I'm setting the bias on an amplifier, manual states 32mv at 25oc now it's cold here in the UK at the moment so is it reasonable to divide 32mv by the 25oc then times by the room tempreture and set it there?
No. Bias current must be set when transistors are at running temperature. Offset voltage is usually not so temperature dependant if DC voltage loop is well designed.
IMO, the best way to do this, is to make a first attempt to adjust. Leave the amp, say, 15~20min and check. If it went from optimal adjust again and repeat 3~4 times. If at once is stays quiet, task finished. Be sure that bias trimpot is OK (no dirty not weared, not broken, not altered its value, not corroded), else the adjust will last only as a flatulence under the water.
IMO, the best way to do this, is to make a first attempt to adjust. Leave the amp, say, 15~20min and check. If it went from optimal adjust again and repeat 3~4 times. If at once is stays quiet, task finished. Be sure that bias trimpot is OK (no dirty not weared, not broken, not altered its value, not corroded), else the adjust will last only as a flatulence under the water.
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So adding half an hour warm up into the equation, the room still isn't at the tempreture as stated in the service manual. Still problems setting it as per original post?
It's not about the temperature of the room, it's about the temperature of the transistors. They will get warm even if the room they're in is a bit on the cold side.
Hi. BUT the rom or enviroment temp will still make a differensial temp to get rid of the power. thus rom temp will stil be transistors diff temp.
Thus tansistors wil be running at a sligtly dirrerent point due to diff temp.
Any thugt abt boxing/ semi box wich should be kind of easy control temp inside by adjuting air flow?
It's usually the absolute temperature (temperature in kelvin) that matters, and that won't be too far off.
You have to keep in mind that the temperature of the amplifier's output transistors will vary dramatically under load, much more than the maybe 10°C or 20°C difference in room temperature. Carefully regulating the amp's temperature is overkill, in my opinion.
The transistors that determine the amp's quiescent bias current are thermally coupled (mounted to the same heatsink) - it's more important that they're all at the same temperature, not that they're at a specific temperature within a couple of degrees. It'd be really bad if you set the bias when the Vbe multiplier is still cold while the output transistors are hot, for example. The waiting time is there to allow the output transistors to warm up the Vbe multiplier so it's all at the same temperature.