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.

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    \$\begingroup\$ Measure the voltage across the anode load; compute the current from that. Likely under 1 mA. \$\endgroup\$ May 11 '20 at 20:01
  • \$\begingroup\$ @ nate .The currents will be low but not very well specified .There was a series of valves made in the 1950s and early 60s that were designed for low plate voltages .These are called space charge valves .I have seen them in old car radios where the Audio output stage was germanium PNP transister single ended class A .I ditched my space charge valves at the ham radio trading table .If you can get these valves the plate current will be better specified at low plate voltages .There were also battery valves that might be relevant but I have not used them . \$\endgroup\$
    – Autistic
    May 11 '20 at 21:29

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.

enter image description here


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


.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

You can find some more info on this particular model here

  • \$\begingroup\$ Awesome, I appreciate it. I was getting a current very close to your simulation but thought that couldn't be correct. \$\endgroup\$
    – nate
    May 11 '20 at 22:33

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.


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.


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