Grid Current of a 12AX7

[loudthud]

A short experiment was performed to see just what the impedance of a 12AX7 grid is when grid current is flowing.

The following setup was used. A JJ 12AX7 was used in a typical Fender preamp circuit with 100K plate load resistors and 1.5K cathode resistors. Grid leak resistors are 1 Meg. Only one side is driven, that side has a 33K grid stopper. A 100uF 16V cap bypasses the cathode on the driven side. A 10 Meg 10X scope probe is connected to the plate and another 10 Meg 10X probe connected to the signal side of the grid stopper. B+ of 300V and 6.3VAC are supplied by an old Heathkit power supply. The signal is the collector supply of a Tektronix 576 Curve Tracer set to AC mode. The center of the screen is zero volts at zero current. Vertical is current, horizontal is voltage. The deflection factors are noted by the readout to the right of the screen.



MVC-028F.JPG shows an X-Y plot of the tube. Horizontal is 2V per division with the center graticule line as zero volts. Plus is to the right. The vertical is the plate voltage, the bottom graticule line is zero volts. At the left of the trace as the input reaches about -2.8V the tube is cutoff, the plate is at 300V. At the right portion of the trace at an input of about +1.6V grid current starts to flow and the tube starts to "saturate". Further increase in grid voltage make only a small decrease in plate voltage.

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MVC-029F.JPG shows the display on the curve tracer. The slight slope as the trace moves to the left is caused by the 1 Meg grid leak resistor and the 10 Meg scope probe. As the trace moves to the right at about +1.2V grid current begins and the trace moves up at a rate that is mostly determined by the 33K grid stopper resistor. MVC-030F.JPG is the same conditions but with different deflection factors.

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MVC-031F.JPG shows the Curve Tracer moved to the grid side of the grid stopper. Grid current shoots off the screen at 2V. There is so much grid current that the cathode cap has charged up to about 2V.

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MVC-032F.JPG shows the X-Y scope plot and you can see that the trace has shifted the the right and the plate can now be driven to a plate voltage of 30V. (MVC-033F.JPG shows even more drive and the plate voltage is down to the neighborhood of 10V.)

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MVC-034F.JPG shows that with enough drive, the grid current reaches 4mA at slightly less than +6V and doesn't show any signs of stopping. Taking delta V over delta I the grid impedance is about 533 ohms. Note that the cathode cap has charged up to about 3V.

http://music-electronics-forum.com/a...-mvc-028ed.gif

http://music-electronics-forum.com/a...-mvc-029ed.gif

http://music-electronics-forum.com/a...-mvc-031ed.gif

http://music-electronics-forum.com/a...-mvc-034ed.gif



[Missing images restored by Steve A. (moderator) 06/20/2016]

[woodyc]

You probably know this, but you can sometimes get the effective grid resistance from manufacturer data. I needed it once for a 12AT7 and used a Raytheon plot of grid current vs plate voltage (which I replotted as grid current vs grid voltage to make it easier to read off the impedance.) Its not as much fun as your way, but on the plus side you get more of a representative-average value, not just a result for one tube.

[woodyc]

I just found the graph I made. I'm going to try to post it here. The lines are based on Spangenbergs current division theory.

[exclamationmark]

Those graphs match Spangenberg's predictions pretty darn closely! You'd probably have a ton more variation based on tube manufacturer as well. Looking at merlin's grid current graphs for several different mfr's there was a HUGE difference between some tubes.

As for an unusual application of grid current...I vaguely remember running a 6as7g at something like 30-40 volts and getting well over an amp(!) out of a single section by running the grids hugely positive. I don't recall if I ran into cathode emission limits, but I didn't bother paralleling both sections. As for the application... a class D OTL amplifier. By running a low voltage and high currents you can get the Rp to extremely low values, and actually get some decent efficiency out of the tube. Alas, driving a triodes highly capacitive grid hugely positive with square waves no less, proved to be a challenge (I needed something that could source 10 amp peaks to get acceptable slew rate)... You can easily achieve it with MOSFETS, but then again, you could just ditch the tubes and use the MOSFETS instead, haha. Realizing the impracticalities, I've long since given up... but just for once I want to see something tube powered operating at 90% efficiency (minus the heater of course). Maybe I'll try it using thyratrons or something.

[Steve Conner]

I remember arguing with someone over the feasibility of building a Class-D subwoofer amp with thyratrons. It wasn't you, was it? :-/

The common mercury vapour type have a very long deionisation time that would limit your switching frequency to less than 1kHz. Hydrogen ones are a lot faster, but also more expensive and temperamental. I think you get xenon thyratrons too, but I know nothing about those.

Real men just use horn-loaded triggered spark gaps.

[exclamationmark]

I don't recall having told anyone about my plans for a thyratron powered amplifier for fear of the mental asylum. I'm well aware of the issues of thyratrons though. I saw a picture of a particular spark gap once, and wondered why it was covered in weird yellow netting. Turns out it was chain-mail protection in the case that it actually exploded!

[R.G.]

Real men just use horn-loaded triggered spark gaps.

In Texas, real men use 100 meter tall natural gas well flare-off flames for speakers.

[Enzo]

REAL red-plating, Red Adair plating.


Or is it Adaire?

Or 'ey, dere.

[imaradiostar]

I love this place. You guys are awesome.

Jamie

[loudthud]

Got a request to replace the attachments that were lost a couple of years ago. Had to crop them slightly and modify the filename but here they are:

[loudthud]

I left out one pic. This one shows how if you remove the grid stopper and can drive a 12AX7 with enough current, the plate Voltage can drop quite low.

Thanks to moderator Steve A. for helping to fix this thread.