Hi Peter,

Thanks for the advice. There's no particular reason for using Tufnol,
except that the original (poor and broken) bobbin around the core was
made from Tufnol. I was about to order some Tufnol tonight, but after
hearing your comments I might not. I had also considered using PVC or
acrylic sheet because you can buy cement which dissolves the plastic
and makes a strong bond, but both are more expensive than Tufnol. Is
SRBP easier to glue than SRBF?

The bobbin will need to fit a core with a 4" x 2.5" cross section, have
at least a 4" length available for the winding and be no more than
4.75" in overall length. If you have anything suitable or know where I
might buy one, please let me know. I'm also in the UK (Shropshire).

I'm winding a 240 V - 415 V autotransformer for a phase convertor. The
core is a shell type.

Best wishes,

Chris