Adding GB pot to Gold Mountain King Cobra

[oldexplorer]

What spec pot do I buy to add a full size external GB knob to my Gold Mountain King Cobra? The photo shows the stock mini pot which is adjusted for average conditions at the factory. I hope to buy the pot from www.maplin.co.uk if such a thing is available.

Once I have fitted it, how do I ground balance the machine correctly (sorry if that's a silly question)

Thanks

 

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