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!

4-65a EIMAC

4-65a EIMAC NOS DHT

Just got a couple of NOS EIMAC which I will be using in my SE design

So did some test on distortion, transconductance and driving them to +22.22dBu output to check the quality of these two ones.

I used similar test rig as before. At some point will be able to get a proper filament supply for this valve, but for the time being I will continue to use the hum pot and the big electrolytic cap across my old bench power supply which can gently provide the 3.5A for the hungry filaments!

I tested them at the limit of my CCS and bench HT supply which at the moment cannot provide more than 360V @ 100mA.

Transconductance is in the region of 3,800 – 4,000 μmhos.

So biasing the valve at -2.5V and over 90mA of anode current, the harmonic profile looks like this:

Both EIMAC measured about 0.12% THD.

12P17L curves and Spice model

A great russian pentode valve similar to 4P1L, but with indirectly heated cathode. You can check the valve specifications here.

I tested transconductance in left-handed triode mode: Gm=9.6mA/V @ Va=150V, Ia=50mA , Vg=-5.6V.

Here are the curves for this valve:

For the ones who want to test the SPICE model here is my take on it:

You can try the model and please let me know your results! You can always email me

Curve tracer finalised

After a long process, here it is. The curve tracer is finalised. It includes the following features:

Valve curve tracer

Sockets: 4 pin, 5 pin, 7 pin, octal, loctal, 9 and compactron (10 and 12 pins)
Anode sweep: 0-330V
Anode current: 100mA (max)
Grid step generator: 8 steps, 0 to -80V and 0 to -5V steps
Grid output for calibration
Oscilloscope: X (x10 attenuation) and Y
Y amplifier:

x1/x10 differential amplifier
1Ω / 10Ω sense resistor
Polarity inversion optional
Image sync adjust (coarse and fine)

Valve transconductance tester & THD meter

Anode current CCS 0-100mA (0.1mA resolution)
Anode voltage 0-600V (1V resolution)
Grid bias: 0 to -80V (0.1V resolution)
Transconductance meter:

0-2,000 μmho scale (1 μmho resolution)
2,000 – 40,000 μmho scale (100 μmho resolution)
Input test signal: 100mVrms @1kHz

THD meter:

Soundcard I/O BNC connectors
CCS load or external load

Finishing the curve tracer

Today I did a bit of extra work on the curve tracer with a view of finishing it. It has been a long and painful journey, but I’m reaching the end of it.

The transconductance tester is working perfect. I need to use the following ranges in my true RMS AC voltmeter:

0-2,000 μmho: 100mVrms scale
2,000μmho-50,0000μmho: 1Vrms scale

It’s probably the DC bias which affects the low scale. As an example when testing a 46 in triode mode (see datasheet for details), I tried the following operating point: Vg=-33V, Ia=22mA and the measure should be around 2.35 mVrms over 220mVdc. But in my bench voltmeter, above 17mA in the 46 doesn’t like it and cannot measure it, so need to change scale. I tested low transconductance valves in the lower AC scale such as CX301a, 26, 4P1L, 71a and then using the high AC scale, used 6e5P, 6C45, 6N6P amongst others.

The tracer now has a common-mode mains filter. This was required as at certain times during the day, specially in the evenings when the mains is really noise or my wife is using the microwave oven!, when tracing curves with the 1Ω sensing resistor and low anode currents (e.g. CX301a) then the noise level was sufficient to impact and distort the traced image. With the common-mode mains filter it works brilliantly.

Now need to place bottom plate and standing feet. Job done then and will move to some proper audio work!

Testing the circuit today, I measured 29 46 valves. Ended up discarding two which measured low and then when tested with the tracer found that curves weren’t good at all. Probably electrode misalignment as they weren’t just with low transconductance. Will upload some examples as it’s very interesting to see the difference

Cunningham CX301a

These RCA Cunningham CX-301a have short pins so had to use an external socket to trace the curves. They look really nice and the pair I got measure nearly 100%!

Measuring Transconductance (Gm)

GM tester jig circuit

After a bit of work, got the transconductance jig working fine. Made an obvious omission which was not bypassing the CCS. The CCS present a very high impedance in AC to the circuit, therefore not developing the current variation on the measuring resistor. Bypassed by an electrolytic presents a path to ground.

Measuring valve transconductance

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

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