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Old 2nd Jul 2010, 9:54 pm   #21
qcda275
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Default Re: AVO CT160. Protecting the meter.

I didn't mean that, I meant to post the URL of the album so a new tab is opened to take the viewer straight to photobucket.
Sorry, I should have made that clearer.
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Old 3rd Jul 2010, 9:51 am   #22
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Default Re: AVO CT160. Protecting the meter.

The trouble with off forum images, no matter how linked to, is that they can disappear at a future date which is frustrating for anyone looking at the thread in the future.
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Old 3rd Jul 2010, 11:30 am   #23
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Default Re: AVO CT160. Protecting the meter.

Guys, URL's, Photobuckets - I'm a 1960's Analogue person. It's taken me(& actually my wife's laptop - she's into photography, Adobe Ph.Shop7, etc) ages just to get round to picture reduction down to less than 200K.
Usually I just send folk pictures full size by email. I wouldn't ken a photobucket unless I tripped over it in the garden!
We should have the pics out later today.
Another reason for re-casing the CT160 - didn't like the idea of reaching across the control unit to fit & remove valves, change selector settings, etc - from a safety point of view. Side by side is just great. Yes, its bulkier, but just fits fine in my wee workshop. Stored upright, then laid down flat when in use. If I'd only a few valves, now & then, to test, then I'd have left things as they were. But have only got round to testing about 25% of the 900 or so.
Also means I've a clam-shell case spare. Guess I should advertise it for sale sometime.
For AVO purists - not the done thing, I suppose. But hey - might even get round to a similar project with my CT446. Already made one or two wee alterations.And I've even heaps more semi-conductors than valves to test. God, I hope my eyesight stays OK till I'm into my 80's, & retain the dexterity in my hands. ( Those of you with a military background -will see a joke in there, somewhere).

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Old 3rd Jul 2010, 8:36 pm   #24
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Default Re: AVO CT160. Protecting the meter.

Hopefully have attached some pics. Jones plugs were used for interconnections.

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Old 4th Jul 2010, 12:19 pm   #25
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Default Re: AVO CT160. Protecting the meter.

Coming back to the original question: protecting the meter, I use two diodes.
I have seen this used commercially, but don't now remember where. I see Dekatron has also seen this, 2 July. However the Avo VCM MkIV uses a single silicon diode in parallel with the meter, conducting forward. I have fitted my CT160 with a silicon diode forward, and a germanium in reverse. I am encouraged that GMB 2 July also refers to this method.
The reasoning is: Meter 30 ua, 3.3k, so 100mV forward. A silicon diode will conduct at 0.6 v. For reverse, the germanium will conduct at 0.3V. So the peak current both ways is limited. The diodes will start to conduct a bit below the traditional turn-on voltages.
I note the discussion upon high peak voltages. I can only report that I have not found the meter readings to be affected. High anode current testing will not alter the current through the meter, as it is shunted to permit the high anode current .
A Schottky will start to conduct forward at say 0.1 volts, which is too low, though will be alright for the reverse.
It is quite interesting to actually measure when a diode begins to conduct, something not normally shown on a data sheet. I reckon 0.1v for Ge and 0.4V for Si, and less than 0.1v for a Schottky..
There is an interesting discussion about shunting meters with rectifier units to give overload protection in Scroggie; Radio Laboratory Handbook, 2nd Edition,1944 to 6th & 7th Edn 1971, using "metal rectifiers", presumably CopperOxide.
The final 8th Edn 1980 has been modernised and uses silicon or zener diodes, with a resistance in series with the meter to bring the voltage drop up nearer the diode turn-on voltage to give a non-linear scale. If these references are not to hand, I can post them. Bill M0WPN.
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Old 4th Jul 2010, 7:22 pm   #26
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Default Re: AVO CT160. Protecting the meter.

Quote:
Originally Posted by WME_bill View Post
I note the discussion upon high peak voltages. I can only report that I have not found the meter readings to be affected. High anode current testing will not alter the current through the meter, as it is shunted to permit the high anode current .
Bill, thanks for the info. You say the diodes make no difference which is good, and not in dispute.
The CT160 works by balancing the dc mean value of a rectified sine wave (ish) from the anode current reference supply, against the dc mean voltage accross the anode current shunt (far from a rectified sine wave, as the current through the valve at any instant in time depends on the instantaneous anode voltage and negative grid voltage). This effect is more pronounced with triodes than pentodes. I used a 6080 with both sections strapped to look into this.
Although the difference in dc mean voltages across the movement is zero at balance and as you say 100mV at fsd, expect a suprisingly large ac voltage accross the movement (3rd harmonic?).
Since the operation of the CT160 is based on an assumption, which generally holds true, there's no reason why other parts of the design were not based on an assumption too.
I shall do a bit more research into this.
Rob.
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Old 4th Jul 2010, 8:25 pm   #27
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Default Re: AVO CT160. Protecting the meter.

I have looked at quite a few documents form AVO regarding the design of the AVO Valve Testers including the CT/VT160 and there has not been one reference to diodes across the meter in the CT/VT160 in any documents that I have found so far, including a handful of Military documents relating to the CT/VT160. If anyone has a document that describes a modification of the bridge circuit with diodes across the meter for a CT/VT160 I would be very interrested in getting a copy of that document. I have also been trying to get hold of any AVO patents regarding the CT/VT160 but not found any so far so that is also on my wishlist!

Since AVO where using diodes across the meter in the other Valve Testers up to and including the MKIV (and also later the VCM163) at the same time the CT/VT160 was manufactured and designed I really think that there must be a good reason for them not to use any diodes in the CT/VT160.

Could it be that a bridge design like the CT/VT160 was negatively affected by the diodes that was available at the time it was designed/manufactured, maybe due to too high leakage current or any other factor of the diodes. Or that the bridge circuit would not work correctly with diodes across the meter due to the non-linearity of the diodes?

Not many bridge designs from the same era uses diodes across the meter if the meter is directly in the bridge circuit (without an amplifying stage between the bridge and the meter).

Unfortunately I do not own a CT/VT160 any more as I sold mine when I got hold of my VCM163 so I can't make any tests myself to check how the bridge behaves with diodes across the meter.
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Old 4th Jul 2010, 9:02 pm   #28
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Default Re: AVO CT160. Protecting the meter.

This is becoming most interesting.
Relating to the comments from Dekatron, the circuit of the CT160 is identical to that for the VCM MkIII, and they were made at the same time. The only differences are the different outputs of the transformers for feeding anode, screen and grid supplies. The basic bridge principle for balancing out the Anode current and giving the Gm is the same.
The VCM MkIV was made later and is the same except for the modification for protecting multigrid valves: the Normal/Unlimited switch. See my previously posted memo from Avo Service Dept dated 28 Aug 1959 giving a mod to the MkIII to incorporate this protection. (It appears under something like " Avo CT160 calibration resistors").
I just wonder if at the time the CT160 was made, the Ministry of Defence would not accept semiconductor diodes (? germanium) for military use, so EB91/6AL5 valves were used.
By the time of the MkIV, silicon was acceptable, and is used everywhere, with no valve diodes.
Yet the MkIV has the forward silicon diode across the meter, and there is no apparent reason with similar circuits, why the CT160 and MkIII could not have it as well. Except perhaps that the MoD would not accept semiconductor diodes at that time for military use.
This third harmonic mentioned by GlowingAnode sounds curious. Particularly as the Anode current is unidirectional single half wave, and the grid voltage while both half waves, is also unidirectional. I assume it was not from the mains supply itself, rather than generated by the circuit. My posting on calibration resistors includes simplified operation diagrams for the various settings of the function switch. I am curious about all these references to a bridge balancing the grid voltage and anode current. Surely the Anode current is backed off, and then when the GM control alters the effective grid bias, the anode current changes, and that is what deflects the meter to read Gm. I will think more about this. I remember that the Avo Valve Testers parapet is a very dangerous place with all these AVO enthusiasts around with sharpshooting rifles. I am sure there are many more expert than me who will carry out tests, and then evolve an explanation. Bill m0wpn

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Old 4th Jul 2010, 9:22 pm   #29
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Default Re: AVO CT160. Protecting the meter.

Hope fully this will clarify. Shown is a trace of the anode current of a triode under test vs a "sine" wave (suitably scaled) to represent the current reference.
You can see the dc mean values are similar. Over the first 1/6 of a cycle the anode current dominates. Over the second 1/6 of a cycle the reference voltage dominates. Over the third 1/6 of a cycle the anode current dominates.
The difference (ac and dc) between these two signals is what the meter sees.
The difference signal varies 1 1/2 cycles over 1/2 cycle of the mains ie. 3rd harmonic.
Displaying the actual waveform across the movement is difficult, as it requires a scope with a high CMRR, which I do not possess.
Rob.
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Old 4th Jul 2010, 11:44 pm   #30
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Default Re: AVO CT160. Protecting the meter.

Some musings. From the scope shot above, suppose this shows the voltage across the meter when balanced. If we imagine the difference waveform, there will be two positive peaks and a negative slump. The combined area of the peaks above the baseline is the same as the area of the slump below the base line, ie. zero dc mean. The amplitude of these peaks is large compared to the dc fsd. A series inductor or parallel capacitor will reduce the amplitude of these peaks, whilst leaving the dc component unchanged. It is this super imposed ac that causes the needle to rattle which cannot be good for the movement. This obviously didn't concern AVO, but then they did sell meters after all.
Since the source impedances are low, I treat the movement as a mean reading voltmeter.
If the positive and negative portions are clipped to the same voltage, the area above the base line will now be less than the area below, i.e. negative dc mean or low meter reading.
By clipping the negative portion to a lower voltage, this effect may be conteracted to a degree. I wonder if this is what you have done purely "by accident". Did you design that yourself or was it published elsewhere?
The peak value is of less importance than rms since the damaging effects of overload are thermal (leaving mechanical damage aside). I suspect the rms value is much smaller.
Clamping diodes are a good way of protecting against gross overload, and work well in dc only circuits.
The harmonics are generated by the non-linear devices, chiefly the valve under test.
Roughly speaking for a triode; for a fixed grid voltage, anode current is proportional to anode voltage. For fixed anode voltage, anode current is proportional to grid voltage. However the grid voltage is decreasing whilst the anode voltage is increasing so non-linearity can only be expected.
Remember we are not dealing with ideal dc conditions or pure sine waves.
Rob.
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Old 5th Jul 2010, 11:30 am   #31
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Default Re: AVO CT160. Protecting the meter.

What Rob has said in his final sentance,concerning the waveforms - echoes my conlusions some time ago in another Forum post. In fact, a scroll back thru "Search" will reveal several similar conclusions from other Forum CT160 enthusiasts. Unsmoothed Full wave & 1/2 wave rectified pulses can be seen on scoped displays.If our eyes could detect such fast movements of the meter's needle "vibrations" - then a 20mS flicker would be clearly seen.
If AVO thought that diodes would protect or improve the CT160's meter display - then I'm sure that mod kits would have been issued in the 60's & 70's(Shotskey, Zenner & other diodes were by then in universal use). During my time on 103MU - no such kits were issued by AVO or Henlow. Nor since, to my knowledge.
In conclusion - I'll repeat - this is a purely AC instrument. Involving waveforms of Pi or 2 Pi radians duration in it's circuitry. Please just accept that. Voltages & currents selected by the switch's displayed ranges are just a close approximation of their DC equivalent. If folk want to test under pure DC conditions( as I do with "Standardised" CV455 & other ECC valves), then build yourself a tester. Or Hybridize your CT160. Or build that excellent Tester that was proposed a few months ago on the Forum.
If we wern't experiencing a spell of grotty weather up in the north of Scotland, I just wouldn't have had the time or inclination to bang-on again about CT160's.

Regards, David
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Old 5th Jul 2010, 1:15 pm   #32
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Default Re: AVO CT160. Protecting the meter.

Quote:
Originally Posted by glowinganode View Post
Some musings. From the scope shot above, suppose this shows the voltage across the meter when balanced. If we imagine the difference waveform, there will be two positive peaks and a negative slump. The combined area of the peaks above the baseline is the same as the area of the slump below the base line, ie. zero dc mean. The amplitude of these peaks is large compared to the dc fsd. A series inductor or parallel capacitor will reduce the amplitude of these peaks, whilst leaving the dc component unchanged. It is this super imposed ac that causes the needle to rattle which cannot be good for the movement. This obviously didn't concern AVO, but then they did sell meters after all.
Since the source impedances are low, I treat the movement as a mean reading voltmeter.
If the positive and negative portions are clipped to the same voltage, the area above the base line will now be less than the area below, i.e. negative dc mean or low meter reading.
By clipping the negative portion to a lower voltage, this effect may be conteracted to a degree. I wonder if this is what you have done purely "by accident". Did you design that yourself or was it published elsewhere?
The peak value is of less importance than rms since the damaging effects of overload are thermal (leaving mechanical damage aside). I suspect the rms value is much smaller.
Clamping diodes are a good way of protecting against gross overload, and work well in dc only circuits.
The harmonics are generated by the non-linear devices, chiefly the valve under test.
Roughly speaking for a triode; for a fixed grid voltage, anode current is proportional to anode voltage. For fixed anode voltage, anode current is proportional to grid voltage. However the grid voltage is decreasing whilst the anode voltage is increasing so non-linearity can only be expected.
Remember we are not dealing with ideal dc conditions or pure sine waves.
Rob.
Could this be why Avo warn against using a reglated power supply for the CT60 due to the possibility of the introduction of harmonics to the supply line ?
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Old 5th Jul 2010, 3:43 pm   #33
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Default Re: AVO CT160. Protecting the meter.

Johnneyt: Yes it is!

Atleast more or less so. I do not have the time for a long explanation here just now since I am on vacation but for the measurements to work correctly in an AVO Valve Tester (except partly for the VCM163) you need to have as pure as possible AC voltage from the wall socket and preferrably also as stable as possible input voltage. It is crucial that the input voltage is both stable and of pure sine wave when you do the calibration, otherwise you will get the wrong results.

I will explain all better later when I have some more time, so please bear with me for now.

This is also partly the reason why the AVO Valve testers work worse near the limits of the input voltages, the transformers gets too saturated and the pure sine wave is distorted so the measurements get wrong!

I have tested both of the effects above and checked how it looks with an oscilloscope and it is not "beautiful" when you have impure AC voltage in regards to sine wave and voltage levels. I have also tested this with different "voltage stabilizers" from magnetic stabilisers to other voltage converters. The far best have been servo controlled variacs and also some true-sine wave converters. Otherwise the voltage from the wall socket is quite stable and as long as you monitor it while calibrating your valve tester it works well. Remember that the calibration process is looking for voltages as low as 0.52V and twice that.

I also agree with forum member "David Simpson" in post #31 that we would have seen a kit to upgrade the CT/VT160 if AVO would have thought it necessary for any reason, but both he and I have not found any reference to such a kit.

I'll try to post more in a few days time when I get some free time from my vacation.

Until then you could fresh up on the two AVO patents for the Valve Testers if you wish, they can be found in one file here http://frank.pocnet.net/instruments/.../AVOpatent.pdf
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Old 5th Jul 2010, 11:18 pm   #34
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Default Re: AVO CT160. Protecting the meter.

Leaving the theory aside for one moment, I decided to record some voltage measurements using a Keithley 175 bench meter.
The valve under test is a doubled up 6080 triode, at 100Va -27Vg and 100mA.
With the meter balanced in the set zero portion of the mA/V dial, across the movement measured 1.5mV dc and 330mV ac rms. At the 100% mark on the scale I read 74mV dc and 395mV ac rms.
I also experimented with shunt diodes.
Firstly I connected two 1n4007's and a centre off c/o switch across the movement (actually the input of my op-amp) and using the switch I could shunt the movement with a diode in one direction or the other.
The results were startling. Starting from the 100% mark, in one direction the reading went up to 115%, in the other it went down to 45%. With both shunting back to back, the reading increased to 55%.
Interestingly, shunting with a silicon one way and a germanium the other, the reading went up to 90%.
I'll let you draw your own conclusions.
Perhaps another member could replicate these tests, preferably on a tester with the original movement.
Rob.
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Old 6th Jul 2010, 9:09 am   #35
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Default Re: AVO CT160. Protecting the meter.

Rob, did you test with germanium diodes back to back too?
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Old 6th Jul 2010, 9:17 am   #36
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Default Re: AVO CT160. Protecting the meter.

Quote:
across the movement (actually the input of my op-amp)
Could you clarify this please. Is there a direct DC connection between the two points?
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Old 6th Jul 2010, 11:00 am   #37
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Default Re: AVO CT160. Protecting the meter.

I do not want to answer in Rob's (glowinganode) place but I just read his meter replacement thread below.

Looking at one of Rob's (glowingonode's) earlier postings : https://www.vintage-radio.net/forum/...ighlight=ct160 where he described his meter amplifier using an OP-AMP and also looking at the original schematics there is a direct DC-path across the movement.

In the original schematics the movement is parallelled with a 10 KOhm resistor. For some meter replacement kits this resistor should be removed and for some it should be left in place - some meter OP-AMP kits have choosen to put this resistor on the PCB to get a new high tolerance resistor in place of the old one that might have been damaged when the meter is blown.

In the OP-AMP construction used the meter's internal resistance is replaced by a potentiometer that can be adjusted to fit the particular meter internal resistance of roughly 3250 Ohms.
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Old 6th Jul 2010, 8:18 pm   #38
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Default Re: AVO CT160. Protecting the meter.

Thanks Martin, no problem. As you say, I have left the original 10k shunt resistor in place, the op-amp has 3250r across the input to emulate the original movement.
I did not try with two ge diodes back to back, no reason I just didn't.
Rob.
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Old 7th Jul 2010, 11:19 pm   #39
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Default Re: AVO CT160. Protecting the meter.

Out of interest I replaced the 6080 with a 1k resistor with a diode in series.
The ac rms component across the movement was reduced to roughly the same value as the dc component, so it is the valve that gives rise to such high ac rms levels.
If you have to disable any shunt diodes whilst carrying out a mutual condance test, this leaves the movement vulnerable when it is most likely to be damaged (by the valve under test either going s/c or o/c).
I don't know why AVO didn't use a mechanical cut-out as per AVO8.
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Old 8th Jul 2010, 9:26 am   #40
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Default Re: AVO CT160. Protecting the meter.

Rob, what happens if you use another valve than the 6080 and use silicon diodes across the meter? The 6080 tube is not the best tube to perform these tests on since it has a low Ra of 250 Ohms, even AVO mentiones tubes with low Ra as problematic to measure in their testers as it is near the anode measuring resistance of 200 Ohms. AVO says that the 6080 valve then is tested under dynamic conditions and that the readings could be considerably different than from those expected, they say that the best way to measure a 6080 is to compare the results against a known good 6080 and more or less disregard the measured value as long as it is in par with the good valve.

I have just been told of the existance of a document describing how to use diodes across the meter movement, but I have not read it yet as it is on its way to me - so it should be possible according to this document. I will publish this document here with the proper acknowledgement when it has arrived to me. I might put this document in a separate thread and referring to it from this thread as I think it deserves its own thread as it is a separate document describing the diodes and also some other AVO stuff - tell me what you or any moderator thinks about this.

At the same time I will also publish a document/text on the inner workings of the AVO MKIV but which holds true for almost all of AVOs Valve Testers except for where the testers differ too much, like the bridge measuring in the C/VT160 or the VCM163 with its oscillator and measuring amplifier - but there will be much to learn from that document anyway. I'll put that document in a separate thread and refer to it from this thread.
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