Digital AVO Mk4
This is an AVO Mk4, that I bought for parts. The owner told me it was stored very long ago, and from what he was told the meter was broken. However he knew not much about tube testers, and I got it for a reasonable price. I thought it was for the parts only.
Indeed the meter was bad. Water has come in the meter, and due to the dust proof construction of all panel meters, the water took to long to get out and the meter was all green oxide inside. But for the rest I saw no big defects. All switches were stuck, but with high isolation cleaner oil these went smooth soon. Well and then the back off pots were oxide inside, which is pretty normal with those. I was lucky to be able to repair then nicely, otherwise you have a problem. They run smooth as new. So... I mounted a multi meter instead of the panel meter, and at least the mains calibration point appeared to be fine. All other tests too, but not accurate, and when you tapped it, the reading was instable. No solution.
So I decided it was scrap. I removed the top deck, because that is a beautiful collectors piece. As you perhaps know, you can veeeeery nicely use that to make a more universal tester from any other tester. For instance from the Amplitrex AT1000. On the back is nice strip of connections, for Plate, Grid1, Grid2, etc etc. So all you do is, mount the deck in a nice box, with a cable on it and an octal connector, and plug that into the AT1000 in an octal socket. So you have to program only once a artificial tube called "AVO-1" or so. Then take any setting from the AVO book, for instance for an RE134 or AD1, and there you are it is always nicely connected to the Amplitrex. All rectifiers too. So far so good, that was my plan. It seemed then the roller switch was totally smeared with dirt. So I took it apart and cleaned it, and mount it back in the top deck. Since the chassis of the AVO Mk4 was still on my bench, I just tried what is would do with the cleaned deck, and bang... it worked as perfect as you can dream of. Not even a need to calibrate it, it indicates all tube accurately.
I have been looking for a meter on Ebay for half a years... No luck.
Then I decided to make it a digital AVO. It is not finished yet, but tests are very very good.
I tested a 6SN7 at 250V /-8V
On the knobs you can read the settings. This is a 6SN7 System one at 8.7 mA.
This is System2 at 8.1mA
This is the good / bad test, and transconduction part. On the original AVO metere is a red/green scale. With this digital meter, a reading of "7" means the beginning of "green". 10 is in the middle and 13 is where it ends. At the AVO is the center mark "1" in the middle of the green scale. So we have 10 here now. It works the same way. So this tube is nicely in the middle of the green scale.
For a transconductance reading you need to set the reading to "10.0" using the right dial, and then you read from that dial what is the transconductance, same as the original method, where you have to set it to the "1" mark . Here the test result is 2.5mA/V as you can see on the dial. I should have set the digital meter to "0.0 but that is a small difference on the dial reading.
Disadvantages? Yes. A switch is needed to switch from Plate Current to transconductance reading, but that is all! You can even correct that, but it means you need to change original resistors inside. Don't want to do that now.
Advantages? Oh yes, many