Monday, April 24, 2017

Not fully satisfied with the usability of my curve tracer, especially its 'quick test' function


I discovered that one of the final tubes, a 6146A, had a cracked envelope and the anode was disconnected. The first final tube measured decently, but the second was bad. I then grabbed a tube that appeared to be a 6au8, which is how I set it up, but it didn't measure any plate current at all. I then decided it was a 6au6, swapped the pin settings but still zero. 

Old tubes tend to have faint or virtually nonexistent markings, which would make this risk unacceptable. I will need to try each tube in my TV-3 tester, go through all the shorts testing, and look for leakage before I attempt a tube in the tracer. 

Yikes, when I stuck in yet another tube, no plate current. Doing more testing, I did find that the reservoirs produce good voltage, the tests run and curves plot, although I found several tubes where the current through the tube was much too low at the 'correct' grid bias, but a higher (less negative) bias gave me closer readings. 

Measurements were far enough off to douibt that I happen to have a pair of tubes, one a dual-triode, that are that worn. I knew I had to do some recalibration when I swapped to the power brick from my lab power supply, but these odd results force the issue.

After recalibration, the results were about the same. Everything was running lower at the same settings in these tubes. I fortunately have a set of unused NOS tubes, which I know have never been powered up because the chain of custody is clear. They were shrink wrapped in an old kit (actually, portion of a kit) I bought years ago and I have just removed the parts within the last week. 

The new tubes are producing similarly confusing results - the 'quick test' that should recreate the characteristics defined in the tube data sheet are not working out. I did notice that many of the tube datasheets include a cathode bias resistor in the circuit, which changes the effective grid bias compared to my tracer which hooks the cathode up to 'ground'. 

For example, a characteristic given for -3.5V bias isn't reached until the setting is nearly 0 on the curve tracer. I need to investigate this further. The curves being drawn are more reasonable, although for the new tubes I can find no datasheets online that include curves. 

The cathode resistors are for biasing the circuit and produce the bias voltage by the drop across the resistor from the current flow. Thus, when spec sheet lists the characteristics at some plate and screen voltage with a given cathode resistor value, I should be able to figure out the effective bias. 

That is, if they give the expected plate and screen current, those sum to the cathode current. Using ohms law, the voltage drop is the sum current times the resistance. These tend to give me values close to what I am observing when I experimentally set various grid values to try to achieve the reported currents. 

I found some recommendations in the curve tracer manual having to do with manually setting some ranges and averaging values in order to get a more accurate quick test result. I will experiment with this to see if I can get quick test results that are close enough to spec to give me confidence I can tell a good tube from a mediocre or bad one. 
I got closer, but invariably the specs are at a low negative grid bias and the current and other results are low unless I bump it up a bit. For example, a new 6CL8A was low on both plate and screen current at the spec sheet bias of -1V, but when I set to -0.7 I got almost exactly the right current. The transconductance came out close, but the plate resistance was off by quite a bit and the amplification factor was almost nonsensically high (869x). 

One hypothesis is nonlinearity and inaccuracy at low negative voltages, perhaps combined with too much resistance in the wiring, switches and tube socket, if the grid voltage is not delivered at the value I intended. If it is errors calculating at very low values of grid bias, I am kind of stuck since most characteristic reports I have found are at low negative grid bias voltages. 

I also don't know how precise the tracer can vary the voltages for the quick test - if the grid bias is -0.8V then the variance is 0.08V which may not be achievable by the circuitry. If the delta V is off, all the derivatives will be off. 

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