Various sage advice on fusing.

Mark · Post by **Mark** » Mon Jun 17, 2019 7:18 am

I was looking at ways to protect an AC-30 from destroying itself.

There have been a few opinions out there on the subject.

Greg Fryer who did work on the Brian May AC-30's has a 500mA fuse on the centre tap of the power transformer secondary and a 500mA HT fuse. Fryer also has fuses on each leg of 6.3vac heater windings. Fryer also has seperate cathode bias resistors for each pair of valves and each pair is fused to allow the amp to keep running when a pair blows. That seems like a good idea.

https://fryerguitars.files.wordpress.co ... -stage.jpg

https://fryerguitars.files.wordpress.co ... er-amp.jpg

The next one is Merlin Blencowe's Valve Wizard site. Merlin stated fuses are best in ac paths and not DC paths. Thus Merlin has a fuse from either side of the power transformer secondary going to the diodes. Merlin is not in favour of putting fuses on the 6.3vac supply due to the inrush current at start up.

http://www.valvewizard.co.uk/fuses.html

Last but not least is R.G. Keen's articles in Premier Guitar. R.G. has fuses on each leg of the diodes as does Merlin Blenecowe. R.G. liked the inline DC supply fuse to protect the output valves as well as a fuse on the 5vac rectifier tapping, a fuse on the 6.3vac tapping too.

https://www.premierguitar.com/articles/ ... ash-pt-2-1

My thoughts on the matter for what it is worth is. I like Merlin's fuses after power transformer secondaries and prior to the plates of the rectifier valve. I also like Fryer's idea of having seperate fuses for the output stage, though I would probably omit the HT fuse. I can't say I've heard enough talk about issues with the 5vac rectifier heater supply to include a fuse, the same goes with the 6.3vac heaters. The issues I've seen with 6.3vac heaters is in relation to audio power valves using an octal socket. I have seen HT (B+) on pin 3 make it's way to pin 2 (the heaters), the heaters didn't have a centre tap and used 100 ohm resistors, these were blown apart in spectacular form. I think the better fix would be to use ceramic sockets.