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Testing tubes with a 1khz signal
Testing tubes with a 1khz signal
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Old 22nd October 2017, 04:18 PM   #1
Dlav is offline Dlav  United States
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Default Testing tubes with a 1khz signal

Hello all, Iíve attached a schematic of a tube tester I completed, itís only for receiving tubes. A few changes Iíve made to tester, was adding a varic to keep the plate and screen supplies accurate, put ferrite beads on all tube sockets, put a 100uh inductor for parasitic issues. It test tubes at DC static characteristics per tube manufacturer data. I would like to test dynamic characteristics, I have a calibrated hp 201C audio oscillator. Iíve read some tube testers use a 1khz at 100mv signal. Can I just inject the test signal into tubes under test of course with the bias voltage? Will there be any impedance issues between the oscillator and the tubes being tested? Notice in the schematic there is no plate resistor in the circuit, will one be required? Iím building this for interest and learning. Any advice will be appreciative. Thanks
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Old 22nd October 2017, 11:45 PM   #2
audiowize is offline audiowize  United States
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I wouldn't use the variac, but rather perhaps just regulated supplies.

If you want to test dynamic characteristics, a load will be required.

To keep things simple, consider testing tubes as triodes. A selectable load of 100, 1000, 10000, and 100000 ohms will get you a decent selection. If you have a regulated power supply that can go from 300-600V, all the better! A negative bias supply will make things easier too. I'd recommend getting one of those cheap bench supplied on eBay as a bias supply. You can also put six AAA batteries in series, then a few 9V in series with that to get -1.5V, -3V, -4.5V, -6V, -7.5V, -9V, -18V, -27V, -36V (add more 9V if you need more bias voltage). Don't forget a grid leak resistor and input cap to keep your generator happy.

For a tube like a 12AX7, use the 300V supply, the 100K load, and dial up 1.5V of bias, then send the 1khz signal in and see what comes out.

Last edited by audiowize; 22nd October 2017 at 11:56 PM.
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Old 28th October 2017, 11:47 AM   #3
Dlav is offline Dlav  United States
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Thanks Audiowize for the advice. Late response back on the thread, I had auto accident. One question about the load resistors, would the different values be for different tube types? By the way, my tester has a bias voltage from 0 to -80vdc on the tester. For dynamic characteristics, don’t mean to sound ignorant, what am I measuring, DC plate current, AC plate current, are the AC voltage output across the load resistor? Thanks for your time.

David
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Old 28th October 2017, 03:47 PM   #4
audiowize is offline audiowize  United States
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Yeah, you would want different load resistances for different tubes. You'll want to put a capacitor on the plate and measure the AC voltage coming out as well as the plate current for a given tube.
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Old 29th October 2017, 05:21 PM   #5
v4lve lover is offline v4lve lover  Netherlands
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Testing tubes with a 1khz signal
The soviet L3-3 tube tester works in the following interesting way:

There is a Wien bridge oscillator whose output is fed into a precision devider. this divider is used to adjust the sensitivity of the circuit.

The anode regulator has a 445 Ohm 0.1% shunt in the feedback loop, hence it keeps the output voltage at a fixed point, however in front of the resistor an AC signal is picked off and fed into a two stage bandpass filter that drives the panel meter.

During S calibration the 450mV signal is fed directly into the bandpass filter and the gain of the filter is adjusted to the calibration mark.

Due to the nature of the tester the the accuracy comes from the high stable anode shunts and S calibration resistors. Other components influence the drift but not the absolute accuracy

Schematic here:

http://www.jacmusic.com/Tube-testers...agram-L3-3.jpg

Im currently thinking of building a modular tube analyzer using the same principle, however it is to have switchmode pre regulators, and solid state lineair regulators. All fed from a 24V bus.
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