Vacuum Tube 6EJ7 Test

I assume you have the set up as follows...

Anode and screen tied together AND both taken to a low impedance 240Vdc supply.

Cathode to 0V.

Grid 3 taken to 0V,

Grid 1 taken to a low impedance variable voltage supply centred around

-3Vdc.

Is there a 10k resistance between the grid 1 and its variable biasing Vdc supply?

If the grid 1 voltage is made mode positive say to -2.5V, and you say that Ia "runs away", ie after a short time the Ia becomes much greater than the gm should make it, then what voltage is across the series 10k g1 R?

What is screen current in relation to anode current at g1 = -3V?

Is it anode current or screen current that "runs away"?

If there is a 10k R between Vdc g1 bias supply, does the Eg1 go more positive than the bias supply?

Ie, does a largish Vdc appear acros the series R to g1?

Is the tube oscillating at some RF ?

Is the anode and screen supply well bypassed with C placed close to A and g2 and cathode/0V?

I am sure had you more thouroghly measured everything properly and more quickly, and tried several samples of this tube, that you'd have found all the answers your'e looking for.

If gm was around 10mA/V, expect Ia to increase 5mA with a +0.5V change at g1.

What happens to the screen current?

Looking closely with magnifying glass at the tube, does the screen appear to glow red/orange?

Data for the tube can be seen at

The tests shown include Ra curves, ie, Ia vs Ea curves which indicate Ra for various Eg1 values AND on 3 graphs, with Eg2 = +140V, 170V, and 200V, but not

The data says that with Ea = Eg2 = 230Vdc, Ia = 10mA, Ig2 = 4.1mA, and Eg1 would probably have to be -3V or more. So Pda for anode = 2.3W, and PdG2 = 0.943W.

What did you measure?

What currents have you observed?

To measure the currents, two 100 ohms can be used as sensing R in series with g2 and A, without affecting results much.

( 5mAdc change across 100 ohms = 0.5Vdc only ).

Try several different samples to make sure you have not got a dud tube, because your experience looks like you have got a dud.

You can of course use ac to test the gm.

If 1Vac ( Vrms ) at say 1kHz from a sig gen is applied to g1 via a coupling cap and low value Rg, then if gm was 10mA/V, expect to see 10mArms of Ia at the 100 ohms, ie, 1Vrms. Your CRO should show slight 2H distortion, because the load of 100 ohms is near enough to a vertically drawn line on the anode curves. Ac Screen current will also be seen, and you should measure and observe its magnitude and distortion nature.

Patrick Turner.

Hard to tell because your description of the test setup is sketchy but at 240V plate and screen with Ec1 -3V a bogey 6EJ7 would already be at or over max plate and screen dissipation and increasing Ec1 to -2.5V would put both almost twice max.

Thanks Patrick. I need to review this a little. I am away from the circuit at this time and can't do much with it.. I will review what you hasve provided and apply Monday. I will post back Monday evening with results.

Thanks again, Don Ennis

I had a little more time to look at the circuit today and make some measurements. I reset the plate and screen grid to 200 vdc. I varied the g1 voltage from -5 to -2.4 and recorded Ib at normal levels. Ic2 also seemed normal. I have g1 connected to cathode through the 10k resistor. I need to look at a few more things as I am told that these tubes were not outputting up to spec. I can't recreate the circuit they were used in so I'm just doing the best I can to verify they have low output. So far they seem ok. Any thoughts?

Thanks, Don