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AmadeusMozart · 10th Jan 2010, 11:46 am

I have just completing collecting all the parts for the RAT tube tester when someone pointed out to me this thread. The RAT tube tester is based on a CCS in the cathode lead and this results in lower g2 and anode voltage as measured to the cathode which I was going to change to using a negative grid voltage. The advantage of the CCS is that it is easier to adjust.

I would make two changes and this is based on observations I made while restoring some Mercury 1000 tube testers and feedback from someone else who has the RAT tester.

The first one is that I would use some ferrite beads at either tube tube sockets or at the switches to try to prevent parasitic oscillation. The second would be to use a setup for valve sockets like the Mercury 1000 / 2000 is using (circuit diagrams are to be found on the web). What is done in these valve testers is to have multiple sockets for the same base but which have different filament connections. Then hardwire the filament connections and do not connect the pins that have a filament connection to the switches. In that way a short of the filament across the switch is avoided (basically that position on the switch for that particular tubebase is n.c.).

There are only something like 4 different filament connections for the Octal and 4 for the Noval and 2 for the 7 pin. The Mercury 1000 has only 2 for the Noval. A worthwhiloe safety measure!

I am using 10 turn bourns pots for the voltage adjustments and am using a 5 A, 15 V variac (that I found on a local auction website) for the filament adjustment (have a 15 V winding on the transformer). This will allow me to drop the voltage on the filament and see the corresponding drop in transconductance which is an indication of how much life in the tube is left (see the RCA tube manual on this).

Anyway this is an elegant solution and I like the layout of the whole. Just wish I could get a similar cabinet locally, my metal working skills are not that wonderfull.

Thanks for publishing and congrats on making something so good looking, well done.

AM

10th Jan 2010, 11:46 am	#46
AmadeusMozart Triode Join Date: Oct 2009 Location: Masterton New Zealand Posts: 17	Re: Homebrew Valve Tester I have just completing collecting all the parts for the RAT tube tester when someone pointed out to me this thread. The RAT tube tester is based on a CCS in the cathode lead and this results in lower g2 and anode voltage as measured to the cathode which I was going to change to using a negative grid voltage. The advantage of the CCS is that it is easier to adjust. I would make two changes and this is based on observations I made while restoring some Mercury 1000 tube testers and feedback from someone else who has the RAT tester. The first one is that I would use some ferrite beads at either tube tube sockets or at the switches to try to prevent parasitic oscillation. The second would be to use a setup for valve sockets like the Mercury 1000 / 2000 is using (circuit diagrams are to be found on the web). What is done in these valve testers is to have multiple sockets for the same base but which have different filament connections. Then hardwire the filament connections and do not connect the pins that have a filament connection to the switches. In that way a short of the filament across the switch is avoided (basically that position on the switch for that particular tubebase is n.c.). There are only something like 4 different filament connections for the Octal and 4 for the Noval and 2 for the 7 pin. The Mercury 1000 has only 2 for the Noval. A worthwhiloe safety measure! I am using 10 turn bourns pots for the voltage adjustments and am using a 5 A, 15 V variac (that I found on a local auction website) for the filament adjustment (have a 15 V winding on the transformer). This will allow me to drop the voltage on the filament and see the corresponding drop in transconductance which is an indication of how much life in the tube is left (see the RCA tube manual on this). Anyway this is an elegant solution and I like the layout of the whole. Just wish I could get a similar cabinet locally, my metal working skills are not that wonderfull. Thanks for publishing and congrats on making something so good looking, well done. AM