08-11-2015, 06:36 PM
(05-11-2015, 10:21 AM)Crackle Wrote: I rechecked the heater current and it had dropped slightly to around 120mA but the radio was still working well even though it did take a long time to warm up.
I should think it would, Mike!
120mA @ 240V gives an overall heater chain resistance of 2000Ω.
For 150mA @ 240V it should be 1600Ω. Either someone has replaced one or more components in the heater chain with the wrong value components or the barretter is showing its age.
I would suggest a thorough check of the dropper components and replacement of any wrong value parts or, if they are already correct, consider a modification to increase the current.
Moving the heater adjustment to the 200V tap will give an effective 1667Ω - which is probably close enough, being ~100Ω higher than required for your 235V mains supply - so would be a quick way out providing that the HT tap is separate and can be left on the 240V tap.
Recheck the heater current after any modifications, of course!
