You are here at Analog Testers, Part2

Centrad 752 - The French "AVO"!

This was an exceptional find. I will describe it later. I noticed it's from the city of Annecy, same as were the Metrix U61B comes from. That relation is perhaps no coincidence. This tester looks a bit like a Taylor 45C. It has one defective selector switch, but that looks easy to repair. Just need to have time. So I cannot select every tube, but almost any. This little pearl is based on the circuitry of the 45C, but it has more and better options, also it had the selectors for any random tube, same as the AVO's have. All together this is a mix of AVO Mk1, Taylor 45C, and the designer saved absolutely not on nice options. It has for instance the professional leakage test method, same as the AVO's have. This means a real ohms measurement, but with a high voltage as bias. These French designers were aware, sometimes the one, sometimes the other method is better. Heater voltage very interestingly done, with a coarse and a fine adjustment. Well, I have not really looked at it in full detail, but I will probably restore it completely. Some incandescent lamps are used as a fuse, same as Hickok does. Fact is, the tester works, and is precise. It can do many of the European B4B5 socket tubes like PX4, PX 25, etc. Easy to handle and a nice design.

There is the original French manual, and a very well made translation in Dutch. This Dutch translation you can enter in the Google translator and it nicely gives English instructions.

As a pre-test it has a way to test the cathode all alone, so without any function of the rest of the tube. They write in French, this indicates a troublesome tube, but for the rest the transconductance should be tested to judge the tube.

VOLTAGES: Grid voltage can be set from 0...15V. Anode and Screen voltage are selectable from 6...250V, in well choose steps and combinations. Heater voltage can be set for almost every possible voltage. This method is very clever, and results in a relatively simple switch. Moreover, you can varied all voltages up and down 1V or 2V, which is really very handy to test emission quality.

TRANSFORMERS: It has a separate heater and anode transformer. Many better tube testers have this.

Many things of this tester makes me realize they some very talented people tried to do a good job, and they succeeded also. This this tester gives you: A pre-test, a transconductance test, and a "Quality Test". Whereas the CONAR (below here) only presents a COLOR. Yes, you get only a color, so it's red, yellow, or green, and that's all this silly CONAR will tell you. What a difference with the CENTRAD!

VALUE: There are very little of those around, so there is little reference. But you have to make good note, it has an excellent variety of useful tube sockets, both USA and European, it has all the heater voltages, and 1:1 setting of the socket connections, just by the tube data sheet, to test for any new tube that would be invented later. The panel meter itself is robust and of excellent quality. Operation and many details remind me of the Taylor 45B, and price should be in that range. Given that people pay 250 Euro for such a silly and useless CONAR as pictured below, the Centrad 752 is worth 3x as much. So we talk about 600...800 Euro, similar like a 45B. Though the Centrad can do more than the 45B.

Size and weight: It is larger as it looks on the picture, and actually quite heavy, the metal handles make sense.

ManualFrench - ManualDutch - MeterPicture


CONAR TUBE TESTERInfamous brand tube checker. The CONAR brand should not be confused with CANOR, which is a high quality digital tube tester. The CONAR uses all tubes as a rectifier, and load that rectifier circuit with a variable resistor. It's the 'D' knob here. This tester works at too low voltage. The good function of a tube tester is not to show that new tubes are 'good', and broken tubes are 'bad'. The question is, when the tester says the tube is somehwere in between, how the magic works. It works at very low level and makes many mistakes. So when the tube is in the green it may be good indeed. Or may be bad as well. I would say it is worth 50$ for the fun of owning it, but you cannot test tubes with it in a reliable way. No stop! I am wromg about the 50$. It's worth a lot more, because you can sell it on Ebay easily for 200$, and when you picture it with an EL34 far in the green, you get even 250$ for it. Or perhaps you have even an original Telefunken in it's box. Picture it with that tube on it, and you get even 300$. Yes, that's what you should do with it.

This tester wins the Silver Lemon Prize.

Picture series here


DIY tube testerMany DIY testers exist, and it is interesting to see sometimes 50 years old versions of those. Some are really nice done, and some not so nice. Today, often the testers are made as digital products, and you can see the maker getting deeply into software issues, and the hardware getting larger as intended. For that reason, a nice way is use digital panel meters, and for the rest set up the tester in the good old analog way. From my own experience I can say, use only industrial meters for this. The Chinese Ebay meters have unlinearity problems. Suppose you calibrate it for 1000V full scale , and measure 99V. So you switch to the 100V range to get more accuracy, but you measure suddenly 98V. (Mmmm ?!) The reason is panel meter internal unlinearity. You can't calibrate that away. The expensive meters don't have this problem. Since the tester is an instrument, you need to make the full concept before you start, keep the wiring simple, yet prevent to forget functions that you have to add later. So the thing gets serviceable later. With the concept being 'final' from the beginning, you can make a nice front plate, use nice knobs, and paint it professional. Here is one of those products, which I think is extremely well done. It was made by a private person. He is 78 years old, (while I write this in 2011). He admires tubes for the industrial design 'art' in them. So this is somebody with time and experience, and the right heart beat to build something ultimate. The result is the most beautiful analog tester I ever saw. Read more about it.



A picture of his DIY tester tester was send to me by a Frenh DIY enthousiast. I have no more information than the picture, but click on it for a enlargement. You can see many interesting details



This tester was build by a German person, who knows a lot a about software and test equipment. On Ebay you can buy used VXI boards. When 5 1/2 digits is enough for you these boards are cheap! I see them go for 50$ often. This is an older standard. These are plug in versions of regular Hewlett Packard instruments. The difference is, they have no display and no knobs or buttons. The control is done only via PC, and reading the measurement data also. Because many do not know how to use this, these VXI boards can sometimes be found cheap, and yet represent a beautiful Hewlett Packard multimeter. Moreover, the boards take little space. So many fit into a rack. This is a so called VXI rack. This standard was also adopted by some other vendors like Siemens and Tektronix. The boards get their power supply from the VIX rack, and computer interface is also with the VXI rack itself. So you can just stick 5 multimeters in, and then access the VIX rack via your computer. You do need the right measurement software for it. I use "Agilent VEE" for instrument control, but there is also freeware around. If you can get hold of a r copy of VEE, that may be realy nice to use with those older HP or Agilent VXI cards. With such a program you can write to the multimeters, like set the range, Volt, mA, or Ohms, etc. And then read the result from it. On card can measure current and voltage of a device at the same time, because the banana plugs for that are another. Just as with a normal multimeter, you can also hook it up to a circuit and measure current and voltage at the same time, you just need to push some of the button "Volts" and "DC", and read volts. Then push "mA", "DC" and read the current. This pushing of the buttons is done in software. So for a tube tester you need probably 4 such cards. You need also programable power supplies, which this person build himself. For your won DIY tube tester that will be the harder issue. You have to consider this something similar to the Roestest, which works not with VXI, but basically the same way. For normal people the Roetest is probably better.