![]() This voltage appears directly between grid and cathode, causing the valve to increase conduction, but the generatorĪlso drives a current around its own loop. To see how this works, imagine the generator gm = dIa/dVg.īut we can also measure gm using a bridge circuit, as shown in the diagram. The brute-force method of measurng gm is hold the anode-cathode voltage constant, then vary the grid voltage by a smallĪmount and measure the change in anode current. In the Test Tube this is acheivedīy inserting a small resistor in series with the anode current and amplifying the hum voltage developed across it, and sending Therefore, all we have to do is monitor the anode current and adjust R1/R2 until the hum is nulled. If the ratio of R1Īnd R2 is just right, the increase in conduction will cancel out the decrease in conduction and the anode current will remain Increasing, the anode-cathode voltage is decreasing, causing the valve to decrease its conduction. (ignoring the high voltage supply which is constant). However, a voltage V2 is also developed across R1 and appears between cathode and anode This causes a voltage V1 toĭevelop across R2, and this voltage also appears between grid and cathode of the valve, causing it to increase its conduction. Works, imagine the audio-frequency voltage generator drives a current though R2 and R1 as indicated by the red arrow. However, there is a more subtle method, which is to place the valve in a measurement bridge circuit as shown on the right. Mu: The brute-force method of measurng mu is to feed the valve with a constant and current, then vary the grid voltage by a smallĪmount and measure the change in anode-to-cathode voltage. Of operation can be tested, assuming it is pin compatible and the heater current is within limits of the transformer. Any valve that falls into this general range ![]() The valve is cathode biased, so the anode current will vary depending on what type you plug in, but is typically aboutġmA for the ECC83, 10mA for the ECC88, and something in between for the others. Measurements are made at an anode voltage of 100V. (You could of course adapt the circuit by adding other sockets or switches to allow other valve types to be tested.) The Test Tube will measure the amplification factor (mu) and transconductance (gm) of most popular dual triodes, including: ![]() But seasoned builders should have no problems with that It yourself to get it working with the parts available to you. However, a safe (transformer isolated) version of the tester is shown at the bottom of the page, but you will need to tweak And because I am willing to take my own life in my hands. This because I live in Britain, which has the safest mains plugs and domestic wiring in the world, so I can be sure that It is presented only as a proof of concept. Test Tube: The World's Smallest Valve Tester?įirst, let me say my prototype circuit is NOT SAFE for you to copy! It runsĭirectly off the mains and does NOT comply with appliance safety regulations.
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