I brought this unit quite cheaply a few weeks ago. It was very knocked about and filthy dirty, but seems to be working OK. It is mostly complete only the front and side handles missing. I will give it a test and smarten it up. I've not worked on, or used one of these units. They are quite rare, and they seem to attract a huge price, I've never owned one until now. So I'm interested to see how it works.

Avo have developed a very novel design, which avoids complicated stabilization circuitry. The basic circuit is shown below. It uses an A.C. method of operating, and the exact mains taping on the instrument is set before use. The valve is tested using raw A.C. mains at set levels. And a rectified mains derived negative grid volts supply. The meter readings convert it to the necessary D.C. equivalent characteristics.This makes it so much simpler than the Mullard valve tester. which uses stabilised supplies, combined with accurate mains supply settings. The accuracy of the AVO instrument is dependent on the precise setting of the mains supply input. Before a test is carried out the mains supply is measured by the instrument, and the suitable tapping on the mains transformer is selected.

 

The circuit is very simple and easy to service. There is one electrolytic cap and 2 paper caps to replace, and 2 diodes which did not seem to read correctly. Once this was done, the set up can be attempted. This was a bit tricky and time consuming the first time, but all seems well and when testing all sorts of valves the results are as expected. I have found the most difficult thing to get right in the setup is the negative grid volts. The Avo method involves a factory calibrated valve volt meter. Just not available any longer. The modern electronic meters although extremely accurate will not give the correct reading when measuring this very special wave form (Rectified a/c ) The best compromise is to use a good scope, set to d.c. and divide your peak reading by 3.14 to change it to MEAN D.C. Avo use mean D.C. to calibrate their instruments, for each negative grid volt on the Avo will be equivalent to 0.525 Volts (mean D.C.) So With the grid volts set at -10 on the AVO, the scope measures just over 16.5 volts peak. Now divide by 3.14 this will equal 5.25 volts( Mean D.C. ) This is correct. Now rechecking with the standardized valve it works out just right. Note, the 160, Mk 3 and 4 use a full wave rectified negative grid volts. It's important to select the smaller of the 2 half sign waves to measure.

When using the Scope to set up. Note the 160, Mk 3 and 4 use a full wave rectified negative grid volts. It's important to select the smaller of the 2 half sign waves to measure. Thanks to Gary Tempest for his guidance with the maths.

To check that all is well I have constructed a test Jig as recommended in the A.v.o. manual. An old regulated power supply from a Philips G11 TV is used, this is set to exactly 200 volts and no mater how the load current changes it will be stabalized at 200v. The heater voltage is set at exactly 6.3V.

To calibrate a STANDARDIZED valve . The characteristics have been noted for a series of grid volts, with anode and screen set at 200 using a D.C. method. Here my Fluke meters give excellent high accuracy readings. This Standardized valve can then be used to check with absolute certainty the accuracy of the A.v.o. set up. Plug the Standardised valve in the AVO unit and see how accurate it really is. This works fine and I can now trust the readings given from the tester. You will see in the AVO manuals how AVO recommend the use of a standardized valve to check the calibration of their instruments. AVO use a CV491 ( ECC82 ). I have used a brand new Mullard ECL80 or an ECL86, each valve is individually put through the standardization procedure both the triode and the pentode section. 10 readings on the pentode, 4 volts – 12volts, and 6 readings on the triode, 1 volt – 6 volts.The readings are extremely accurate with the use of top quality Fluke meters.

If anyone would like to have a standardised valve for there own tester, then let me know and I will prepare one for you.

See above the special rectified A.C. grid waveform. With the grid volts set at -10 on the AVO, the scope measures just 16.5 volts peak. Now divide by 3.14 this will equal 5.25 volts ( Mean D.C. ) This is correct. Now rechecking with the standardized valve it works out just right. Note the 160, 163 Mk 3 and 4 use a full wave rectified negative grid volts. It's important to select the smaller of the 2 half sign waves to measure. You can see in the scope the 2 half sign waves with the lower trace showing the anode volts, this coincides with the smaller of the half sign waves, so this is the one used for measurment.

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I've decided to give it a birthday and smarten it up. Remove all the knobs and enclosure . Knock out the dents fill with body filler and respray in a satin black. Clean all the knobs, and repaint all the white pointer lines on the knobs. Next repaint the white AVO logo on the meter, assemble and test. The handles were missing on the side, I've made 2 new ones from plastic and sprayed them to match .

 

 

 

 

 

 

The finished article . Not bad?

A quick tip on the safety of the AVO meter movements. They are particularly fragile in the AVO 160 model, the full scale deflection is only 30 micro amps. It is so easy to overload the meter movements on these models. There is a simple mod that can help protect the meters, I use 2 silicon diodes in parallel but opposite ( cathode to anodes ) connect across the meter terminals. Any serious overloads will be shunted away by the diodes. a switch must be wired in, so the mod can be disabled during the Mutual Conductance check as it will affect the readings. I bring the wires to the switch out of one of the unused valve holders, and insulate it well, as there are very High voltages present.

Many questions have been asked as to the best instrument for testing valves / tubes. AVO or the Mullard, they really don't compare, each instrument has it's different job to do. The Mullard will test a set of radio valves in a few minutes with results you can trust. In particular the grid current, or gas check, on the Mullard it is much more sensitive. On the Avo a test and check can take 10 to 15 minutes for each valve and great care must be taken with the AVO settings, It is so easy to wreck a valve or the meter movement with a careless settings. This can't happen on the Mullard, apart from selecting the wrong card it is pretty fool proof. The great facility of the Avo is the valve performance can be accurately measured and compared with other similar valves, so it is a simple matter to select valves for matching, in an amplifier for instance. The gas check however is very insensitive and a reading of 5 micro amps should be considered a bit suspect. Once you have your tester working well It is important to keep it in a dry atmosphere, as they are stuffed with transformers and chokes, which will all stay healthy if the moisture is kept out.

To The AVO 163 valve tube analyzer I must be mad to take on another one of these.

Back to home .

My friend Yutaka Matsuzaka's AVO website in Japan.