Home Brew Tube Tester

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JoeTele
Posts: 127
Joined: Sun Oct 27, 2013 2:20 pm
Location: Milwaukee, WI

Home Brew Tube Tester

Post by JoeTele »

Just thought I share info about the homebrew tube tester I started last October and is now up in running. I had decided by that point I would like to have a tube tester and the more I researched and pondered, the more I thought this would be a cool project, especially to push my knowledge of both tubes and electronics in general, and that it did. Though by no means an efficient solution, it gets the info that those in the know seem to prioritize. The attached images show a Shuguang 12AX7 I had laying around under test at typical operating conditions (per the datasheets). Note that the panel meters are approximate, and deliberately set slightly high. Final voltages are dialed in and read on the external meter pictured, either via tapped jumper wires or dedicated test points. Plate voltages are in blue, plate current is underneath (millivolts across Vishay Daly 1.00-ohm [measured by me] 1% resistors), grid voltage on external meter, and heater voltage is on the analog panel meter. The spreadsheet screen captures show transconductance data for the triodes along with that derived from the datasheets. The transconductance is a formula so I just punch in the numbers and Open Office Calc does the math.

The tester starts with a power transformer and a 4-stage capacitor/choke/resistor filter network. The stages are jumpered to either of 2 Hall Amplification style variable voltage regulators adjustable from about 45 volts to just over 500 volts and which feed the tube's elements. That transformer also supplies the heater voltage for the tubes, which is adjustable from about 5.5 volts to 6.7 volts, ensuring correct voltage regardless of the tube's filament current, and allowing for that current to be reduced for the "life" test. A second transformer supplies the negative bias to the grids, which is adjustable from -250mv to -72 volts via 2 Bourns 10-turn pots for course and fine control. This transformer also has a choke/resistor/capacitor filter circuit which, along with full-wave rectification, knocks the ripple voltage down to levels sufficient for the low preamp tube grid voltages. All voltages are manually jumpered to the tube sockets via banana connectors so that any voltage the tester can produce can be applied to the elements of any tube that fits in a 9-pin, octal, or 7-pin socket (all I care to deal with, at least for now).

Other tests are:

Heater to Cathode Leakage:
With the heater on, the maximum allowed voltage (per the datasheet) is jumpered to the cathode across a 1k resistor. A meter measures for AC voltage drop across this resistor.

Microphonics: A test jack, capacitively coupled to either of the high-voltage output nodes via an on/on switch, allows for easy connection of an oscilloscope to the tube plates. With the scope connected, tubes are tap-tested for microphonics.

Life Test: This is done by reducing the heater voltage to 90% of the their standard operating voltage and monitoring change in plate current. That value is noted.

"Gas", i.e grid current, is examined with dedicated nominal 1-meg pots in series with the grid circuit (in reality they top out somewhere above 900k). If a tube has a maximum grid circuit resistance below the 900k, it is tested at that level.

Would love to hear thoughts or if there is anything wrong, etc. Full disclosure, I intend to sell a few tubes online as I find them. Won't likely be super profitable, but it gives me an outlet that does not result in accumulation of stuff, and that is to me very worthwhile.

Thanks!

Joe
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xtian
Posts: 6990
Joined: Mon Apr 19, 2010 8:15 pm
Location: NorCal
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Re: Home Brew Tube Tester

Post by xtian »

Look at you! Awesome job, Joe!
I build and repair tube amps. http://amps.monkeymatic.com
Rockwell666
Posts: 136
Joined: Mon Nov 21, 2016 4:56 pm

Re: Home Brew Tube Tester

Post by Rockwell666 »

This Blew my mind!!!
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