guitar amp ht fuse

i have just restored a nice ampeg r12 guitar amp that had a burnt out secondary ht winding.

the transformer has been sent for rewinding to transformer equipment ltd. great work at less than a new transformer. totally recommend them for transformer repairs

now i am thinking it could be useful to put a fuse on the secondary, at least on the same ht tap that failed before

wondering the correct place to put the fuse. two fuses one on each side would probably be the safest option, but since the amp is cathode biased the ht winding is not shared by other taps, placing a single fuse between ht centre tap and ground could be enough  ? this is actually like the classic Marshall JTM45 , and that amp is fixed bias instead. I have read in this case a single fuse from ht ct to ground is not entirely safe. would be great to hear your thoughts

If you put it before the first cap you will probably burn them when you turn on the amp.  The charge of he cap draw a lot of current. it is almost a short circuit when the cap is empty. I think between the first and second cap is a typical place.

isn't better placing the fuse on the AC side of the circuit ?

beatnik said:

isn't better placing the fuse on the AC side of the circuit ?

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Yes. A lot of guitar amps used to have a fuse after the rectifier before the first filter cap. Most "modern" tube guitar amps have the HT fuse or on the AC side of the circuit.

single fuse between HT centre tap and ground would be enough ?

if I understood correctly, this method is safe if one side of the rectifier gets shorted, but when BOTH sections of the rectifier are shorted, this still creates harm to the power transformer. but probably is very unlikely that both sections of the rectifier tube fail at the same time ?

beatnik said:

single fuse between HT centre tap and ground would be enough ?

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If the rectifier diodes fail short it won't protect the power transformer. If that happens the primary fuse should blow so how you fuse the HT winding won't probably make much difference.

when I received the amp the 5Y3 tube rectifier had one section shorted and unfortunately the mains fuse didn't save the HT secondary from blowing up. so I don't really think I can rely on the mains fuse for protecting the HT secondary