• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Shorted secondary windings of transformer

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Unfortunatelly I shorted the secondary windings of my power transformer. I was attempting to solve a hum issue and thought I was connecting the transformer shield to ground but instead I connected a tap of the B+ winding to ground by mistake. The transformer is a Hammond 300BX with a 400-50-0-400 winding. This caused both the 50V tap and the center tap to be grounded. After about 1 min after powering on the amp, the fast blowing 3 A fuse blew( I live in europe 220V and my amp has 2x 300BX, one for each channel).

I'm trying to understand what damage I may have caused and I dont understand why the fuse didn't blow immediatelly. The power supply is fairly standard: transformer, rectifier, cap, choke, cap... the rectifier is a 83 mercury rectifier if that matters.

hope you can help me find some damage or suggest what to test before powering on again, thanks
Probably no damage in one minute. But check the resistance from the 50V tap to ground against the other transformer - if it's less, you have shorted turns.

Do you have a separate fuse for each transformer? That would give better protection...

Protecting transformers fully is NOT easy. For example, if the 50V winding has 20 Ohms resistance, it will draw 2.5A when shorted - that's 125 VA or about a half amp at 240V... hardly a noticeable increase! Worse yet, the wire resistance goes up when it heats, so the current DROPS from there. The loss of bias (if you used it) would cause the tubes to draw lots of current, and that may blow the fuse.
I' going to remove all tubes and test the secondary voltages.

Tom, I was using the same fuse for both channels. I will add separate fuses if I manage to get the amp working again, good idea. The amp uses cathode bias which was probably upset, I think your theory is correct. It wasn't the shorted secondary that drew enough current to blow the fuse it may e.g. have been the output tubes because of wrong bias.

Will test voltages and let you know, how it goes.
Tested the resistances of the windings and one of the transformers seems to be broken. Resistance of 400 to CT is 10 ohms but the other side is about 50 ohms. The other transformer shows about 50 ohms on both sides of the center tap. Will test voltage tomorrow - need a new fuse... And yes was using the same fuse for both channels, good idea to have separate fuses for each channel.

Tom, i think your theory is right, bias got upset, amp uses cathode bias.

Thanks for the help. Hope the output tubes are ok.
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