Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Tom_Hull on March 30, 2012, 03:28:01 pm
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hi fresh start mentioned this subject ..
should I or we use this method to protect the power transformer ???
http://www.premierguitar.com/Magazine/Issue/2008/Aug/The_Immortal_Amp_Mods_Pt_2.aspx (http://www.premierguitar.com/Magazine/Issue/2008/Aug/The_Immortal_Amp_Mods_Pt_2.aspx)
tom
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I am laying out my new project now, and incorporating the 1 fuse per winding, plus the fuse that feeds the B+ rail. I am figuring on using a 1A slo-blo in all 3 but haven't really done math on it. My thought is that that should be large enough to not blow willy-nilly, but small enough to blow quickly in event of an actual problem. If I create more problems for myself than I think it's worth I'll let you know. :laugh:
This is one of those things I've gone back and forth on. On one hand I think of all the amps out there (a million maybe?) that have never had the fuses with no problems. On the other hand I think of that $80 PT I'd rather not have fried if a tube melts or whatever.
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Yes, RG knows what he's talking about, and all the mods in those articles are worthwhile.
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I've read if the PT secondary wind has 2 wires then 1 fuse is fine, but if it's a 3 wire wind (HT with CT, heaters with CT) it should have 2 fuses, 1 in each leg. :dontknow:
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I've read if the PT secondary wind has 2 wires then 1 fuse is fine, but if it's a 3 wire wind (HT with CT, heaters with CT) it should have 2 fuses, 1 in each leg. :dontknow:
That is correct....look at the "paths" the current can flow...if only one leg of a CT'd 2ndary (3 wire) is fused, there is still a path thru the other leg and the CT. If the CT is the only thing fused, there is still a path thru the "legs" for an over-current situation.
On a non-CT 2ndary (2 wire), only 1x leg is fused, because if the fuse is opened, there is no other path for the current to flow.
G
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That's really interesting because, Merlin/Valve Wizard, an advocate of secondary fusing, doesn't make this point clear on his site or book. I think he assumes that designs using a bridge rectifier won't be center-tapped, however in many amps, such as Marshalls, they are. Of course, his diagrams don't show a CT for the bridge recto, but I've already many people state that 'bridge rectifiers only need a secondary fuse in one leg'. If I understand Geezer correctly, then power supplies with bridge rectifiers only need a secondary fuse in one leg if there is no center-tap.
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I wonder if one could use a fusable link in the winding lines.
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I had a 5Y3 short in a Marshall clone which took out both the cap can and the PT. I think those mods are worthwhile!
Dave
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I put the rectifier protection diodes in all my tube-rectified amps and a HT fuse in all my amps.
But I haven't got my head around how to put heater fuses in whilst keeping the lead dress looking tidy