Any input for a tube noob's first attempt? Flea Watt 12b4, 6dj8, low B+ - diyAudio
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Old 30th December 2013, 08:01 PM   #1
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Default Any input for a tube noob's first attempt? Flea Watt 12b4, 6dj8, low B+

Click the image to open in full size.

I've got an extra isolation transformer with 6.3V heater taps that came to me somewhat serendipitously and so I've been looking for a design that could utilize it in a novel way. Finding very little out there to make use of 150V B+ (other than preamps that I really don't need), I decided to attempt to cobble something together.

See said (sad) attempt above.

This is based on one part

A Single Ended 12B4A Integrated Amplifier. 1.5 Watts Of Pure Single-Ended Triode Power!!! (Part 1) by Gaby Levinson

and other part

The Suitability of the 6DJ8 for Audio

Would anyone be willing to take a look at this and let me know where/if I may be going entirely wrong? Any input on the 6DJ8 operating points (two suggested)? I guess what I'm worried about is how easy it seemed to put something like this together with my fairly rudimentary tube know-how.

I'm hoping this will be a fun and inexpensive (junk-bin) build for anyone with high efficiency speakers.


EDIT: I should add that I have considered using a voltage doubler, but the simplicity of rectifying the 120V secondary of an isolation transformer is keeping me from pursuing that unless 150V is totally ruled out.
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Old 31st December 2013, 03:49 AM   #2
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Default I would suggest building a proven design!

Hi, Unless you are really knowledgeable about tube Audio Design, I sure would not attempt on trying to fit my components I had on hand to a 1/2 sure schematic or less... I would take the 12b4 Amplifier you saw, Order the exact parts, etc, and build it... You are going to have enough on your hands just to get the mechanical and grounding scheme to be optimal, much less you need to lift the heaters dc to the Ct filament winding, there are a lot to building a tube amp and make it right!! sound right!! and Make sure it is not going to smoke or blow up... Just some tough love so you don't waste time and money on something you don't even know is going to put out what you want out of it! I hope you do not take this the wrong way, it is just, there is just sooo much a person needs to know in order to avoid shock, bad design, etc..Pianolydia Ps That Gaby Levinson is a Go!! I would build that! He already even tested it and gave you all the results! What more could you ask for! So it will cost a good $300 to build this Nice amplifier; you would pay $1,000 for the same in a Jolida

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Old 31st December 2013, 06:12 AM   #3
zobsky is offline zobsky  India
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150v is too low for the 12b4 running as a power tube. Follow the original schematic - I built it a few years ago and it's great
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Old 31st December 2013, 12:23 PM   #4
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Hey Zobsky, thanks for chiming in. I did come across your build.

I went with 150v because it is called for on the data sheet and the watts (150v @ 35mA vs 200v @ 27mA) stays pretty close. I don't quite understand how/why the higher OP was chosen. The article just says something about distortion remaining low and output increasing to 1.5W. I don't know how this is calculated?

Enjoyed your blog, btw. Your traffic spike yesterday was me reading the whole thing
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Old 31st December 2013, 12:30 PM   #5
zobsky is offline zobsky  India
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Read up these links , they should help you understand

How to calculate distortion etc on the load line How to "Screw Around" Your Tube Load Line

How to design a tube amp from scratch: http://www.wavelengthaudio.com/bugle.pdf
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Old 31st December 2013, 01:45 PM   #6
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Thanks!

I've read through the first (very entertaining) but not the second.

I like the Einstein quote in your sig, btw. Reminds me of "Those who know, do. Those who understand, teach" (usually credited to Aristotle).

Thanks for the tips
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Old 31st December 2013, 02:15 PM   #7
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Default Crude SE amp design technique, in a single post...

Well, you haven't gone totally wrong...

As was mentioned, you will get much better performance with a higher B+ voltage.

Assuming you're not doing this for the ultimate in fidelity, but rather as a cheap learning exercise, then yes, it will work.

Here's a rough output stage load line for 150V B+.

Keep in mind that the "plate voltage" is the voltage across the tube. Using a cathode bias resistor, you eat up some of the available B+ voltage. In this case, the operating point I chose is 135V, with 15V on the cathode for bias (135 + 15 = 150). This results in a plate current of about 35mA. To drop 15V with 35mA, the cathode resistor needs to be about 428 ohms. I would use 470 ohms to start, then maybe change it later.

Size the bypass cap to have a reactance of < the cathode resistor, at 20Hz. Xc = 1 / (2*pi*f*C), so 470 ohms at 20Hz means > 17uF. Bigger is generally better, I'd use 47uF or 100uF.

This load line into a 5k OPT will get you just under 1 watt out. You will get almost 100V of swing, x 0.707 to get RMS, then V ^2 / R...

On the driver, the plate voltage plus the plate resistor drop (plus the cathode resistor drop, but its small enough to ignore) add up to B+. Usually you try and more or less split the B+ between tube and resistor (though there are often reasons to move it one way or the other). So let's assume 75V across the tube and 75V across the resistor.

On this low B+ voltage, you need to bias the input tube at pretty low current, otherwise you will drive the grid positive (relative to the cathode) before the output stage reaches clipping. So, let's say 3mA...

75V and 3mA means that the plate load resistor is 25k. 22k, or 27k, would work. From the curves, 3mA and 75V means you need around 2.2V bias... 2.2V and 3mA gives you 733 ohms. I would experiment, something between 750 and 1k will work. Or better yet, just use a red LED, then you don't need a bypass cap.

By the way, this just might perform better than one would think. With both tubes operated at low B+, both will have a lot of 2nd order distortion. But the 2nd will cancel somewhat between the two stages, so at low power THD may be pretty low - sort of a case of "two wrongs making a right". You can tube this by shifting the driver operating point, while looking at an FFT (using a sound card and software like AudioTester).

Pete
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Old 31st December 2013, 04:15 PM   #8
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Pete-

You're the man. You're quite right about building this as a learning exercise and not for the ultimate in sound.

If I'm reading your response correctly, here's where it would stand:

Click the image to open in full size.

Your post explained a lot. In truth, I was interpreting the tube charts and operating points in the original design/article incorrectly and so I ended up lost when trying to apply them. I understand more about choosing an OP for the output tube thanks to your help. I can see now why a higher B+ would help put the tube into a more linear part of the chart!

One thing I don't understand though is how one determines the slope of the load line after choosing an OP for the 12B4 (though I do understand how this happens with the 6DJ8). For example, I understand why there are hashes on the load line in your diagram at 0V and -30V (+- 15V bias), but not why they are exactly where they are. In short, why do x,y = 300, 0 and 0, 60 on the load line?

I'm still wrapping my head around how this tube amp design process works, but I've cut my teeth by building amps from several tube schematics/kits in the past. When it comes to tubes, I know plenty of the 'whats' but not many of the 'whys'. Actually, the Starving Student was my first tube build completely from scratch. I did two of them point to point very shortly after you released the design (when 19J6's were still easy to find). You have yourself to thank for this tube noob asking all the tedious questions!
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Old 31st December 2013, 05:39 PM   #9
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Let's say I can get 160V to the 12B4 (120V secondary SS rectified to 169V, followed by a CLC or CLCRC filter). If I choose a 140V OP with 20V bias, I end up with a plate current of only 15mA (1k3 cathode resistor), right?
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Old 31st December 2013, 06:30 PM   #10
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Quote:
Originally Posted by Sodacose View Post
For example, I understand why there are hashes on the load line in your diagram at 0V and -30V (+- 15V bias), but not why they are exactly where they are. In short, why do x,y = 300, 0 and 0, 60 on the load line?!
The left end is set by Vg = 0V. This is the point where the grid would start to draw current, and for normal class A1 designs, it's the limit. The right end is just the same distance (in terms of grid voltage) on the other side of the operating point.

Quote:
Originally Posted by Sodacose View Post
Let's say I can get 160V to the 12B4 (120V secondary SS rectified to 169V, followed by a CLC or CLCRC filter). If I choose a 140V OP with 20V bias, I end up with a plate current of only 15mA (1k3 cathode resistor), right?
If you have more voltage, you would take that same 5k load line, and shift it ~20V to the right. Now the op point will be 140V plate voltage and -20V grid bias. That will put you right about 32mA, so use a 620 ohm bias resistor.

Pete
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