Got a GR 1559-B Microphone Calibrator

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Wanted one of these for a while. It's pretty well tuned up and I've put up a page on it here- General Radio Corp. Collectors Area
The 1559 is a few items down, under the hummer.

What I'm really hoping is that somebody might have a manual later than the 1965 manual available online. It lacks any calibration info. Though most of it is easy, there's one dial vs. pot setting that's quite difficult to get right.

Thanks & enjoy,
CH
 
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I have one of those as well. What we will need is adaptors for modern microphones and something to interact where the vintage GR slm would be. Also you will need a high voltage signal source (30V+ I think) to get into the target range.

There really is no calibration involved since its based on physical constants like the properties of air. You do need a barometer like the ones that come with B&K pistonphones.

I believe that in essence you measure the drive into the reciprocal transducer (microphone) and measure the output from the other microphone. The reverse the process and derive the absolute sensitivity from those measurements. The GR allows you to add a third microphone to the mix and get a sensitivity from the interactions of the other two. But after that blurry description their instructions left me cold, which is why mine sat for 10 years. I think their instructions are geared towards someone who is already very familiar with the process and the measurements.

The whole reciprocity process is more than a little bewildering to read through. The mechanical calculator built in is pretty trick and the 6 position switch that also controls the clutches is typical GR with its exquisite mechanical execution.
 
Wow, that's three! Mine, yours and one guy in the Yahoo GR groups forum. You're right, just getting microphone sensitivity doesn't require any calibration, assuming the pot is tweaked correctly, something nobody should have touched since it was built. What does need calibration are the three pots that adjust the meter to match the PZT SPL, and the position of the smaller dial in relation to the log pot. That's what's giving me fits, as I had to do a full disassembly to get the mechanism working. The big polystyrene current sampling cap also needs to be correct, though I don't know how close. Spec is 0.25% and mine is about 0.5% out.

I have two Fortin mercury barometers but need to refill them. No question about local pressure with those. I also have a lathe, so if you need any adapters, let me know. It's in the garage, so nothing happens until warm weather.

What year is your manual?
 
My scan doesn't have that, so please see if yours says anything about adjusting the three pots for the meter cal, or especially anything about setting the small dial. See if the resistance chart still has the error I talk about in my web page. Also, the date!

I've found pretty much everything online that exists, save for anything NIST might have buried. Their site of offline because of the shutdown.

Thanks,
Conrad
 
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What I have is a good scan but its as thin on details as yours most likely. Where should I upload it to share it?

As far as I can tell mine is almost unused. I can check various things if that helps. Looking at the schematic there are adjustments for meter sensitivity and a trim across a cap but nothing else specific. The microphone sensitivity knob has a clutch I think and has continuous rotation. The various rings are engaged depending on the measurement phase. . .

Next step is to figure out how to interconnect it with external instruments. You need to be able to insert a voltage in series with the mike capsule. The PZT mikes are provided for. Others are not so easy. More research needed

I'll try to make some sketches of the adapters needed. Unfortunately they say its not usable above 8 KHz and needs corrections above 2 KHz. And the corrections they provide are not necessarily valid for other microphones or applications. If I can get it running I can compare with my several pistonphones to validate its operation. Not only are adapters needed for inserting microphones adapters will be needed to connect it to usable instruments.
 
A measurement would be fantastic! I'm not sure what switch position would be best, but the idea is to set the small dial to something like center (-60 dB). The position of the large dial doesn't matter. Then measure wiper to each end of the pot. I lost my clocking when I serviced it- shoulda put a mark!

I have an ancient Beat Frequency Oscillator that goes to 50 V output, and the 1311 bridge oscillator they say to keep on the 30 V range. Both work well with the unit. I'm guessing if somebody didn't have an old tube oscillator, maybe a modern unit with a 2:1 or 3:1 transformer could work. I can't remember the output of the old HP 200 series, but that might get close as well.

Various things will work as a detector. The old HP3400 rms meters are good. It's probably desirable to make up low capacitance cables no longer than needed.

What the date on your manual? You could try to email it (don't know what the limit is). Address is here- Personal History


Thanks,
Conrad
 
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I think the frequency limit is partially because of the mic diameter. When I read about mics for loudspeaker testing, they say anything above .5" diameter will be limited on high frequency performance. I was surprised to see that 1" mics are really 0.936" +/- 0.002". Kinda like a 2x4.

I have a question about couplers and cavity volume. I know the volume has to be accurately known for the reciprocity measurement, but when just using it (or a regular calibrator/pistonphone) as a source, how critical is the volume. IOW, do adapters have to put the mic face in just the right spot, or is the cavity SPL valid over a range?
 
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First- the numbers you are looking for. When the knob is at -54 the resistance from the wiper (measured at the wiper) to the far end of the pot is 504 Ohms. At -60 its 203 Ohms. While its open I lubes the contact surfaces with the Progold. After all these years its appropriate. Still this looks unused.

The HF issues are twofold. First the difference between pressure, diffuse field and free field. The B&K curves are enough to make you pretty careful abut any accuracy on high frequencies especially for 1" mikes. Second issue is the size of the cavity. Above 2 KHz?? the wavelengths start to become such that the pressure is not uniform. By 8 KHz if you plot it you will see some serious fluctuations. Same as headphone measurements. The cavity is about the same size as a headphone. For HF calibrations NIST fills the cavity with hydrogen. I'm not up for that if I could figure out how. There is something online about The US Navy doing some stuff with the cavity but I forgot what. That was also in the 1960's.

I'm building an electrostatic actuator drive setup and it needs a 30V source so a little tweaking of the unit should be enough for this as well. Both a 100V supply and a "100W" amp module from eBay for not much. That's a project for next week.
 
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Both this and the pistonphone need pretty precise cavities to work correctly. The pistonphone has a fine wire in the cavity to adjust its volume to that that final .1 dB. It a larger cavity its less critical. .1 dB is roughly the same as 1% so a 1% change in volume is significant. It gets messier when you need to figure in the equivalent volume of the microphone. Those are published for B&K and similar. Not for the more generic microphones.
 
Much better- everything agrees now. I don't understand they they offer two ways of exciting the cavity, the reciprocal mic and the PZT cylinder. They both seem to rely on the meter calibration and ultimately have about the same accuracy, but that Bonk was a clever fellow, so maybe I'm missing something with the calibrated mic method. Anyway, thanks for the measurements! Now I can stop obsessing over this thing, at least a little.
 
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Much better- everything agrees now. I don't understand they they offer two ways of exciting the cavity, the reciprocal mic and the PZT cylinder.

You hit the reciprocity thing. The point is to measure the loss path two ways and then extrapolate from those measurements and the physical constants the sensitivity of each transducer. This article may make it easier https://www.bksv.com/media/doc/bv0051.pdf and it shows the more standard version with two transducers and three separate measurements and a computer to do the calculations. The meter on the box is secondary to measuring the sensitivity. Its more for gauging the drive level. The sensitivity comes from the matrix.
 
Seems to be working well, though I'd need a calibrated mic to really prove it. I've updated the page and included an Excel spreadsheet that has some useful info.

Another question- I notice that the standard GR mics end up flush or even slightly under the edge of the mic port. The sliding clamp can't do much other than keep a mic from falling out. Was there any thin gasket or o-ring at the bottom of the port? I also had to shim the standoffs that hold the sliding clamp as it was binding severely. Don't know if the plastic shrank a bit, or if somebody left out some washers or if something else is going on.

Thanks,
CH
 
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The .936 is from the Western Electric 640AA which was the standard at the time. The B&K 1" is the same physical dimension.
If you can find a picture of the earlier GR mikes they are larger and the cylinder gets smaller at the right place for the clamp. I'll post a picture that should explain this later.
If the .936 is tight there may be an adapter sleeve in their already. It should be almost airtight.
 
I'm probably not making myself clear, not an infrequent event. My GR mics, 1560-xx are the same size, 0.936". They neck down as described for the clamp, but they aren't proud of the port, so no real clamping is done. The 1559-B port fits them fine and snug diameter-wise, within the tolerances of decent machining. There are actually three tiny ball plungers in the wall of the port to insure snugness. AFAIK, that's the basic size of the 1559-B port, and it can't accept anything larger. No doubt various adapters exist for smaller mics, but I haven't seen them.

What I'm wondering is two-fold. The best fitting mic can still shift a few thousandths, especially if it has a gooseneck on it, and this affects the results slightly. The length of the port is such that the back end of the mic will be flush or even slightly recessed. That means the sliding retainer isn't providing guaranteed seating. It seems if there were some very thin gasket or o-ring, it would take care of both issues. OTOH, one doesn't want to change the cavity volume, which is why I wondered if GR originally used any kind of gasket.
 
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I'll be home tomorrow. I had a GR mike but misplaced it in the last move. I'll take some measurements of mine and we can compare.
The Omnical has an O ring seal. The older calibrator I have has the ball detent. I'll collect some details on all of those.
 
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