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PostPosted: Fri Sep 23, 2022 10:07 pm 
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Joined: Tue Mar 27, 2018 8:44 pm
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Hi All,

Building a 18 watt TMB Mojotone head. I looked at a bunch of schematics for the 18 watters and some come with the HT fuse and others w/o. I have only blown a HT fuse once because of a tube short in a Marshall. What else does this fuse protect against? It seems simple to add one, between the transformer and ground. Any help would be appreciated. I'll post some pics when it's done. I have buddy building a curly maple cabinet for it.

Thanks!


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PostPosted: Sat Sep 24, 2022 11:40 am 
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Location: Albany, NY
Lower wattage amps generally don't have an HT fuse, and none of the three 18-watters I've built have had them. But from what I've read, there's absolutely no downside to having one in the circuit. So go with your gut.

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PostPosted: Sat Sep 24, 2022 12:52 pm 
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Joined: Tue Mar 27, 2018 8:44 pm
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Hi duhvoodooman,

Thanks for the info. The more specific question is, will the HT fuse protect against not plugging in a speaker. I know the output transformer goes into an infinite "loop" if you don't plug in a speaker which will cause an issue with the output transformer or other parts. Does this increase the draw from the power transformer possibly triggering the HT fuse to blow? The 18 watt amps that have the HT fuse are the Tube Amp Depot one and the Marshall 1974x, I don't know about the vintage Marshall 18 watter. Since I may not keep the amp and my buddy that is building the cabinet might, I was thinking if it protected against not plugging the speaker in it might be worth it. I think I learned my lesson on the Champ when I just burned up a cap and 2 resistors.

Thanks


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PostPosted: Sat Sep 24, 2022 2:33 pm 
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My understanding is that the main thing that the HT fuse protects against is a shorted output tube, which is a common cause of blown transformers. But I would think it would also afford some protection against the OT drawing too much current in a no-load condition.

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“My favorite programming language is SOLDER” - Bob Pease (RIP)

My Website * My Musical Gear * My DIY Pedals: Pg.1 - Pg.2


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PostPosted: Sun Sep 25, 2022 11:44 am 
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Thanks again!! It's easy to add if I mount it in the inside of the chassis like the Brit amps. :lol:


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