Making my own strobe disc from first principles

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N8Nagel

Addicted Member
Good afternoon fine folks,

I decided that I was going to finally get at and do a little something that I've been meaning to do for a while. I want to make a CAD template for a strobe disc (actually a strobe ring) for a Dual 1229/1229Q but with a twist - adding a 78 RPM track in between the 33 and 45 RPM tracks. However, I don't have anything to start with so I had to figure this out. My reasoning was as follows:

f=60Hz obviously
I know that "33" is really 33-1/3 RPM so
ω = 33-1/3 RPM or 33-1/3 * 360 / 60 degrees/second = 200 deg/s
(200 deg/sec )/(60 Hz) = 3-1/3 degrees per event
360/(3-1/3) = 108 lines

likewise
360/(4-1/2) = 80 lines

then
360/7.8 = ~46.15 lines?

now this is irrelevant for my current purpose but I got curious
3600/(16-2/3) = 216 lines - well of course that worked dummy, it's exactly half the speed of 33-1/3.

however I downloaded a random strobe disc and counted the lines on the 78 track and it appears to have 92, OK, that's twice 46 so that makes sense I guess the NE-2 would brighten both on the "positive" and "negative" peak of the AC line? so really f=120Hz. So does that mean that 78 speed isn't exactly 78 RPM but is really 78.26 RPM? (doing the math backwards)

And then what happens in 50Hz countries? I never really thought about this before but at least 45 RPM doesn't work with an even number of lines on a strobe disc for 50 Hz like it does for 60 Hz, and the closest approximation for 78 RPM isn't going to be 78.26 as above. Does that mean that in 50Hz countries using a typical NE-2 strobe with a strobe disc is going to mean your turntable speed is going to be ever so slightly off unless you're playing at 16 or 33 because those actually work exactly for both line frequencies?
 
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Also, I now am going to have to try the strobe app on my phone at 120Hz not 60Hz because it didn't work well enough to use a strobe disc when I tried it before.
 
I made this, along with a couple standard strobe discs. Is this a useful thing? I haven't seen anything like it but I thought it might be helpful for guys archiving really old discs. What range would be most useful, if this is useful at all?
 

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Here's my bog standard strobe disc. Any suggestions would be appreciated. I don't have a good light source to use this so if someone felt like printing this out and testing it I'd really appreciate it. I made all the tracks 50/50 light/dark if there is a better ratio I'd be happy to change to make better. Next step is what I originally set out to do if this one works well.

Edit: I made this one to print on 8.5x11 and for some reason it's not printing to PDF correctly. I usually deal with drafting type sizes so I don't have a premade page setup for this and I think it's time for me to quit for the night so I'll come back to this later. You get the idea though.
 

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N8Nagel said:
I made this, along with a couple standard strobe discs. Is this a useful thing? I haven't seen anything like it but I thought it might be helpful for guys archiving really old discs. What range would be most useful, if this is useful at all?
Click to expand...

KAB sells something similar in concept with a range from 70 to 90rpm. It is definitely useful to have a range of speeds around 78, although I know that some of the early records I have were recorded at speeds down to 70 or even lower. While there are fewer at higher speeds, Columbia and Edison records were nominally recorded at 80rpm and some records are even faster. Most early Victors tend to be around 75 or 76.

Bill
 
Bill H_NJ said:
KAB sells something similar in concept with a range from 70 to 90rpm. It is definitely useful to have a range of speeds around 78, although I know that some of the early records I have were recorded at speeds down to 70 or even lower. While there are fewer at higher speeds, Columbia and Edison records were nominally recorded at 80rpm and some records are even faster. Most early Victors tend to be around 75 or 76.

Bill
Click to expand...

I'm gonna have to get myself an actual strobe light to test this, if it works though I can take the concept and make more that cover a higher or lower speed range if necessary. I really only have a couple albums of 78s but I just knocked this together to warm up a little before I pull the platter on the 1229 and start working on my actual project. I've been using AutoCAD for something like 25 years now, 20 of it regularly and professionally, but mostly in an architectural context so I almost never use polar arrays, I figured this would be a skill builder for me while I'm making something useful.

Edit: looks like you're referring to the KAB "speedstrobe" which I was previously unaware of, but that looks like something I may have to look into. Based on the speeds listed on the disc it appears that that is in fact a 120Hz strobe so matches everything I'm working on but presumably quartz locked not tied to mains frequency so that's pretty cool.

Edit2: it looks like the "speedstrobe" disc jumps every other speed that I have on my disc. I just started at 78.26 RPM and added or subtracted one mark/rev and made it that way,. I could easily edit that design to make 2 mark, 3 mark etc, jumps if that's useful, or keep it the way it is with the absolute minimum speed increments given a 120Hz strobe and make several discs to cover wider speed ranges. My intent is you could print these on standard printer paper and take them to Staples etc. and have them laminated and then cut them out and use them on your turntable given that you have a "60Hz" (really 120Hz) strobe light to use with them.

Edit3: for the 78 disc it might be a cool idea to make it a double sided disc. But what would be the desired speed range? As I've mentioned, I only have a handful of 78s so my immediate interest is just locking on to design speed at this time.
 
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Thought: if there is any interest in this if I end up with designs I like I could possibly have them silkscreened on aluminum or stainless steel discs, I have connections to do that. Could do custom sizes like 12", label size, etc. in that case. I made the 78 disc 10" because a) that's the size of a 78 and b) it can be printed on 11x17 paper and the normal one 8" for no reason other than it could be then printed on 8.5x11 paper with a little bit to trim off, and it seems like a good usable size. Although maybe it would be better to make it 7" so you could file it with your 45s?
 
This is a cool endeavor. Thanks for the post.
When you mentioned 2 sided printing my guess it would be pretty tough to actually keep the center holes aligned. Printers vary a lot and when you flip the paper you just doubled any offset there was originally

Gary at TVOM used to sell a nice large size strobe disc. I believe it was a Rec-O-Kut.
 
revox-b77 said:
This is a cool endeavor. Thanks for the post.
When you mentioned 2 sided printing my guess it would be pretty tough to actually keep the center holes aligned. Printers vary a lot and when you flip the paper you just doubled any offset there was originally

Gary at TVOM used to sell a nice large size strobe disc. I believe it was a Rec-O-Kut.
Click to expand...

yeah I will have to experiment with that. I also need to get myself a proper strobe light (one of my turntables is an AR XA so the only way I have to check the speed on it now is by an app on my phone). I was just thinking if I made the 78 disc double sided you could then have all the speeds say 60-81RPM printed on it so you could cover a wide range of vintage stuff but only have two items to file, a regular 4-speed disc and a 78 disc. I would have to ask my annunciator guy (I design fire alarms for paying work, that's why I can get stuff silkscreened) about alignment if I were to do something that didn't involve paper and lamination :) I have no idea what his process is, it might need to be hand positioned.
 
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