PT HT fusing mod question

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PT HT fusing mod question

Post by Beano69 » Sat Aug 27, 2022 12:43 am

Hello.Newbie question. I'm part way through a JTM-45 build and in need of some advice please. Though I've happily worked away on my own amps for years, this is my first complete build - but it's far from being completed. I would love to stick to original specs, but with my goal of a JTM that's as versatile and fail safe as possible I've included a few mods. This may end up being the only amp I keep and I don't want to be shelling out on expensive tubes and transformers because I've made dumb mistakes.

I chose to incorporate an Immortal Amp / Valve Wizard mod and to fuse both AC sides of the PT HT winding to protect the PT in case of a rectifier fault, instead of just using the CT fuse.( I've also put in protection diodes at the rectifier socket ) I found the best place to mount the fuse holders was on the side of the chassis, and I've read of others doing this on JTM's, so I went ahead, mounted them and soldered it all up. Then I had a DOH moment. I don't have a dedicated bias winding on my PT - output stage bias supply was going to be tapped via Pin 6 of the rectifier socket - after the fuse.

This is where my lack of knowledge and experience comes in. I don't understand what exactly will happen with a rectifier fault. If the rectifier fails will it affect the whole GZ34, blowing both fuses, cutting off the high voltage altogether or will there be a chance that it only causes one side of the HT fusing to blow? I don't want a situation where only the PT side supplying the bias circuit for the output stage blows, I lose bias and the power tubes run away on me ( even though it would be half-wave rectified AC ).

I need to understand if this is a possibility. If it is, I figured that I might be able to tap bias supply on the PT side, before the fuse and run it from there? - but this raises the problem of it no longer being part of a twisted pair with resulting interference issues.

Any help you can give me would be much appreciated!

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