Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: jackplug1 on February 25, 2022, 01:36:01 pm

Title: Filament transformer fuse specification advice needed
Post by: jackplug1 on February 25, 2022, 01:36:01 pm

In my amp build I have a separate filamant transformer to supply the heaters, what fuse should I choose to protect the transformer and valves
I supplies two 12ax7 and one 6v6gt so 1.05a at 6.3v  so only 6.6watt.
it is a 12va transformer operating at 250V should I opt for a 250v 50ma fuse or should I take surges into account and go higher?
I can't find any lower than 50ma.

Title: Re: Filament transformer fuse specification advice needed
Post by: uki on February 25, 2022, 01:53:08 pm

This link have solid info !!

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

Title: Re: Filament transformer fuse specification advice needed
Post by: jackplug1 on February 25, 2022, 04:02:48 pm

btw - I have seen that but I'm refering to a fuse on the primary side, mains input not the secondary winding

Title: Re: Filament transformer fuse specification advice needed
Post by: sluckey on February 25, 2022, 04:17:51 pm

I would use 1/10^th amp fast.

Title: Re: Filament transformer fuse specification advice needed
Post by: PRR on February 25, 2022, 04:59:07 pm

I have seen a very small transformer not hold (unloaded) on less than a 1/4A fuse at 117V, due to cheap design and excess reactance current. So there is a bit of suck it and see.

Fusing small radio parts is not a precision job. What's the worst can happen? A 5A fuse will blow before the carpet is set on fire. I'm not inclined to "fuse for the transformer" because if the transformer is sick, why protect it? A 12VA tranny won't make a lot of smoke or flame.

(I just got a 10-port USB charger which feels SO cheap that I am afraid to plug it in again. Maybe if it was a nosehair trimmer I would risk it, since it would be in my hand in use; a USB charger is sucking while we are sleeping.)

Title: Re: Filament transformer fuse specification advice needed
Post by: trobbins on February 25, 2022, 06:53:43 pm

Can you measure the primary current with suitable accuracy and safety?