Avo CT160

[mikko rintala]

Mikko from Finland hi. I am quite new here and i have a guestion.
I wanna put digital negative grid voltage meter my Avo ct160.
What point i should connect that is measuring correct voltage.

[G4_Pete]

Mikko,

Welcome to the forum,

If you search CT160 on this site there is a wealth of information on the CT160.

https://vintage-radio.net/forum/showthread.php?t=121855 See post 12.

However in reply to your specific query the waveforms in the CT160 are not DC see above post. Therefore the readings on any digital meter would have to take the form factors into consideration and be calibrated to compensate.

Pete
P.S. If you have not done so already it is advisable to look at the sections on this site with regard to meter protection as these things are rare and expensive to repair .

[David Simpson]

Hello Mikko,
Welcome also to the Forum, and to the valve tester enthusiast's fraternity. As Pete says, CT160's voltages are not straightforward. For calibration purposes, AVO in their Servicing Manuals quote a DC measurement of -ve Vg, but it can be jolly misleading. As CT160 parts are very expensive(I've heard of £200 being asked for a replacement Grid Volts 10K Potentiometer(RV2) and for the 30uA meter), like Pete - I would encourage you to look through "Search" at the mass of information in past threads.
Have a look at the simple pictorial diagram(not a circuit diagram) which I drew a few years back, to help folk to understand the CT160's AC functioning.

Regards, David

[mikko rintala]

Hi all. i am allready make new meter that Avo with Martin Forsberg advice and thank to him that works now great.
But i just wonder is it easy put grid voltage meter that Avo?

The RV2 is so sensitive that its affect so easy tubes measurment.
Its can be analog panelmeter also.

Best Regaird Mikko

[Station X]

RV2 is sensitive, but it hardly matters except when you're zeroing the meter prior to making a gm measurement at a given anode current. I don't see how a grid volts meter is going to help you with that.

[ms660]

I might be miles out but the only way I can see of measuring the grid voltage with a DM is to rectify the half wave pulses that are fed to the grid, then reservoir capacitor, then a pot divider to do an RMS fudge, the input impedance of the DM might do for the bottom half of the pot divider?

Lawrence.

[David Simpson]

Looking at my diagram, it can be seen that what looks like a Full WR waveform is on the Grid. In fact it's two separate 1/2W rectified pulses from the 55 & 66 V(RMS values, not P - P) windings. Only the 55V derived pulse is used when a tested valve is conducting. Therefore the scaling of RV2 from 0 to 40V(in DC terms) is somewhat misleading. Some folk say that RV2 is non-linear. Wrong - it's three 90deg linear segments(0 - 5V, 5 - 15V, and 15 - 40V), total resistance being 10K. Hence the horrendous cost of a genuine replacement.

Replacement costs for pots and meters, and recent mercenary greed fuelled eBay prices for CT160's, results in me and other AVO VCM enthusiasts banging-on about folk being very cautious when attempting to repair/calibrate these 60 year old testers.

Regards, David.

[ms660]

There's some stuff on replacing a duff grid pot and fitting a digital grid volts meter here:

https://vintage-radio.net/forum/showthread.php?t=85697

Lawrence.

[mikko rintala]

Thanks to all. It nice that this forum is so kind people to help another.
Just keep on study to how it s work best way.

Best regaird Mikko from Finland.

[David Simpson]

Hello again Mikko, I guess if you keep in touch with min ven Martin in Linkoping, he'll keep you right. As Kauhava is in the west of Finland, perhaps you can speak some Swedish. Jag tala svenska inta braw.Velkomen til CT160's.

Regards, David

[mark-jacobs]

You can connect a DC meter between the cathode and the grid on the deck, and scale the meter to read 0-40V corresponding to the grid dial markings. I mapped out my grid pot at various points and then adjusted the meter to give the most accurate results across the measured points. I set the grid voltage with the dial, and then fine tune it with the digital meter while the tube is under test. I have several calibration tubes and the results have been accurate between testers under the same operating parameters.

The Grid bias meter is activated by a push button, but must not be in the circuit for other tests like leakage. I have done this with two CT-160's that both are converted to use SS diodes. There are separate meters that connect via a plug to the deck connections and provide plate current, bias V, line V and heater voltage. The Heater voltage is important, I also have a 3 way switch for the heater voltage which allows an intermediate volt range, one to get a more accurate heater voltage. In the pictures below it is in the center position to give 6.2V, otherwise the voltage would be either to low or high with the stock CT-160.

[mikko rintala]

Originally Posted by David Simpson

Hello again Mikko, I guess if you keep in touch with min ven Martin in Linkoping, he'll keep you right. As Kauhava is in the west of Finland, perhaps you can speak some Swedish. Jag tala svenska inta braw.Velkomen til CT160's.

Regards, David

Nej,jag tala svenska inte Maybe if i live 60km to north,then ja tala swenska

Med vänliga hälsningar Mikko

Originally Posted by mark-jacobs

You can connect a DC meter between the cathode and the grid on the deck, and scale the meter to read 0-40V corresponding to the grid dial markings. I mapped out my grid pot at various points and then adjusted the meter to give the most accurate results across the measured points. I set the grid voltage with the dial, and then fine tune it with the digital meter while the tube is under test. I have several calibration tubes and the results have been accurate between testers under the same operating parameters.

The Grid bias meter is activated by a push button, but must not be in the circuit for other tests like leakage. I have done this with two CT-160's that both are converted to use SS diodes. There are separate meters that connect via a plug to the deck connections and provide plate current, bias V, line V and heater voltage. The Heater voltage is important, I also have a 3 way switch for the heater voltage which allows an intermediate volt range, one to get a more accurate heater voltage. In the pictures below it is in the center position to give 6.2V, otherwise the voltage would be either to low or high with the stock CT-160.

Something like that i think too. Some one asked how a digital grid panel meter would help with the help sensitive grid pot problem. I think that a voltmeter would help with repeated test on the same type of valve. I mean that it helps that the grid voltage is the same. For example if I test 10 ECC83 valves in a row.

T Mikko

Originally Posted by mark-jacobs

You can connect a DC meter between the cathode and the grid on the deck, and scale the meter to read 0-40V corresponding to the grid dial markings. I mapped out my grid pot at various points and then adjusted the meter to give the most accurate results across the measured points. I set the grid voltage with the dial, and then fine tune it with the digital meter while the tube is under test. I have several calibration tubes and the results have been accurate between testers under the same operating parameters.

The Grid bias meter is activated by a push button, but must not be in the circuit for other tests like leakage. I have done this with two CT-160's that both are converted to use SS diodes. There are separate meters that connect via a plug to the deck connections and provide plate current, bias V, line V and heater voltage. The Heater voltage is important, I also have a 3 way switch for the heater voltage which allows an intermediate volt range, one to get a more accurate heater voltage. In the pictures below it is in the center position to give 6.2V, otherwise the voltage would be either to low or high with the stock CT-160.

Hi. I tested and but leads in deck,and little tunign. Works fine.
But if negative grid marks is -2v and digital meter shows -2v ,then dial number -40v meter shows -45v ?


Best Regaird Mikko

[Station X]

Originally Posted by mikko rintala

Hi. I tested and but leads in deck,and little tunign. Works fine.

But if negative grid marks is -2v and digital meter shows -2v ,then dial number -40v meter shows -45v ?

Best Regaird Mikko

Perhaps you need to calibrate your CT160?

[mark-jacobs]

The grid voltage at 40 will be 47V (47.3 per others) with the calibration link closed, 20.8 with the link open. Others have outlined in detail the operating properties of the AVO testers, so the actual voltages do not coincide with the setting (dial) voltages. You need to use a scalable voltmeter where you can adjust the scale to read 40V DC when the grid scale is at 40 but the measured voltage is ~47V. Since the grid pot is wound with 3 fixed linear sections, the grid dial is also adjusted relative to the grip pot, and there are dead zones at either extreme of the pot, the accuracy can vary through the range. The position of the dial, and to some degree the calibration adjustments are compromises to get the best results across the dial range.

I use Simpson M235 and M245 digital meter which you can scale the output voltage, I also relocated the scaling adjustments so they are accessible after the meter is mounted in its enclosure. The digital meters make up for linearity errors in the grid pot, this can be fairly significant at certain points of the scale. Values below 2.0V are very sensitive but also other portions specific to each pot/tester. I also have a range function for my digital meters, so one setting is 0-19.99V and the other is 0-199.9V, the former is used for low grid voltages. With these changes along with using SS diodes, the CT-160 results are very comparable to standardized tubes tested in jigs and my curve tracer.

[Dekatron]

I never dived into making a meter for the grid volts potentiometer, I simply replaced the grid-volts potentiometer with a 10-turn potentiometer and a 10-turn scale that I could lock, I just had to correct for the total resistance of the 10-turn potentiometer (the one I used was correct so no correction actually had to be done). I didn't have to drill any holes or modify any mechanical parts this way, just use the original parts in a different way - mounting them like in the photos in the thread below. With this 10-turn precision potentiometer in place I just had to remember that each turn corresponded to 0.4V. I accidentally burned one of these 10-turn potentiometers so a fuse in the grid volts circuit would be a good idea.

You can read a short note on it here: https://www.vintage-radio.net/forum/...2&postcount=15