Newly acquired a (variable speeds) Goldring 88. It needed a new idler tyre plus general lube and fettle. It's run for a few hours now so thought abt a re-check of the speed using the accompanying Goldring strobe disc and an LED table lamp.
Then I remembered Gridwatch. A single page giving real-time power grid data including exact mains frequency for nerds like me. True, engineers try to keep the total number of cycles per day accurate but the frequency throughout does vary fractionally. The needle on the dial showed more deflection than I'm used to though I've seen it more off than that. The frequency was 49.85 Hz. If you translate that to concert pitch 440Hz that would play at 438.7Hz. I reckon a guitar tuner would pick that up but I wouldn't. Point is, if I see the strobe lines move, whatever the mains is doing its caused by the deck!
If you are setting the speed using a strobe you're locking it to the mains frequency at that time. Best to glance at what you're locking it to, or possibly just get a life....
Then I remembered Gridwatch. A single page giving real-time power grid data including exact mains frequency for nerds like me. True, engineers try to keep the total number of cycles per day accurate but the frequency throughout does vary fractionally. The needle on the dial showed more deflection than I'm used to though I've seen it more off than that. The frequency was 49.85 Hz. If you translate that to concert pitch 440Hz that would play at 438.7Hz. I reckon a guitar tuner would pick that up but I wouldn't. Point is, if I see the strobe lines move, whatever the mains is doing its caused by the deck!
If you are setting the speed using a strobe you're locking it to the mains frequency at that time. Best to glance at what you're locking it to, or possibly just get a life....
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Here is the insane solution to your questionnement:
Just realised the last part I wrote is wrong!
If you lock the turntable to the mains when the mains is slow the strobe bars will be stationary at that slow speed, but when the mains is bang-on strobe wiould still be stationary at the correct speed. No, it doesn't matter when you set the speed, the turntable will always follow the mains speed and will be nominally correct.
Pitch change with mains variation part is correct tho.
Re Elvee's post, I wondered about using the 7th harmonic of the mains (very easy to generate) fed into a guitar tuner looking for an 'F', but that arrangement would have the tuner looking for 349.2 Hz and not 350Hz.
If you lock the turntable to the mains when the mains is slow the strobe bars will be stationary at that slow speed, but when the mains is bang-on strobe wiould still be stationary at the correct speed. No, it doesn't matter when you set the speed, the turntable will always follow the mains speed and will be nominally correct.
Pitch change with mains variation part is correct tho.
Re Elvee's post, I wondered about using the 7th harmonic of the mains (very easy to generate) fed into a guitar tuner looking for an 'F', but that arrangement would have the tuner looking for 349.2 Hz and not 350Hz.
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Shirley, that's why one should use:
1. a battery-powered strobe, and
2. a motor speed controller that generates the 50Hz / 67.5Hz frequencies for the motor.