29-07-2015, 02:44 PM
(This post was last modified: 29-07-2015, 02:46 PM by Radio Fixer.)
Well one way would be to find an old transformer, of enough VA, without pitch or any encapsulation and pull the laminations out of it (carefully noting how they go and keeping the sides in order, they are coated to prevent eddy currents). Hopefully you will be lucky and the heater windings will be on the outside of the former (that is usually the case).
Say there are a couple of heater windings one 6.3 and the other 5V. Unwind them both noting the number of turns for one of them. This will give you the turns / volt of that transformer (normally around 5). Then rewind with suitable new wire (enough for the current) bringing out taps for 1.5, 2, 4 etc. Just bring out a loop and carry on winding.
You would need to devise a method of terminating them or take them direct to a switch or better IMHO sockets and have a fly plug for selection.
For varying mains voltage, supply the primary from a Varaic with an accurate meter on the heater winding in use.
No need to worry about mains surges and spikes that's how they are in radios and they are pretty robust devices (why they still work 80 years on).
It might occur to you that you could use a 6.3V transformer and just drop that voltage with the Variac, its possible if the winding has enough current capability but more dangerous than selecting a voltage on the secondary side.
Gary
Say there are a couple of heater windings one 6.3 and the other 5V. Unwind them both noting the number of turns for one of them. This will give you the turns / volt of that transformer (normally around 5). Then rewind with suitable new wire (enough for the current) bringing out taps for 1.5, 2, 4 etc. Just bring out a loop and carry on winding.
You would need to devise a method of terminating them or take them direct to a switch or better IMHO sockets and have a fly plug for selection.
For varying mains voltage, supply the primary from a Varaic with an accurate meter on the heater winding in use.
No need to worry about mains surges and spikes that's how they are in radios and they are pretty robust devices (why they still work 80 years on).
It might occur to you that you could use a 6.3V transformer and just drop that voltage with the Variac, its possible if the winding has enough current capability but more dangerous than selecting a voltage on the secondary side.
Gary
