[arnoldc]

I totally agree with Eli, speakers are the key to low power amplification. Do not sacrifice power and I've had experience with several Bob Brines speakers (a friend licensed them) and I'll tell you, they sound best with power.

[Jeb-D.]

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]

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

[Eli Duttman]

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.