Filament fuse blows only when all tubes are installed

[Erik Aa]

Hello. I have acquired a vintage EKO mod Evc 10 tube amp. It has two eec83 tubes and two el84 tubes. When I bring it up with my variac through a current limiter, the filament fuse (1a) blows. The fuse does not blow when I bring it up with the preamp tubes and one of the el84s. I can bring it up with either el84 tube, so it is likely not a bad tube. It is only when both el84s are in the amp that the fuse blows. Can anyone please help? Is the fuse wrong (1a glass wire fuse) or is it something else? Thanks!

[sluckey]

Wrong size fuse.

Each ECC83 filament draws .3A and each EL84 filament draws .76A. That's a total of 2.12A.

[Erik Aa]

Thank you so much, Sluckey. What would you recommend? 2a slow fuse? Or something else? Thanks again.

[Latole]

If tubes need 2.12 A a 2A fuse will blow

I'll put 3 A fast blow

[Erik Aa]

Thank you Latole. I will try this then. Best regards.

[Latole]

Do you have the schematic ?
I never saw a fuse on heater on old amps.
I never work on these Italian amp It is original ?

 

[Erik Aa]

There is no schematic. I have vacuumed the internet for it. But I do have some pictures of the amp and the circuit.

[Latole]

There is no schematic. I have vacuumed the internet for it. But I do have some pictures of the amp and the circuit.

We like pictures !

[sluckey]

Thank you so much, Sluckey. What would you recommend? 2a slow fuse? Or something else? Thanks again.

I would use a 4A. Maybe even a 5A. If it was my amp I'd just put a jumper wire across the fuse holder.   

[pdf64]

Assuming UL rated fuse types, I suggest 3A antisurge / time delay / sloblo, as there will be an initial current surge at power up. Due to cold filaments having a lower resistance.
For IEC rated fuse types, T2.5A.

[Erik Aa]

Here are some pictures of the amp.

[Erik Aa]

One more picture.

[Erik Aa]

Close up of circuit.

[uki]

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

[Erik Aa]

Thanks everybody for your inputs. Very helpful indeed. Regards.