Series one 104 EL34 blowing Mains Fuse

Discussion - Series One amplifiers. The New #1.
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Posts: 4
Joined: Fri Apr 14, 2017 8:44 am

Fri Dec 15, 2017 4:05 pm

Hello I have the BIAS set to on CON1 to 140mA/460VDC, it sounds very good. Valves are new.
But i have the problem that the Mains-Fuse (T2A) blow after playing 2-2 1/2 hours in a silent time.
Is this normal? Please can you help me?
Greetings from Germany,

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Posts: 1273
Joined: Wed Jan 29, 2014 6:42 pm
Location: Virginia, United States

Fri Dec 15, 2017 7:26 pm

I don't own one nor have I see a schematic, so I might be wrong about this....but you might have the bias set way to hot.

Based on a plate pf 460V and assuming you want to run ~70% of max dissapation, you should have around 35-38mA per tube.

The issue might be that CON1 is not showing you the bias voltage across all four tubes. Instead, it's more likely that it's showing you the bias voltage across one push- pull pair. Meaning that you are running the tubes at around 130% of max dissipation. It probably sounds awesome, but'll blow fuses.

Easiest way to find out (short of finding a schematic...good luck) is to measure the bias current at the tube pins.
WARNING: Tube amps contain LETHAL voltages, even long after they have been unplugged. If you are not familiar with how to safety work on high voltage circuits, then PLEASE don't die...take it to a tech.

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