JH turntable with new (thinner) drive band plays a semi-tone higher in pitch

[upster]

5 hours ago, upster said:

Will definitely try that. Thanks

Using the app I measured 35.25 RPM

[upster]

26 minutes ago, Grant Slack said:

yesssss. On reflection I oversimplified.

 

my calc went like this: a semitone difference is a frequency difference of 1.05946:1, so, to increase the radius of the belt midpoint by that amount = 150mm x 1.05946 = 158.9mm, which is (almost) 9mm thicker.

 

However, I didn't take account of what is going on at the smaller drive pulley with such a thick belt: chaos? 

 

Bottom line: it is more complex than I assumed, and to do it properly we would have to make assumptions about the material properties of the belt and how it behaves when wrapped around two pulleys of different diameters. Does it stretch, or scrunch, or slip? etc. Hmm, not today.

 

What I can say is that a semitone different from 33.33 rpm represents either 35.3 rpm or 31.5 rpm. That's a big change. Hence my interest in getting a proper measurement of RPM.

 

cheers

Grant

Thanks Grant - yes I measured it at 35.25 RPM.  Now this is weird as the new band seems thinner than the original. Mind you, the original did have this sort of stylus shaped profile which might mean that the internal side of the band might be thinner. Hard to say. Given AussieVintage's description of the compressing and stretching of a rubber band around the pulley, the inside (being thinner) than the outside which was quite a bit fatter than my replacement could have given an average pulley radius smaller than what the replacement gives.

Anyway. I guess I need a way to find a correct belt, or enlarge the pulley (hard to do), or slow the motor (any ideas)?

Thanks

[upster]

On 6/7/2024 at 6:28 AM, VanArn said:

The easiest solution would be to either reduce the diameter of the motor pulley or increase the diameter of the platter. Alternatively, a variable frequency ac power source would provide a means of pitch control. This would consist of an oscillator and a low power amplifier with a step-up transformer.

Thanks VanArn.  I like the 'variable frequency ac power source' idea. ATM a bit out of my skill range. 

[upster]

On 6/7/2024 at 6:51 AM, southern_tango said:

About the correct rubber belt, I have a brand new mechanism around with the original belt still around the platter and as far as I remember it is like a thick dental floss so thin I recon is the way it was design.

JH reproduces tables tend to run a bit faster as you noticed dust bugs should help because of it's drag.

Another issue is the platter itself as it is pretty light way and thin aluminium tends to resonate, what kind of mat are you using?

The original JH mat had like little lumps of what looks like EVA rubber that sticks in various spots, to separate the record from the platter ( Silcrom tables had a thicker spindle and platter and a different mat for example)

Thanks southern_tango. It still has the original 'anti-static' rubber with the little bumps.
The original belt was light grey rubber. Definitely a lot thicker than thick dental floss and I'd have thought slightly thicker than the replacement I purchased.  this should have made the TT a little slower (not faster?)

[upster]

On 6/7/2024 at 6:58 AM, VanArn said:

The original circular cross-section belt (approx. 3mm) was made from white silicon rubber.

Yes - exactly - any idea where I could get one like that?

[upster]

On 6/7/2024 at 7:49 AM, aussievintage said:

Maybe the thinner belt is sitting lower in the groove around the platter, hence spinning it faster.

The platter groove is quite wide and flat so I don't think that would be making the difference.

[aussievintage]

23 minutes ago, upster said:

Yes - exactly - any idea where I could get one like that?

 

My memory says they weren't as thick as that (3mm) and this one is 1.5mm https://www.decibelhifi.com.au/belt-for-jh-turntables/

[audiofeline]

On 05/07/2024 at 7:40 PM, VanArn said:

My memory of the JH turntable was that it ran a trifle fast as it had to contend with some heavy tracking cartridge combinations and the extra drag of a Dust Bug record cleaner. 

And I have read that some turntables were manufactured to run slightly fast so that in showroom comparisons they would sound a bit brighter and more lively when compared to turntables that ran at the correct speed. 

[audiofeline]

Further to my previous comment about making a belt from 3mm o-ring rubber, o-ring rubber can be bought in smaller and larger diameters. 

 

[upster]

23 minutes ago, audiofeline said:

Further to my previous comment about making a belt from 3mm o-ring rubber, o-ring rubber can be bought in smaller and larger diameters. 

 

Thanks Rob - may I ask what sort of vendor sells rubber o-ring rubber?

[aussievintage]

49 minutes ago, upster said:

Thanks Rob - may I ask what sort of vendor sells rubber o-ring rubber?

 

I once bought some from a car parts/spares store. 

[rocky500]

As a test could you put say a lacky on the spindle or 2 to make it slightly thicker, then place the belt on?

Would having a thicker pulley make it faster or slower?

[audiofeline]

2 hours ago, upster said:

Thanks Rob - may I ask what sort of vendor sells rubber o-ring rubber?

I've bought it from eBay.

[rocky500]

Just for testing purposes.

 

Could try bunnings for orings.

https://www.bunnings.com.au/kinetic-assorted-sizes-o-ring-kit-43-pack_p4920305

 

 

I was actually thinking those elastics for teeth braces.

 

Edited July 8, 2024 by rocky500

[Grant Slack]

One could put several #64 or #109 rubber bands from Officeworks around the platter to experiment with slowing it down.

 

cheers

Grant

[upster]

22 hours ago, rocky500 said:

Just for testing purposes.

 

Could try bunnings for orings.

https://www.bunnings.com.au/kinetic-assorted-sizes-o-ring-kit-43-pack_p4920305

 

 

I was actually thinking those elastics for teeth braces.

 

Were you thinking of these for trying different pulley/spindle sizes?

[upster]

On 8/7/2024 at 6:55 PM, audiofeline said:

And I have read that some turntables were manufactured to run slightly fast so that in showroom comparisons they would sound a bit brighter and more lively when compared to turntables that ran at the correct speed. 

The effect of this speed-up (from 33 .33 rpm to 35.25 rpm) is not the same as (say) an orchestra playing in B rather than Bb. Rather the character is markedly changed. Singer's voices sound as if they've been slightly chip-monked. (only by a little) but the character is just not there.

[e83cc]

Speed is determined by the size of the pulley and the size of the platter.

Changing belt thickness, type, colour, or gender will have no effect.

 

In theory to slow the speed you would need to DECREASE the size of the pulley.

 

I have successfully done this to my Garrard 301 pulley - using high quality 2000 grade wet and dry, water and a little oil on the pulley so its not too aggressive, I managed to trim each speed to get the speed control on the Garrard dead centre. This process was very tedious and took a few hours because you can't overshoot, so you have to go very slowly and measure regularly.

 

For the JH I don't have a picture of the pulley - but you could check if the pulley size varies, try a nylon thread like on the Well Tempered and try moving it up and down the pulley to see if you could find a spot where the speed is more accurate.

 

You could also try a thicker oil in the main bearing to put some drag on the platter.

 

More costly alternatives would be 

Add a speed controller

Buy a replacement motor kit such as origin live, that includes speed control.

 

Given that the original marketing hype on the JH touted the advantage of using a very light 12 pole motor, it would be nice to maintain originality and use a speed controller with the original.

 

A third option would be to get an engineer to make a new pulley. ( Preserve the old pulley as is ).

 

All you need to do is accurately measure the pulley and the speed error, and recalculate the correct size based on the adjustment required.

 

 

Edited July 9, 2024 by e83cc

[BioBrian]

1 hour ago, upster said:

The effect of this speed-up (from 33 .33 rpm to 35.25 rpm) is not the same as (say) an orchestra playing in B rather than Bb. Rather the character is markedly changed. Singer's voices sound as if they've been slightly chip-monked. (only by a little) but the character is just not there.

 

It's funny to see this. I've been reluctant to join in here as I gave up TTs decades ago, but this strikes a chord for me.

 

About 1970 my older brother bought a JH TT and arm, but by the time I could afford one, the TT had been superseded (at least in the Melbourne shops) by Silcron, which seemed to be designed on the same principles - extremely light platter and motor. I love the uni-pivot arm in its silicone damping fluid, but the ultra-light platter and motor is a worry.

 

I think @VanArn nailed the problem early on here, pointing out that as you add the arm the thing slows down, and with the fashionable Dust Bug used at the time, it would slow down even more. But as the arm and Dust Bug got closer to the centre pivot the relative drag decreased, so the platter would increase in speed and the pitch would rise. I have an enduring memory of playing an Elgar Symphony which has a slow movement that's so long, they had to cut it at the end of Side A and continue on Side B. Well, with my setup, Side B started about a semitone flatter than what I'd just been deeply immersed in, on Side A. That was a horrible shock!

 

A related problem that will not go away, even with CDs and digital files, is that a lot of 'artists' play sharp, as in higher in pitch. Especially violinists, and they deliberately play sharper than the piano or orchestra, just to 'be noticed', which possibly doesn't worry a lot of people, but for me as a budding professional with a developing 'perfect pitch' or whatever it's called, it caused a lot of anguish. Do I copy them and play sharp all the time, or play at the pitch of our family piano, which is tuned to the benchmark A of 440 Hz?

 

I had to do without a TT for a few years due to not having the power on, but when I got one again, I made sure it had variable speed (Yamaha, with DC motor). At last I found I could slow the platter down, and listen to Menuhin playing Paganini's 2nd Concerto at the right pitch! The orchestra sounded a bit flat, but hey it didn't feel like I had a mouthful of raw lemons, and I could focus and listen for the finer details of what he was doing.

 

So in my opinion the JH and Silcron TTs are a failed design, and it's easy to see why modern TTs are much heavier (did the terms Wow and Flutter come before or after the JH??), and presumably have motors strong enough to keep the same speed whatever you do to the platter (or however many arms you want to use at once).

 

The problem is doubly alleviated these days with the move toward truth in musicians. Finally we are blessed with artists like Isabelle Faust who play perfectly in tune, so I just select recordings that don't grate, and hopefully other buyers vote with their dollars, and the 'playing sharp' disease dies out. Can always hope.

[aussievintage]

1 hour ago, BioBrian said:

So in my opinion the JH and Silcron TTs are a failed design

 

 

A system such as used by the JH, can be brought up to speed (pun intended) by a speed controller that uses feedback from the platter's actual speed and corrects the motor speed to attain the desired setpoint.   There are modern turntables that do this.     It's probably not worth the effort though.  I have a JH (I think) on the shelf that I have never bothered to use.

Edited July 10, 2024 by aussievintage

[Grant Slack]

6 hours ago, e83cc said:

Speed is determined by the size of the pulley and the size of the platter.

Changing belt thickness, type, colour, or gender will have no effect.

 

Hi Nick, so what do you think did change?

 

Speed was right => puts on thinner belt => speed too fast.

 

cheers

Grant

 

PS not sure I agree with your words in bold above. There is such a thing as the effective diameter of a pulley with a belt wrapped around it. Belt engineering generally ignores it, because precise speed of the driven shaft is so rarely critical. A precise drive ratio is often important, but in such cases a toothed belt is always chosen.

[aussievintage]

I have been trying to think of a different way to explain belts around pulleys.

 

Can we agree that if you bend a belt around a curve, the inner surface will compress and the outer will stretch? 

 

If so then the pulley is driving against a compressed surface.  This means that a certain distance around the circumference of the pulley will actually correspond to a slightly greater distance on the belt (and so move a longer length of belt for a certain amount of rotation)  because the belt is compressed and will be actually longer when it unbends.  This happens on both the pulley and the platter, but the amount it happens is different due the different curvatures. 

 

Now make the belt thicker.  The compression and stretching on the two belt surfaces has to be greater for the same curvature due the thickness, and  the difference between metal surface and compressed rubber surface being driven, is greater for the greater curvature around the smaller pulley.  Hence the pulley now moves an even greater length of belt for the same rotation.   The platter sees the same effect, but to a lesser extent due to the lesser curvature, so there is a net gain in speed.

 

Better, or worse?   

[VanArn]

Since there is a groove machined into the platter rim to hold the drive belt in a fixed position, a belt with a larger cross-section diameter, in some circumstances, it will ride higher in that groove and hence slightly reduce the rpm of the TT.

Edited July 10, 2024 by VanArn

[e83cc]

Quote

 

Hi Nick, so what do you think did change?

 

Speed was right => puts on thinner belt => speed too fast.

 

cheers

Grant

 

 

Firstly he has never said that it ever ran on speed - he said the belt disintegrated.

 

So I checked the net and indeed the thickness of the belt can affect speed - bit of a surprise - effectively I presume the "apparent pulley platter sizes" are increased slightly.

 

Personally I run silk thread on my main TT - no stretch - 26kg platter and can adjust speed from 0-85 on both "33" and "45".

 

I see that the original belt was 1.5mm round. So he should find a belt that meets that spec - there are plenty around - JH Silicon. Decibel/LP Gear and many others.

 

He could try a thread belt ( thin ) and see where that gets him.

 

 

 

 

[upster]

On 10/7/2024 at 7:50 AM, e83cc said:

Speed is determined by the size of the pulley and the size of the platter.

Changing belt thickness, type, colour, or gender will have no effect.

 

In theory to slow the speed you would need to DECREASE the size of the pulley.

 

I have successfully done this to my Garrard 301 pulley - using high quality 2000 grade wet and dry, water and a little oil on the pulley so its not too aggressive, I managed to trim each speed to get the speed control on the Garrard dead centre. This process was very tedious and took a few hours because you can't overshoot, so you have to go very slowly and measure regularly.

 

For the JH I don't have a picture of the pulley - but you could check if the pulley size varies, try a nylon thread like on the Well Tempered and try moving it up and down the pulley to see if you could find a spot where the speed is more accurate.

 

You could also try a thicker oil in the main bearing to put some drag on the platter.

 

More costly alternatives would be 

Add a speed controller

Buy a replacement motor kit such as origin live, that includes speed control.

 

Given that the original marketing hype on the JH touted the advantage of using a very light 12 pole motor, it would be nice to maintain originality and use a speed controller with the original.

 

A third option would be to get an engineer to make a new pulley. ( Preserve the old pulley as is ).

 

All you need to do is accurately measure the pulley and the speed error, and recalculate the correct size based on the adjustment required.

 

 

Thanks for that detail.  Yes - I'm quite sure the belt would have to be thinner or I make the pulley circumference smaller.  I just can't believe that the new belt I got was thicker than the original. I guess I'll try to find a way to measure the thickness of the new belt and see if I can order something a little thinner. At $50 a pop it would be quite an expensive trial-and-error