GTA 300B SE Mk.II Power Transformer

korneluk · 17th December 2007, 01:50 PM

Folks,

I have a 300BSE Mk.II (6SN7, EL34, 300B) that instantly trips the breaker when it is turned on.

With the power switch on, I measure 1.6 ohms across the hot and neutral AC pins. There are no shorts between hot/neutral and the ground pin, and the internal fuse F1 on the PCB is not blown.

I am afraid that my power transformer primary may be shorted. Can someone ohm out their unit and confirm my measurements?

TIA,

-- josé k.
jose_korneluk AT bellsouth.net

dshortt9 · 17th December 2007, 03:03 PM

I have a power transformer from that amp. I removed it and do not remember which leads are the primary. If you can tell me which leads to check I can do so.

korneluk · 17th December 2007, 03:09 PM

The primary are four black leads (connected in parallel for 120VAC) that do not have a center tap. There is another winding that comes out the same side as the primary which has a center tap for the 5V rectifier tube filament. The ten thicker leads on the other side are all secondaries.

BTW, if you do not plan on using that tranny, I'll gladly buy it off you. Please PM offline.

Thanks and best regards,

-- josé k.
jose_korneluk AT bellsouth.net

dshortt9 · 17th December 2007, 03:41 PM

The two primaries read 2.0 ohms seperately or 1.0 ohm in parallel, right at the transformer. You probably have an extra 0.6 ohm in the power cord and connections.
My transformer has a shorted secondary which is why I removed it in the first place.

kmtang · 17th December 2007, 05:09 PM

Remove all the tubes and see if it can be powered up first.

In case it still blows the fuse, surely there's something wrong with the power transformer.

Johnny

17th December 2007, 01:50 PM	#1
korneluk diyAudio Member Join Date: Jan 2005 Location: South Florida	GTA 300B SE Mk.II Power Transformer Folks, I have a 300BSE Mk.II (6SN7, EL34, 300B) that instantly trips the breaker when it is turned on. With the power switch on, I measure 1.6 ohms across the hot and neutral AC pins. There are no shorts between hot/neutral and the ground pin, and the internal fuse F1 on the PCB is not blown. I am afraid that my power transformer primary may be shorted. Can someone ohm out their unit and confirm my measurements? TIA, -- josé k. jose_korneluk AT bellsouth.net

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17th December 2007, 03:03 PM	#2
dshortt9 diyAudio Member Join Date: Apr 2001 Location: Milwaukee, Wisconsin	I have a power transformer from that amp. I removed it and do not remember which leads are the primary. If you can tell me which leads to check I can do so.

17th December 2007, 03:09 PM	#3
korneluk diyAudio Member Join Date: Jan 2005 Location: South Florida	The primary are four black leads (connected in parallel for 120VAC) that do not have a center tap. There is another winding that comes out the same side as the primary which has a center tap for the 5V rectifier tube filament. The ten thicker leads on the other side are all secondaries. BTW, if you do not plan on using that tranny, I'll gladly buy it off you. Please PM offline. Thanks and best regards, -- josé k. jose_korneluk AT bellsouth.net

17th December 2007, 03:41 PM	#4
dshortt9 diyAudio Member Join Date: Apr 2001 Location: Milwaukee, Wisconsin	The two primaries read 2.0 ohms seperately or 1.0 ohm in parallel, right at the transformer. You probably have an extra 0.6 ohm in the power cord and connections. My transformer has a shorted secondary which is why I removed it in the first place.

17th December 2007, 05:09 PM	#5
kmtang diyAudio Member Join Date: Nov 2004 Location: Vancouver	Remove all the tubes and see if it can be powered up first. In case it still blows the fuse, surely there's something wrong with the power transformer. Johnny