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Old 21st Jul 2019, 11:16 pm   #1
AndyGilham
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Default Avometer Model 7 46-range and 50-range

A question for the sublimely knowledgeable....! (You know who you are!!)

The model 7 went from - 46-range instrument, to a 50-range instrument..... But what were those additional 4 ranges added.....? I count the same for both! What am I missing??

Andy
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Old 22nd Jul 2019, 4:27 pm   #2
The Philpott
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Default Re: Avometer Model 7 46-range and 50-range

Dunno!..Does the PF Power Factor facility account for one range?...or more than one?
Dave
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Old 22nd Jul 2019, 4:43 pm   #3
AndyGilham
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Default Re: Avometer Model 7 46-range and 50-range

I did consider that Dave, but the 50-range was available prior to the PF sockets being fitted.
PMM........ we need your help!

Andy
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Old 22nd Jul 2019, 10:05 pm   #4
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Default Re: Avometer Model 7 46-range and 50-range

I might be able to look at some of mine as I do have some early 7's and see what the ranges are. Some are packed away, so this won't be done quickly. You'll probably get your answer from someone else sooner.
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Old 26th Jul 2019, 6:38 pm   #5
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Default Re: Avometer Model 7 46-range and 50-range

This might help,

Quoted from ACWEECO's own literature:

50-Range Model 7

This improved instrument, introduced in 1942, has the following
modifications:-
Power and Decibel ranges now of 5,000 ohms impedance and scaled
0-2 W., also -15 to +16Db., 0 = 50mW
The 10V. and 100V. ranges can be used as a 200 mW. range.
Frequency response improved. B.S> 1st grade limits to 2 Kc. and
suitable for audio-frequency to 10 Kc.
The instrument is now provided with a moulded case and stout push on
test prods.

It is also mentioned that the Power and Decibel Extension Unit is only
compatible with the 50 Range.


Ian
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Old 26th Jul 2019, 10:58 pm   #6
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Default Re: Avometer Model 7 46-range and 50-range

Andy and everyone,

I think that the answer is probably in the question "What is a discrete range"?

The first attachment is an advertisement for the Model 7 from "Electrical Times" September 1939 in which this model is claimed to have 46 ranges.

Refering to the little table at the left of the page there are six lines of text each of which might be considered a group of ranges. I have anotated this with the number of ranges in each group.
The 'mainstream' ranges are:
22 voltage ranges;
16 current ranges;
5 resistance ranges;
making 43 ranges of voltage, current and resistance.

There are then what may be considered three supplementary ranges - Capacity, Audio frequency Power, and decibels, making 46 ranges in total.

The capacitance range is clearly additional to the volts, amps ohms ranges, although whether it can be claimed to be a "direct" reading is open to debate since an external AC supply is needed. This is also true of the two higher resistance ranges, but let us allow these for this argument.

This is a rather strange interpretation in that there are in fact 3 power ranges although only one has its own knob setting, the other two using 10V (AC) and 100V (AC) ranges. The instructions for measuring power (attachment 3) explain how these measurements should be made and the meter has to present a load of 5000 ohms for the 2W range (1) [Power and dB switch position], 500 ohms for the 200mW in 500 ohms for range (2) [10V AC range] and 50,000 ohms for the 200mW range (3) [100V AC range]. On the 10V range (range 2 for power) the meter has a sensitivity of 50 ohms per volt giving the required 500 ohms and for power range 3, the sensitivity is 500 ohms per volt making a load of 50,000 ohms, again as required. On range 1 however, the load impedance needs to be 5000 ohms and this is not already available as part of the normal voltage ranges. For this reason, the special "dB and Power" range with a load of 5000 ohms has to be provided. So, it seems to the 1939 marketing mind-set the provision to read power and dB constitutes just one extra range.

The provision to read decibels, referred to 50 mW, is just an alternative scale for the power measurements described above, so if my reasoning is correct, it is only the provision of the power scale which justifies the claim to the 46th range.

[Superscope - your information may be very relevent here; what is the source of the 1942 reference to a "New Improved 50 Range Model 7 please? I wonder if the modification was just extra scale markings to allow the exploitation of the 10 and 100V AC ranges to give extra pseudo ranges? Would you agree that no circuit changes would have been needed? This is quite intriguing because ACWEECO continued to run advertisements featuring the Model 7 throughout the War years although very few civilian organisations and even fewer individuals would have had Government clearance to buy these meters at the time, and then probably only if they were engaged in essential War Work.]

By the late 1940s or early 1950s, it would seem that someone noticed that there were in fact three ranges of power measurement, even if two of them were ordinary AC volts ranges with values read from the "Power" scale. [See also paragraph above]. If that argument worked for the power ranges, it was equally valid for the Decibels ranges, or not, depending on your point of view. This reinterpretation gives us four extra ranges, making 50 altogether.

You might think that providing for Power Factor and Wattage measurements and should constitute quite a few more ranges but that is beyond the scope of this question.

It could also be argued that the 50mV and 100mV ranges are not really valid as part of the count because they are in fact just the 1mA and 2mA ranges read off the voltage scale. So should the Model 7 really be considered a 44 range meter?

I must admit to having never having actually used the audio power, decibel or Power factor ranges; while their inclusion is ingenious, their implementation is quite convoluted, time consuming and requires a concentrated awareness of the circuit set-up and parameters.

I believe the late Steve Chellingworth (SPCH) tried them all to improve his understanding of the Model 7 design and he was a great admirer of the engineering capability of Donald MacAdie and his son Hugh S MacAdie. Mr. MacAdie senior attributed the Model 7 design to H. S. MacAdie, so it seems it is him we should thank for the circuitry, although possibly not the count of the ranges. H. S. was with the company when the claimed number of ranges changed, but I have no way of knowing if he initiated the claim or approved of it.
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File Type: pdf Model 7 Mk I Block Magnet W and dB instructions.pdf (56.2 KB, 42 views)
File Type: pdf P24 Circuit.pdf (83.3 KB, 48 views)
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Old 27th Jul 2019, 12:39 am   #7
AndyGilham
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Default Re: Avometer Model 7 46-range and 50-range

PMM
I guess we all knew you would have the answer! Thank you for the concise details of those "wafty" ranges.
It does feel very much like the DC Avominor change from 10 range to 13. No change to the schematic, but clever use of the 6mA connection to double voltage ranges.
I do wonder if these extra ranges were "discovered" by users, and that information fed back to ACWEECO at the time, or the clever engineering bods worked it out. Either way, it's a great marketing plow, an improved instrument without additional engineering costs!

Andy
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Old 27th Jul 2019, 10:38 am   #8
The Philpott
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Default Re: Avometer Model 7 46-range and 50-range

Post No.7- Andy, the second para. was exactly my thoughts on the discovery of extra ranges on avominors. In addition to the DC example you mention, either an employee or a customer must have discovered late in the production run of the AC Universal Avominor that a Low Resistance range was hiding in the design.

It simply needing a modified scale plate and an extra lead to short two of the terminals. The main criteria for accurate readings is that all four wander plugs must make good contact in the meter (not always the case now unless thoroughly cleaned!) and the short lead is removed afterwards to avoid draining the cell.

A quirk of the wander-plug system as opposed to selector switches. I do like the system, mainly because it cuts down on the curse of moving parts- the death of so many machines.

Dave
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Old 27th Jul 2019, 3:02 pm   #9
pmmunro
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Default Re: Avometer Model 7 46-range and 50-range

I think most of the credit for solving this mystery should go to Ian (Superscope). His post, which I read only after I had written mine set me thinking, so I have done some more research and found further information.

Ian's post suggests that there really was in fact just one power range on the 46-range Avometer, with a corresponding decibel range. I have consulted a 1939 Model 7, its instruction plate and a sales brochure from the late 1930s. Please see the first attachment which is the range table for the 46 range Model 7.

Only one power range, 0 to 4 watts, having a 4000 ohm internal resistance is mentioned. The corresponding decibel range is given in the brochure as -10 to +15 dB which covers 5 to 1500 mW with respect to the 50mW reference level. +15dB is the highest figure on the dB scale on the scaleplate. 15 dB is about 64% of full scale and in fact the decibel scale arc has radial lines, but no figures, going to 100% which would be 4 W and +19 dB, which needs some explanation. I have run a spreadsheet on these values and this range is in effect an alternating voltage scale with a FSD of 126.5 volts. I have only one Model 7 of this age to refer to so it might suggest that some figures were missing - the other were red and are now badly faded, but the instruction plate and brochure both state that the maximum value is +15dB.

This verifies the information in Ian's post and refutes my aspertions implying that the New Improved 50-range Model 7 was in fact no different to the 46-range version.

The other comments on the verifiable facts about the 50-range version remain valid. The basic power range has an internal resistance increased from 4000 to 5000 ohms but the maximum power is cut to just 2W. The decibel range then became -15 to +16 wrt 50mW.

The additional ranges, which are, as already established, really just the 10 and 100 V (AC) ranges but read of another scale with a range of -25 to +6 dB wrt 50mW - except that I can't find such a scale on any of the Model 7 meters which I have to hand. Perhaps someone can point out what I'm missing here as otherwise these decibel scales are not of much apparent use.

I think Andy is probably correct that the additional power and decibel ranges could result from someone just "playing around", in an informed way to seek out all the facilities of the Model 7 and it really would be nice to know if it was a customer or a member of staff. Alternatively, the higher range could have become necessary with the increased ra values of newer output valves when pentodes replaced triodes. Realusing that this could be done

I think three questions remain:

1 Why is the decibel scale above 1.5W not acknowledged by dB values on the 46-range meter?

2. Where is the -25 to +6dB scale on the 50-range Model 7?

3. For whom were the power and dB scales intended?

In regard to question 3, would many radio set owners have been interested? It seems more likely that power/dB would interest public address engineers but 2 W is not much in this application, even by the standards of the day.

PMM
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Old 9th Aug 2019, 2:49 pm   #10
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Default Re: Avometer Model 7 46-range and 50-range

I consulted a 1940 edition of the ACWEECO book "Radio Servicing Simplified" in the hope of finding more information on how the Model 7 was intended to be used to measure decibels and power but it only makes a brief reference to these ranges with no further explanation. It does however refer to the Model 7 as having 50 ranges, so the change must have been before or during 1940.

PMM.
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Old 12th Aug 2019, 12:09 pm   #11
AndyGilham
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Default Re: Avometer Model 7 46-range and 50-range

PMM,
It was the information in the 'Radio Servicing Simplified' book from 1940 that originally got me asking this very question...! I noticed the comment in there, and the dates did not tie up with the mark 2, so i wanted to understand the range additions.

Andy
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Old 13th Aug 2019, 7:33 pm   #12
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Default Re: Avometer Model 7 46-range and 50-range

PMM

Just seen your posts!

It was that very Book I took the quote from on post #5, however my copy is
the 10th Edition from 1945.
I don't have the 1940 copy which I believe is the 7th Edition.

I can only imagine that the 10th Edition has a Typo or the 50 Range although
planned earlier, didn't actually make it out of the ACWEECO Door until 1942!

Ian
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