Plate current use of 12AX7 vacuum tube at low voltage

Question

Does anyone know the current used by at 12AX7 (or similar) vacuum tube when in normal (triode) operation using a plate voltage no more than 10VDC? I'm not considering the heater current (~300mA at 6.6V), just the grid to plate current or the current drawn from the power supply. I've simulated the tube in MicroCap with little luck and I don't have my equipment to physically test it.

Answer 1

Looks like anode to cathode current will be in the 400uA range with zero grid voltage ("Idss"). 6AV6 has the same characteristics as the 12AX7 according to the databook.

Edit:

LTspice with the RCA12ax7 model yields a plate current of 417uA.

Model:

.subckt RCA12ax7  1 2 3 *
*   *   *   *   *   *   *   *   *
eGIogVpc 20 0 value={log(v(1,3))}
rGlogVpc 20 0 1 
eG0 10 0 poly(1) <2,3>   -3.7694e+00   1.9947e+00   5.9432e-02    
eG1 11 0 poly(1) <2,3>   -3.2024e-02  -4.1443e-02  -4.8236e-03 
eG2 12 0 poly(1) <2,3>    1.9127e-02  -1.2189e-02  -1.5526e-03 
eG3 13 0 poly(1) <2,3>   -1.1354e-02   4.9339e-03   6.1016e-04  
rG0 10 0 1
rG1 11 0 1
rG2 12 0 1
rG3 13 0 1
gG   2 3 value={(exp(v(10)+v(20)*(v(11)+v(20)*(v(12)+v(20)*v(13)))))/170}
*
eP0 110 0 poly(1) <2,3>    -9.9158e+0  1.9145e+0 -2.8135e+0  1.8661e+0     
+                                                1.5643e+0  4.7240e-1  6.4276e-2  3.3101e-3 
eP1 111 0 poly(1) <2,3>     9.5428e-1  3.2558e-2 -8.3349e-1 -4.8578e-2  
+                                                2.6213e-1  1.0492e-1  1.8921e-2  1.3632e-3 
eP2 112 0 poly(1) <2,3>     9.5766e-2  2.5192e-2  2.2391e-1 -1.7040e-1 
+                                               -2.4952e-1 -1.0960e-1 -2.0981e-2 -1.4882e-3
eP3 113 0 poly(1) <2,3>    -6.6107e-2 -3.9657e-2  7.5560e-2  3.1025e-2  
+                                                2.4265e-2  1.7002e-2  4.2512e-3  3.4761e-4
eP4 114 0 poly(1) <2,3>     8.4148e-3  4.7989e-3 -1.3258e-2 -1.9288e-3  
+                                                5.2888e-4 -5.6853e-4 -2.4727e-4 -2.4359e-5
rP0 110 0 1
rP1 111 0 1
rP2 112 0 1
rP3 113 0 1
rP4 114 0 1
gP    1 3 value={(exp(v(110)+v(20)*(v(111)+v(20)*(v(112)+v(20)*(v(113)+v(20)*v(114))))))}
Cgc 2 3 1.8p
Cgp 2 1 1.7p
Cpc 1 3 1.9p
.ends

You can find some more info on this particular model here

Answer 2

It's important to understand that 12AX7's and their cousins were never designed or intended to be operated at HT+ voltages as low as 10 volts - they are inherently ~300V devices. That's not to say they can't operate at voltages that low, but you have to understand that you're entering a very different world. A few issues off the top:

1. The datasheets give little useful information for operating these tubes in that region.

   Specifically to your plate current question, the published plate characteristic curves are effectively useless. They're both inaccurate at that low of a Va and simply don't have the resolution needed even if they were accurate. E.g. take a look at the published 12AX7 characteristic curves: note that Va = 10V is the very first tick in the lower left corner, a mere 0.3% of the X-axis.

   You can find low-voltage characteristic curves which have been measured by people on the internet, but I only know of one such set myself. I know of no SPICE models which attempt to correctly model operation in this region.

2. Grid current is always significant, hence:

   • Biasing has to be done much differently.
   • You don't have a high-impedance signal input anymore.

   Traditional cathode biasing, with a grid leak to ground and a cathode resistor, simply doesn't work anymore. At a minimum, the grid leak needs to be replaced with a low-impedance, positive-with-respect-to-ground source, e.g. an op amp stage. The op amp can also address the issue of driving the low grid impedance.

   A perhaps simpler option is something called "pull-up grid-leak biasing". It's pretty much what it sounds like: no cathode resistor, and a pull-up (vs. the normal connection to gnd) on the grid. This does nothing for the low input impedance issue however.

Answer 3

my PDF from GE databook shows 100uA at -0.5v grid bias at plate of +10v.

Only the 0v and -0.5v curves are drawn continuously down to Zero Plate voltage.

The shape of the -0.5v curve is the same as all the more negative grid biases.

Answer 4

Be careful! At such low plate voltages you have an 'electron cloud effect' (negative charge buildup) near the cathode; and mathematical models for higher voltages fail. Reduce filament voltage as well to reduce thermal emission to range of that very low plate current to prevent that cloud effect.