04-06-2012, 09:30 PM
Many pre-war radios had mains energised speakers, so if the speaker is a permanent magnet one, even if it didn't have a date on it, if it looked to be original, it's a safe bet that set is post war. If the dial has station markings, anything that has the Third programme on is post-1948. I believe that it wasn't until after the war that they could make larger permanent magnets with sufficient magnetism to power the larger speakers.
Many pre-war sets with Mains energised speakers have had them replaced due to the field coil burning out. The field coil consists of thousands of turns of fine wire (about 48SWG) and as it doubles as the smoothing choke carries all the HT. If for any reason (eg leaky caps), excess HT is drawn, the field coil can't cope and acts as a fuse! They can be re-wound at a price, but many restorers, or even service engineers when the sets were in normal daily use, prefer the easier, cheaper and quicker solution of fitting a PM speaker and putting a high wattage resistor across the smoothing/reservoir caps to substitute for the field winding.
Many pre-war sets with Mains energised speakers have had them replaced due to the field coil burning out. The field coil consists of thousands of turns of fine wire (about 48SWG) and as it doubles as the smoothing choke carries all the HT. If for any reason (eg leaky caps), excess HT is drawn, the field coil can't cope and acts as a fuse! They can be re-wound at a price, but many restorers, or even service engineers when the sets were in normal daily use, prefer the easier, cheaper and quicker solution of fitting a PM speaker and putting a high wattage resistor across the smoothing/reservoir caps to substitute for the field winding.
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
