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Old 22nd Oct 2018, 12:14 pm   #1
Paul JD
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Default GEC AC37 volume pot.

Hi, I am working on a GEC AC37 that needs a new volume pot (R3). The original pot is a 7k that also incorporates the mains switch, the closest I can find with a suitable switch attached and that will fit is a 10k. I was thinking it might make sense to change the value of R5 that is in parallel with the pot. Currently the pot is 7k with a 6.6k across it, with a 10k pot changing R5 to about 5k should keep the same overall parallel resistance? Does this make sense?

Paul
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Old 22nd Oct 2018, 1:03 pm   #2
Boater Sam
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Default Re: GEC AC37 volume pot.

The 6.6K is not across the volume control.
A 10k pot as a direct replacement will be fine, shunt it with a 22k if you wish.
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Old 22nd Oct 2018, 1:24 pm   #3
Paul JD
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Default Re: GEC AC37 volume pot.

Thanks Sam, much appreciated. I was struggling to visualise what the 6.6k R5 was for but thinking about it again I guess it could be to safeguard against the pot going open circuit?
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Old 22nd Oct 2018, 4:37 pm   #4
'LIVEWIRE?'
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Default Re: GEC AC37 volume pot.

Maybe I'm being pedantic, but isn't R3 technically an RF gain control, rather than a volume control, per se. Surely, whatever name it is given, R3 is-in effect-in parallel with r5 via the winding L2, assuming the 'bottom' end of R5 to be earthed, that is.
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Old 22nd Oct 2018, 6:20 pm   #5
Paul JD
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Default Re: GEC AC37 volume pot.

Yes, the service sheet describes it as gain control which varies the grid bias and also acts as an aerial earth shunt.
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Old 22nd Oct 2018, 7:00 pm   #6
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Default Re: GEC AC37 volume pot.

It both increases the bias and lowers the sensitivity, quite common in old TRF sets.
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