Can someone check my math!

[Thermioniclife]

Ok here's the deal. I've always struggled to understand load lines and I think I may be getting close but I'm sure somebody will shoot me down, don't worry I can take it.
I have a power supply that I think will deliver 325vdc I also have a pair of jj2a3-40's and a pair of 3k opt's.
If I want to run the 2a3's at 250 plate to cathode volts I would need to drop 75 volts across the cathode resistor, 325v-250v=75v
at a 60ma quiescent current 75v/.06=1250 ohms cathode resistor.
Do I have this right or am I an Idiot
Thanks

 

[Paul Birkeland]

Load lines are more easily understood if you start with a resistively loaded triode (like a driver stage or a preamp).

What I see that's problematic is that you need to start on the curves to see where 250V P/K and 60mA intersect to determine the bias voltage.  On the 2A3-40 curves, that looks like more like 45V of bias, not 75V.

Another way you can analyze this is to assume that you have a 1250 ohm cathode resistor, then draw a cathode resistor line on the curves that indicates how much current the 2A3-40 would draw with a 1250 ohm cathode resistor.  I've attached such a line to this post.

 

[Paul Birkeland]

So if you have that cathode load line up, you can see that with 325V of total B+ to work with, the 2A3-40 will settle with about 265V of B+, 50V of bias, and about 52mA of quiescent current.  That 265 and 50 doesn't quite get to 325 because I am allotting some voltage drop for your output transformer. 

In order to determine where you would land on the cathode resistance line, it was just a matter of finding the spot where the grid bias volts and the plate volts added up to about 325.

 

[Paul Birkeland]

I added a 3K load line to your operating point here.  This should get you the typical 3.5W of output and would still work just fine with a normal 2A3 plugged in and will have a little bit of headroom on the positive voltage swing (but not much).

 

[Thermioniclife]

Thanks PB.
I am not restricted to running a 1250 ohm cathode resistor that was just a product of my hairbrained understanding. I just want to run the tube at the typical O.P. of 250P/k v and 60ma quiescent current with a 750 ohm cathode resistor. So if I get you right it would suit me to get my B+ down to about 300V. And if I may be so bold this would give me a little more headroom on the positive swing.
And if I can impose further how did you derive that cathode resistor load line?

 

[Paul Birkeland]

With a 3.3K transformer, you can run a little more voltage and a little less current if you'd like.  A 2A3-40 is not going to care about extra voltage.

275V/55mA might be a nice place to be.  I didn't do the math, but that could just mean going to a 1.1K cathode resistor and calling it good.

I wouldn't throw away the extra B+ unless you absolutely have to.

 

[Thermioniclife]

Thanks for the help PB I will look at some resistive loaded curves as you suggested to get a better understanding.
As for the 275v 55ma it looks like -51v bias so 51v/.055= 927 ohms. I'll give that a shot.
Thanks very much.
Regards,
Lee

 

[Thermioniclife]

Does my last post make sense? or should i go up to 1k1?

 

[Paul Birkeland]

I haven't double checked it, but the numbers sure seem reasonable.  You could probably nudge the 927 down to 910 and call it good.

 

[Thermioniclife]

Thanks a bunch.

 

[Thermioniclife]

Thanks a lot PB,
I ordered 910 ohm resistors (they actually measure 923 and 925) and installed them, The amp sounds Great!
running at 53ma quiescent current. Now i can finalize the chassis design and order.
Thanks again.