Hickok OZ-1 (550X) Tube and Set Tester

The Hickok OZ-1 was made for the U.S. Navy and is basically the same tester as the model 550X. It's from the 1940's and the year I've seen mentioned online is 1942.
The front panel of the tester. It's a combination tube tester and radio set tester.
A closeup of the tester's meter. Because the OZ-1 has a seperate Line meter, the Line Test line on this meter isn't used. The OZ-1 is also an ohmmeter, a voltmeter, and can test capacitors. I probably won't use these functions very often.
This is the added AC voltmeter for the line adjustment. The Line Adjustment pot is used to set the needle to the red line.

Pressing the Line Test button displays the actual AC line voltage.
The tubes under the panel are a 5Y3 vacuum rectifer, an 83 mercury vapor rectifier, and two 6H6 dual diodes used in the set tester section. I installed a solid state replacement for the 83 tube, to lower heat buildup in the cabinet. The solid state 83 uses zener diodes to simulate the voltage drop of the real 83 and works OK. But when using a solid state replacement rectifier it is a good idea to add a fuse inline with the rectifer output in case a diode ever shorts, which could burn out the transformer winding. A fuse is cheap insurance.

All out of tolerance carbon composition resistors were replaced. All the wire wound and bobbin resistors were OK. And all the switch contacts were cleaned as well.
Tucked away under a bakelite board in the set tester section were two paper capacitors that were replaced with film caps.
The top of the cabinet had a large section of the black cloth ripped off (forgot to take a photo). I ordered some Tolex to replace the cloth, but it really wasn't a great match. Then I realized I already had a matching piece of cloth just the right size, on the inside of the lid! So I stripped the cloth from inside and reglued it to the top (the dried glue on the right is from someone else's attempt at a repair). I only had to trim a little from one end of the cloth. A perfect match.
Since I stripped the cloth from inside the lid to reuse on the outside top, I used the Tolex to re-cover the inside, where it's less noticeable that it's not quite the perfect match.
The tester still has its original manual and tube charts, which is nice. The serial number inside the OZ-1 manual matches this set.

I tested some tubes, both good and bad, and compared the results to my TV-3 tester, which also uses a Hickok circuit. On the OZ-1, tube test results using the settings in the tube charts are read on the meter's English reading Replace/Good scale.

​To test actual mutual conductance the L pot is set to 60 (marked Gm) for the 3000 scale, instead of the "English Reading" L value on the tube chart. Also, my copy of a 510X manual says that to use the 6000 scale the L pot is set to 71, and for the 15000 scale it is set to 77, but these are penned in values on the manual page so vary from unit to unit. A copy of a 530 manual I have uses 70 and 76. But when I checked the three scales on this tester I found that setting the L pot to 60 gave accurate results on all three.
The micromhos column of the tube chart is only used when making Gm measurements. The 510x and 530 manuals both state this reading is minus 20% of the manufacturer's Gm specification for an amplifier tube, and minus 25% for power tubes. These numbers do not match the minimum Gm values listed in the TV-3 chart. However, the actual tube Gm test results do match the TV-3 results when the L pot is set to the Gm position on the dial.
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For example, I measured a 6F6 tube on four testers. On my drug store tube tester, which is an emission tester, I got a reading of 90%. Pretty good tube according to it. Then on my Precision Tube Master 10-12 it read 84%. Still pretty good.

Next I tested the 6F6 on my TV-3. The minimum Gm is listed as 1260 on the roll chart and the 6F6 measured just under 2000 μmhos. A good and accurate result.

Finally, I tested it on the OZ-1. The controls were set to the values listed under the 6F6, which in this case just happens to set the L pot to 60, so the Replace/Good test also gives the Gm reading as well. And according to the tube chart the minimum Gm for a good tube is 2000.

The tube tests good, right in the Green zone on the OZ-1, and the Gm measures just under 2000 μmhos, which matches the TV-3 result, and makes this a very good tube, right? Well, not according to the OZ-1 chart. Going by it the tube should be replaced since it falls below their minimum value of 2000.
Obviously the 20% number needs to be taken with a big grain of salt since it doesn't come close to matching the minimum on the TV-3 chart. I don't think I'm going to be wasting tubes by replacing them when their Gm is down by only 20%, so I guess I'm going to have to copy my TV-3 data and add it to the OZ-1 manual as a reference for minimum Gm values.

So, the OZ-1 works well and is giving accurate Gm readings, and now that I've worked out how to interpret the results, all is good.
Page created 3/29/2021
Last edited 3/30/2021