Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Erik Aa on February 19, 2022, 12:47:13 pm

Title: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 19, 2022, 12:47:13 pm

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!

Title: Re: Filament fuse blows only when all tubes are installed
Post by: sluckey on February 19, 2022, 01:00:46 pm

Wrong size fuse.

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

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 19, 2022, 01:04:31 pm

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

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Latole on February 19, 2022, 01:35:04 pm

If tubes need 2.12 A a 2A fuse will blow

I'll put 3 A fast blow

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 19, 2022, 02:07:57 pm

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

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Latole on February 19, 2022, 02:12:04 pm

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 ?

 

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 19, 2022, 02:15:29 pm

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

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Latole on February 19, 2022, 02:16:26 pm

Quote from: Erik Aa on February 19, 2022, 02:15:29 pm

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 !

Title: Re: Filament fuse blows only when all tubes are installed
Post by: sluckey on February 19, 2022, 02:30:18 pm

Quote from: Erik Aa on February 19, 2022, 01:04:31 pm

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.   :icon_biggrin:

Title: Re: Filament fuse blows only when all tubes are installed
Post by: pdf64 on February 19, 2022, 02:34:06 pm

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.

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 19, 2022, 03:22:25 pm

Here are some pictures of the amp.

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 19, 2022, 03:23:18 pm

One more picture.

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 19, 2022, 03:32:29 pm

Close up of circuit.

Title: Re: Filament fuse blows only when all tubes are installed
Post by: uki on February 19, 2022, 03:57:35 pm

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

Title: Re: Filament fuse blows only when all tubes are installed
Post by: Erik Aa on February 20, 2022, 11:17:10 am

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