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Old 19th February 2009, 09:36 PM   #1
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Default Bias Pots

Does anybody know what the difference is between an expensive multi-turn pot and a cheap one? E.g. this one: http://www.bluebellaudio.com/potsmultiturn.htm
and this one: http://www.maplin.co.uk/Module.aspx?...=2202#features

Presumably there's a reason why the expensive one is better for a bias adjuster but I can't see what that might be right now!
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Old 19th February 2009, 10:16 PM   #2
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The expensive one is 2 watts power rating, is panel mounted with a standard 1/4 inch shaft, and can be connected to multiturn counter dials etc. It is a proper multi-turn pot. Can be used for things like power supply voltages or tuning a radio or whatever, with a knob.

The less expensive one is a circuit board mounted trimmer, 1/2 Watt rating, screwdriver adjusted. Used for setting something once then forget about it.

Personally I wouldnt use either one for a bias pot, I dont think you need the resolution of a multiturn pot for that, but if I had to, I would use the cheap one, have it inside the amp, set it once and forget it. No need for a panel mounted bias pot unless you want to make a big show out of biasing your tubes.
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Old 19th February 2009, 10:16 PM   #3
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The difference is, if you set your pot the first time for too high current you have a chance to save your equipment adjusting a simple pot, but with a multi-turn one you may not have a time.

The right answer is,
1) Use a high reliability pot (I use cheap pots that were used in computer monitors to adjust parameters with knobs on front panels; they are more reliable than usual trimpots -- you may spot them there on the bottom view picture with open bottom cover: http://cgi.ebay.com/ws/eBayISAPI.dll...m=180327852085 ),
2) Calculate well ranges in which you need to adjust, so your trimpot will cover no more than may be needed,
3) Design your bias such a way so when the pot is open it causes idle current to drop,
4) Before switching on your gear for the first time, or after output tubes were swapped, turn your trimpot such a way so an idle current is minimal.
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Old 19th February 2009, 10:34 PM   #4
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Default Re: Bias Pots

Quote:
Originally posted by AmpBuilder225
Does anybody know what the difference is between an expensive multi-turn pot and a cheap one? E.g. this one: http://www.bluebellaudio.com/potsmultiturn.htm
and this one: http://www.maplin.co.uk/Module.aspx?...=2202#features

Presumably there's a reason why the expensive one is better for a bias adjuster but I can't see what that might be right now!
The expensive one is a precision wire wound multi-turn (they can get more expensive than that), and the inexpensive one is a cermet trimmer pot.

I have the inexpensive trimmer pots in both of my DIY amps with no issues so far (to set bias V and CCS current). The only pain I see with the trimmers is if you want to adjust without flipping the amp over, you will need to align the screw adj with a hole in your top panel (this assumes a chassis with the tubes sticking out of the top deck)
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Old 19th February 2009, 10:34 PM   #5
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I see, Thanks for the responses - taking it all into account and since i know accurately what current I want for the valves, I think i'll opt for an quality accurate one with small range and small price!
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