Millett Mighty Midget - blowing fuses

[lordearl]

I've just finished assembling the PCB and chassis for a 6T10 Pete Millett Might Midget build and the fuse keeps blowing when powering on.

It's a decent pop as well (you can see the damage to the fuses in the attached pictures).

I tried a few sets of tubes but the problem remains, so I assume there is an error in my circuitry.

The circuit and more info is here on the brilliant @pmillett site:

http://www.pmillett.com/midget.htm

What would be the best testing points to determine the error when the circuit can't even power on?

 

[huggygood]

It looks like a short circuit, have you checked the orientation of your diode bridge and your diodes?
Have you checked that nothing is touching the bottom of the pcb?
Have you tried without the tubes?

 

[lordearl]

I just popped the lid to inspect the damage, varistor rz1 is totally blown!

I can't see it in the circuit (attached), what might be causing this?

 

[Tom Bavis]

Did you change the varistor for 240V operation? Should be 280-300V, not 150. Transformer obviously changes, as well.

 

[lordearl]

Of course, yes it's 240V line voltage here in Australia so it must be the problem!

I don't have a replacement varistor rated for higher voltage but Pete Millett sent me a note saying the amp can run without it as it is there for transients like lightning strikes etc.

So to test the amp I could just snip out the damaged varistor and power on without it or will it need to be replaced with a wire link/jumper lead?

 

[Tom Bavis]

Just cut it out.

 

[astouffer]

The varistor has effectively become a jumper.

 

[lordearl]

I cut it out, replaced the fuse and hey presto we have music. Straight out of the gates it is a fantastic sound with Fostex full range speakers!

Thanks for the assistance - a good lesson learned here is to go through every item in the Bill of Materials and ensure you are aware of exactly what the component does!!