Immortal Amplifier Mods

[Tom_Hull]

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

tom

[John]

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. 

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.

[The_Gaz]

Yes, RG knows what he's talking about, and all the mods in those articles are worthwhile.  

[Willabe]

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.    

[Geezer]

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.    

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

[The_Gaz]

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.

[mresistor]

I wonder if one could use a fusable link in the winding lines.

[bluesbear]

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

[tubeswell]

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