Introduction

For some time I’ve been postponing the conclusion of a half-finished project. This is one of the many projects that I have around as many of you, but it was time to complete it as just some minor bits were outstanding.

My interest in measuring valve transconductance was very high since my early days of involvement with hollow state technology. The old valve tester I acquired didn’t measure it, I tried many ways to measuring it with different methods until I settled with using a CCS load and an AC meter as described here.

The problem I found though was that my true RMS AC meter in low scale (i.e. 100mV AC) didn’t like a significant DC voltage drop across the sensing resistor. Not sure why, but either way I wasn’t happy either without decoupling the anode to the sensing part of the circuit when using high voltages.

Solution

The coupling method used in many old testers is a transformer. So I looked at various options and ended up playing with the following circuit:

Although it seems complex, it is just an instrument amplifier formed by three precision op-amps (INA105 and OPA27). The 1:1 transformer is AC coupled by C2 and R2. R1 is the sensing resistor and a set of resistors and pots are used to change the amplifier gain from x10 to x100. Despite working perfectly well, it is just unnecessarily complex. A symmetric power supply and three op amps is way too much.

Rod Coleman suggested a better approach to the above circuit by reducing the DC load of the valve under test. The solution is simply using a step up transformer which will reflect a very low AC load into the anode. The AC voltage across this load is then amplified as in the above circuit, but with a very basic approach. To avoid symmetric supplies, we will use a common LM258 which is friendly for DC supplies. The trick here is to wind your own transformer. I used the RM10 core from Ferroxcube. You can wind your own transformer with this core very easily. I got my 1:10 step up with 5T for the primary and 50T on the secondary. The results are fantastic. Here is the circuit I ended up with:

The amplifier is very simple. It is powered with a single 9V rechargeable cell and has to gain options which will provide sufficient range from 10μmhos to about 50,000μmhos. It is very easy to calibrate and the precision is very good.

The R2 load is reflected into the anode as:

So the above design is very simple and will provide a very accurate measure of the valve transconductance. Question is how can we calibrate this circuit? There is no need of recurring to any DUT with a known transconductance. We can simple calibrate it with a test signal and an oscilloscope (or a true RMS AC meter). Here is how:

By using a calibration resistor (RCAL) in series with the primary we can then easily load the source oscillator. If we ignore the reflected secondary resistance the primary AC current will be:

We can then apply the calibration signal to reflect the 1V or 100mV AC signal on the output of the meter depending on the scale used.

The only limitation found on this design is the inevitable saturation of the core. I found on mine to be about 55mA for a 5T primary at 1kHz. If you were to get 2T:20T probably the DC capability would be better or change testing frequency up to 10kHz (see below). I’m fine with 55mA maximum as only the high current output valves are limited with this design. I can test any valve from 10μmhos to more than 50,000μmhos very accurately (and more than you will easily get with most of the valve testers out there).

Did some quick tests with 26, Cx301a and a 300B today. Very happy with the results!

If you are looking for a transconductance tester, here is a simple way of achieving it without a big spend and accuracy is as good as your AC voltmeter.

Let me know your thoughts and if you build it as well, would be interested in your comments.

Cheers, Ale

Updated (3th January 2014)

After revisiting my CCS load for heavy duty use, I re-calibrated the transconductance tester but did it on 10kHz signal. I found this to provide a better response of the RM10 core transformer and as expected due to higher frequency the current frequency response was much better at higher bias currents. The only challenge is that our friend Mr Miller will be more evident at the gate of our triode DUT given the higher frequency. Is different testing at 10kHz compared to 1kHz. Some valve testers used this frequency and compensated for Miller shunt capacitance. I found that my waveform generator which has Zo=50Ω can easily manage 100mV rms  setting at the gate with high-mu triodes. I therefore re-calibrated the transconductance tester circuit to read up to 50mA/V at 100mA currents easily. I can now test 300Bs at their full operating range.