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Tuesday, February 21, 2012

Testing Miniature Pentodes with the Tek 575

This post is a follow on to my January 6th post “Testing Dual Triodes with the Tek 575”.  With the great success of my dual triode test fixture making it possible to go through boxes of tubes I decided to take on making a fixture for the 6AU6 which is the next most common tube I encounter in much of the antique test equipment that I restore or repair.  The 6AU6 is a miniature pentode and as such would require a screen grid supply of about 100 volts to test properly.  I was torn between making a small switcher and just using back-to-back filament transformers.  This was incentive enough for me to create a breadboard to test out the concept and to see what the volume would be of the components being used.  Here is a picture of the breadboard under test.  The bench supply is for the added digital meter used to monitor the screen grid supply.

Once I was satisfied with the design and the performance I shoehorned it all into the same SERPAC A-42 enclosure that I had used to package the dual triode tester.  As you can see there was a lot more to fit inside, including the screen grid supply, 5 volt supply for the meter, and the meter.  I also found it necessary to add ferrite beads to the wires coming from the Tek 575.

Here is the completed unit ready to go.  As with the previous test fixture power is supplied through a standard IEC power cord.  It’s also worth noting that I added a second 7-pin socket that allows me to switch between two tubes for the purpose of matching.

This is a picture of the pentode test fixture in use. 

Here you can see that the results are quite nice and comparable to the data in the RCA tube manual.  This picture was taken with a modified C-12 camera mount described in an earlier post on this blog.

I was also pleased to discover that just like the dual triodes there is a large list of miniature pentodes that have the same or similar pinouts that can all be used with this fixture.  I’ll add the list at the end of the documentation for this project just as I did for the dual triode fixture.  It will most likely be posted on the Tek Scopes forum;

As usual if you decide to try this on your own I offer the following disclaimer:

Some of the circuits described on this site use or generate potentially lethal electric currents and voltages, and if not treated with care, respect and intelligence, they can result in fatal injury. If you use the information on this site to kill yourself, your friends, family members, acquaintances, total strangers, pets, electronic devices or burn down your house, it is not my problem.  That said, have fun!