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Old 30th March 2011, 06:38 PM   #1
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Join Date: Feb 2011
Location: Devon, UK
Default tone arm (re)construction

I had decided to rebuild a Lenco 75 tone arm some time ago, and bought a broken wand off ebay. Although I quickly replaced the broken stub with a carbon tube with aluminium tube overlaid, and although I turned it into a reasonable unipivot to show 'proof of principle', the headshell is a bit of a joke, looking really anachronistic today. So I bought an ADC headshell from ebay again, and have all but completed the parts to assemble, the main arm wand is carbon fibre.

Two questions:

is the mass required just a matter of adding or subtracting some with a particular cartridge in mind? (I can, & do, measure cartridge/arm resonance frequency)

how accurate (and linear) are the mass and thread bias compensators ? or could a more accurate one be constructed (does it need to be very accurate?)
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Old 30th March 2011, 07:56 PM   #2
AMV8 is offline AMV8  United Kingdom
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Join Date: Jan 2005
Location: near london
Hi
I did own a GL75 and I can confirm that on mine the weight and bias compensators were neither linear or accurate. In fact they seem to be inaccurate on most arms.
I am not sure what you mean by adjusting mass but I think the only accurate way to set the playing weight is by initially setting the arm tube level and the stylus at the correct angle and then using a balance scale under the stylus and adjusting the GL75 weight compensator untill the correct playing weight is shown on the balance scale. It does not matter if the GL75 adjustor is linear or not.
I have found that the only way to set the bias adjustment is by setting the stylus on a blank disc and adjusting the bias adjustor until the cartridge is stable in the centre of the playing area. Again I do not think it matters if the bias adjustor is linear or not. It will be difficult to set the bias properly if you do not initially set the arm level, the stylus at the correct angle and set the playing weight correctly.
I think this answers your question but ask again if this is not what you were looking for.
Don
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Old 30th March 2011, 09:12 PM   #3
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Join Date: Feb 2011
Location: Devon, UK
Hi Don,
thanks for your reply. By mass, I mean the effective mass of the arm, which, when considered along with the cartridge compliance, gives a resonance frequency (somewhere between 7Hz and 15Hz, or so). For a given cartridge, and therefore compliance, one could tune the resonance frequency by adding (or subtracting) mass. To add mass, I have some tungsten loaded putty (used in angling to replace lead shot), which has the added benefit of being good at damping.

For bias compensation, I'm wondering whether the current designs are good enough for the job. I, too, thought that setting up was possible using a blank disc (I bought a 12" video disc for this) but it appears that the bias is a product of groove friction, so needs a groove to be set up correctly. I suppose I'll have to get my oscilloscope out and set it by looking at the sine waves generated from a suitable disc.
Thanks again.
KR, Cat
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