Bulb Replacement

Hi,
I want to change some bulbs on an old Rotel receiver but on looking the bulbs are wired directly. Anyway, I am interested in changing them for LED’s but don’t know which resistor to use. I’ve had a look at some 5mm LED’s and they are 3v.
According to the service manual, the setup shows 8v and 150mA. I’d be grateful if some could tell me what value resistor to use.
Thanks
 
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Check the supply voltage type before calculating anything. Most incandescent bulbs were powered from a separate AC winding on the power transformer. LEDs need DC so you likely need to add a small bridge rectifier and an electrolytic cap of about 1000uF/25V. The DC will be higher, at about 11V when loaded so that has to factored into the supply voltage for calculations

Note that the important parameter here is current and the safe limit for standard 5mm LEDs is about 20mA or 0.02A. See this video and following one if you like detailed explanations with visuals: YouTube
 
The voltage source for the lamps may be AC. Need to be careful that peak reverse voltage on the LED does not degrade it if the source is AC. If the source is AC, use a rectifier diode in series with the LED to block the reverse voltage. A cap across the LED will reduce the stroboscopic behavior of the LED as it goes on & off 60 times a second.