Operating Point of 12AT7

[Grainger49]

It's 3:50 AM and I stifled the laugh when I read "It shouldn't.  But I'm pretty sure it does anyhow,"  Yes, everything seems to matter. 

[rlyach]

The spending continues... I just ordered a pair of matched JJ 2A3's and a gold pin JJ 12AT7.

[rlyach]

OK... I just bought a JJ gold pin ECC81 for the pre-amp stage. The curves for this tube are attached. Basically it shows good linearity with a 2V p-p input. The output should swing -57V to +58V relative to the operating point, give or take drawing error. Since the mu of the tube is 60 everything seems in order. the only difference is that my operating anode voltage will be 192.5 instead of the manual's 230V or the schematics 200V. I think this should work well. Also, this operating point is well away from the low and high limits of -1.0v Vgk and the dropout of 330V (with safety). There seems to be room to move the operating point up a bit, but I think I will leave it at the designed -2.5V. Is my analysis correct?

[Paul Birkeland]

Yes, but you have to factor in that the curves are for a bogey tube; your specimens may show 192V on the plate, or they may be up at 220V (220V is somewhat typical of what I've seen).

From what I know of the Soviet tubes, I think you may find that they are closer to 220V for 100 hours or so, then there might be a little drop in the plate voltage, as I'd imagine that JJ would not use a super fresh tube to draw the curves from. There's a lot of learning to be done with all of this, and much of it transfers to other kits pretty well.

-PB

[earwaxxer]

Hey Granger - I like the new 'avatar'. My pic was taken on a pirate ship. We weren't much in the way of pirates, but I love the feel of the old sailing vessels.

[rlyach]

Ok... This was a little more difficult. Using the JJ 2A3-40 datasheet I arrived at the following: The operating point is 59V at 49mA. I ended up making a spreadsheet to model the cathode resistor of 1194 ohms, and a plate load dc resistance of 600 ohms. Looking at the intersection of the resistors and the 2A3 tube I was able to generate a loadline. The peak to peak voltage swing of the 2A3 at full signal should approach 178V. I hope I did all that right. I understand that all these numbers a just paper calculations but it helped me to understand how all this stuff works. I started with a 2V p-p signal and ended up with a 178V p-p swing, giving close to 40db of gain. The 2A3 is an amazingly linear tube. Also the JJ ECC81 is much more linear than the original RCA 12AT7 I was looking at earlier. Now I think I am ready to build this thing. I was surprised at how large the 2A3 tubes were. Thanks for all the help and training. I now know enough to get myself in real trouble. I hope you guys will still be there to bail me out when I plug the amp in and it doesn't work...

[Jim R.]

Randy,

40 dB?  That sounds somewhat high.

-- Jim

[rlyach]

Well.... Thats the theoretical limit, 20 Log10 (178/2) = 38.9 db. At this level the tube is maxed out at 380V. I also assumed maximum gain from the first stage. So this is the best of the best. Since this is the first time I have ever analyzed this type of circuit I could easily be way off. I was following Morgan Jones 3rd ed. At least I have had fun!

[4krow]

I really love it when customers ask intelligent questions!
[/quote]

Is this the reason you don't return my calls?

[rlyach]

Oh yes... I was calculating the gain at the primary side of the output transformer. If you assume that the output transformer is between 80% and 90% efficient you get a total gain of 37db to 38db. There may be something else I am missing.

[Paul Birkeland]

There is a fair amount of gain until the output transformer steps it down (how much depends on how the secondary is configured).

I would have my doubts about the JJ 12AT7 being more linear than an old stock 12AT7.  The only way to know for sure would be to create some curves on your own (we have likely send you an old production 12AT7).

[Paul Joppa]

OK, now it gets more interesting  :^)

The load line on your curves is an ~1800 ohm resistance, which is not relevant to the audio signal.

For the audio, the plate load (assuming a resistive speaker load of the appropriate value) is 4000 ohms - that's approximately the speaker resistance (8 ohms on the 8 ohm tap for instance) times the transformer turns ratio squared. Since there is little AC current through the plate choke or the cathode resistor+capacitor, they don't affect the AC load. If you draw a line with a slope of -4000 ohms that goes through the operating point (50mA at 300v, nominal) you can identify the two endpoints at zero grid volts and at -120 grid volts. I get 105mA at 95v, and 10mA at 470v.

From this is is possible to calculate not only the power but also the second harmonic distortion.  I get 4.4 watts and 2.7% second harmonic distortion. You can google "load line triode distortion" or something similar and find articles with more depth.

[rlyach]

Paul,

Thanks for the tutorial. I couldn't figure out how to incorporate the AC load and the DC load. So if I understand correctly, You start with the operating point in DC and then draw a loadline based on the AC load and place it on the graph on top of the DC operating point. I will post a new loadline graph with a new gain calculation later. Then I will look up the other calculations you discussed. I really appreciate the help.

[rlyach]

OK...  One more try... Here is the updated loadline. I calculated max power of 4.1Watts and distortion of 2.69%. I also added the power rating of the tube. The usable gain still comes in at around 39db. I think I am getting in over my head here. By the way... Can we then use the max usable output and calculate the rated power and distortion from the same graph?

[Paul Birkeland]

There's a 22:1 voltage step-down through the output transformer (when wired for 8 Ohms).