Built a 12AX7 test jig

[duckduck]

The purpose of this posting is to share my wonder and excitement of doing something new and challenging in electronics (I still consider myself a noob).

I built a test jig for (half of a) 12AX7 vacuum tube. The 12AX7 is one of the most popular small-signal amplifier tubes. The 12AX7 contains two amplifiers, but I only used one of them. I have been looking at tube testers for a while but I want full power testing and I want to see the input and outputs on my scope. I decided to build my own jig to see a 12AX7 working.

I used the noval breakout board from Electro-Resales (1,2) (no relation, just a happy customer). In order to build my jig I used the info on the 12AX7 datasheet from General Electric (3). I took reasonable care to not short anything and double-checked my build before giving it some juice. I was super jazzed when I saw a big sine wave coming out of this thing. I am grateful to the many smart, hard-working son-of-a-guns that invented and refined vacuum tubes.

I inverted the (duplicated) input signal on the scope for ease of comparison. Both channels of the signal generator are outputting same signal and are sync'ed. Of course the amplifier is inverting.

Input signal (blue, scope channel 2) is 1kHz sine @ 3.0V P-P and output signal is (inverted) 180V P-P. Does this mean the voltage gain of my amplifier is -60 V/V?

I notice that as I crank up the input to 4.0V P-P, the top of the output waveform is distorted. I want to bias the input so that I can even that out. I know that's very easy to do, but I've got to hit the books. This is how I enjoy learning: practical problems sitting on the bench in front of me.

I also realized that I need a couple more cables, adapters, and doodads.

What fun!

(1) https://www.electroresales.com
(2) https://www.ebay.com/str/electroresales
(3) https://frank.pocnet.net/sheets/093/1/12AX7A.pdf

EDIT:

I used a Micsig DP10013 high-voltage differential probe in X500 mode to get the tube output to my scope.

[bob91343]

You have the gain calculation correct.

If you clip at the top it means the tube is too close to cutoff and needs a less negative grid bias.  If it's cathode biased that means a lower resistance in the cathode is required.  (It could also mean you are overdriving your measurement equipment.)

[TimFox]

"Full-power testing" of a 12AX7 is usually not relevant, since that high-mu triode is used for high voltage gain and usually not driven to clipping. A medium-mu triode such as the 12AU7, running at higher plate current, is more likely to run closer to clipping.
If you can run a very high plate load resistance, or constant-current source plate load, then you can measure the amplification factor "mu" directly as the voltage gain:  it is reasonably constant over the useful range of plate voltage and current with small signals.  If you saw 60 V pk-pk at the plate with 1 V pk-pk at the grid, then you need to correct that gain for the series combination of R_p (see below) and R_load to calculate mu, which characterizes the internal voltage source in series with the plate resistance.
The other independent small-signal parameter is the transconductance g_m, which requires a very small plate load resistance (ideally, a short circuit).  It is the ratio of the AC plate current divided by the AC grid voltage, and is a strong function of plate voltage and current.  Measuring the AC plate current can be done with an appropriate small plate resistor, so long as the voltage supply is constant (no AC component).
The third small-signal parameter is the plate resistance R_p = (mu)/g_m, and is therefore also a strong function of plate voltage and current.
For American tubes, the GE data sheets in Frank's website are usually the best source of information.

[duckduck]

"Full-power testing" of a 12AX7 is usually not relevant, since that high-mu triode is used for high voltage gain and usually not driven to clipping.

That depends. 
(See image, below)

Of course I understand the point you are making.

A medium-mu triode such as the 12AU7, running at higher plate current, is more likely to run closer to clipping.
If you can run a very high plate load resistance, or constant-current source plate load, then you can measure the amplification factor "mu" directly as the voltage gain:  it is reasonably constant over the useful range of plate voltage and current with small signals.  If you saw 60 V pk-pk at the plate with 1 V pk-pk at the grid, then you need to correct that gain for the series combination of R_p (see below) and R_load to calculate mu, which characterizes the internal voltage source in series with the plate resistance.
The other independent small-signal parameter is the transconductance g_m, which requires a very small plate load resistance (ideally, a short circuit).  It is the ratio of the AC plate current divided by the AC grid voltage, and is a strong function of plate voltage and current.  Measuring the AC plate current can be done with an appropriate small plate resistor, so long as the voltage supply is constant (no AC component).
The third small-signal parameter is the plate resistance R_p = (mu)/g_m, and is therefore also a strong function of plate voltage and current.
For American tubes, the GE data sheets in Frank's website are usually the best source of information.

Thanks. That's very helpful information.

EDIT:

I don't know that group, but they capture the vibe I was trying to convey. Looks like they are using SLO-100s. I'm planning on building a low-wattage clone someday, when my building-fu is better developed.

[David Hess]

Fun times, and much easier than implementing an entire curve tracer.

There are various cross-plot (1) methods for measuring performance starting with a triangle wave that could be applied.  They are used for measuring things like input resistance, common mode rejection ratio, power supply rejection ratio, linearity, etc. of operational amplifiers but they could be applied to tubes.

(1) This means they require an X-Y display device to show their results but that is what an oscilloscope is for.

[rsjsouza]

Congratulations and welcome to the world of vaccuum. Making these glow and work is quite magical, just like transistors as well - although these do not look as majestic as a vaccuum tube (IMO).

Any good reference databook of vaccuum tubes with curves will help you calculate the bias and experiment with it. Fortunately these days the internet is very easy to find this information (I was lucky to find a miniwatt one in a used bookstore during my teenage years in the 80s)

Good luck in your experiments!

[nigelwright7557]

12ax7 is high mu (high gain) so 4 volts multiplied by the mu is massive gain and not typical of how it is used.

In guitar amps they are sometimes driven into clipping/distortion for a different sound.

[TimFox]

A typical use for a 12AX7 triode is to start with a few mV from a phono cartridge, microphone, or guitar pickup and exploit its mu of 100 to amplify the signal to a reasonable level around 1 V, at which level with a quiescent point of, say, 100 V the distortion will be acceptable.  A medium-mu triode running at higher plate current is often (not always) used thereafter to drive a power tube.  The  7247/12DW7 dual triode has one section equivalent to 1/2 12AX7 and a second section equivalent to 1/2 12AU7.