Tag: gm tester

Transconductance tester

Introduction

IMG_0580For 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.

Continue reading “Transconductance tester”

THD, Gm and μ tester

As part of my repairing of the curve tester, I had to do some changes to the transconductance (Gm) meter section. Currently I’m leveraging most of the curve tester to also measure Gm, μ and distortion (THD). Albeit the latter is rarely used as I prefer an external equivalent CCS circuit that is not inside the tester as the output signal comes out cleaner. The curve tester provides all sockets, HT power adaptor, meters and bias supply.

This is my latest circuit:

The additional protection to fuse (F1) is the diode D2 which can protect the LCD panel meter A1 in case of an unexpected anode short. P1 and R4 were chosen to allow a precise setting of the anode current at low levels and some protection to the CCS when P1 is set to zero. M1 is bolted to chassis and is carrying all the effort when providing current at lower anode voltages. M2 on the other extent can be a TO-92 type. R1 was added to allow a bleeding path to C2 when not measuring transconductance. The bias section is a simple adaptation of Merlin Blencowe’s “Power Supplies for Tube Amplifiers”, which I suggest you take a look at as Merlin covers very well the most common valve bias circuits

With this circuit I can measure very accurately transconductance at any desired point.  I highly recommend you Alan Douglas’ “Tube Testers and Classic Electronic Test Gear”, which has a lot of details around how classic valve testers work, challenges around Gm measurement and obviously some good ideas and suggestions for calibrating and measuring Gm correctly.

Obviously adding an amplifier to the Gm tester section could improve the accuracy of low transconductance valves. But that would be for another time!

Measuring valve transconductance

Today I breadboarded the CCS I will use for the transconductance tester jig which is an addition to my curve tracer:

20120527-130504.jpg

Bias circuit is a classic from fixed bias amplifiers. I had the 80V available from the curve tracer circuit. The meter is an external panel AC voltmeter which is a trueRMS meter that will measure accurately the 1kHz signal.

The MOSFET CCS is a simple cascoded which can help setting the valve current and operating point. I source it with my bench variable HT power supply, which also helps in setting out the operating point.

We know that we need to have a small AC signal in the grid to increase the accuracy of the Gm test as the transconductance is given by:

G_{m}=\frac{\Delta I_{a}}{\Delta V_{g}} _{\Delta V_{a}=0}

So can’t feed with 1Vrms, so will use 100mVrms. If a valve has a transconductance of 1mA/V, then the variance in the anode current will be of 100uA (rms). This represents a challenge to measure accurately using an anode resistor of 10 ohms for example since the developed AC voltage across the resistor will be 1mV (rms). Therefore we will use an anode resistor of 100 ohms which will help capturing small transconductance values as this one.

 

Edit:

Found that the CCS bypass was omitted in my first circuit. Also the sensing resistor was reduced to 10 ohms to accommodate the AC true-rms meter I have. See updates on this post here