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You can try a slow blow, that would probably fix it, or install a ICL, like a CL-140 from mouser or an Amtherm also from mouser.
For a quick test, remove C1 and see if your fuse blows. Or install a smallish 22 to 100 ohm resistor before C1. If that works and you have enough fuses, try a 10 ohm and you won't have to adjust R1 or R2.

Thanks, I will get an ICL, as the slow blow fuse hasn't really fixed it.

Ametherm have quite a good website that explains their usage. They seem to suggest using a SL22 10008 (10 ohm) in series with the transformer primary, whereas the CL-140 example circuit shows it after the bridge rectifier. I assume I is best to have it on the primary?

That's where I put them. Try a 10 ohm resistor in the primary first, maybe 2 watts or so, just to see if the fuse blows, so it only needs to be on a couple of seconds. If it still blows try a 22 and work down. Or it might be better to start with a 22 or 33 to save on fuses.

I tried a 23 ohm in the primary, and a SL22 10008 (10 ohm) ICL and changed the 500mA fuse for a slow blow, the fuse doesn't blow now, but the main power switch for the house trips, not the RCD in the fuse box, and so does the circuit breaker I am using which has a 30mA trip current - they go immediately at switch on. Is my earth connection wrong?

Any ideas what I am doing wrong?

If the ICL, a 500mA slow blow doesn't and a 15A house breaker does, I'd say you have a serious wiring oops or a bum transformer

It is wired as the schematic shows, no wiring mistakes. How would I test the transformer?

I have tried tapping the terminals of the mains plug to a 9v battery, and the LED pulses on and off, although the transformer does make a very faint crackle sound.

I think I may have found the problem.... I have just looked up the part number of my transformer, and it is actually an autotransformer! I assume now that because of my earth connection, I am essentially connecting the primary to ground which is causing the problem?


Just tried it without the earth, and it is now working perfectly!

Now I have the field coil power sorted. What do I need for the voice coil? It measures 2.9 ohm, I have a 1626 darling amp with a 4 ohm output, will that be ok?

That would be good for a first try