Stromberg Carlson AU 58B output transformer question

[pontiacpete]

I found this strange. I got this amp up and running and was using clips on the screw terminals to listen to it with a heavy duty 4 ohm speaker (black widow) that I use to test most of the amps in my shop. And it sounded great. Then, since I was hoping to use this as a guitar amp I went to installed a 1/4" speaker jack. So I disconnected the octal socket that has all the secondary OT connections and soldered up the jack with the 8 ohm tap and then used the 'C' black lead for the other end of the winding.
After some time of playing and it sounding really good I accidentally, (zap!) found that there was voltage on the screw lugs where the 16 ohm and 4 ohm leads were still attached and come to find out that my 8 ohm winding also has about 400v on it.

My question is how did my speaker survive that much DC? It still works and sounds fine, I would think that would have blow it.

Unless I'm missing something, my guess is that the OT insulation between the primary and secondary broke down. I finally disconnected all of the secondary and found that there was 74 ohms betw the CT red lead and all the secondary winding taps and 400vdc, except for the winding that is used for the NFB.

Here's a link to a thread that has a good picture of the schematic in case my attachment isn't easy to read.

http://music-electronics-forum.com/t10715/

[g1]

The c/black lead is not grounded, there is probably 400V there too. The feedback winding has one end grounded, thus you don't have the 400V on it.
Not sure why you would see 400V on any secondary leads, like you say, probably insulation breakdown.
Rest assured, had your speaker seen 400VDC it would be toast.

[pontiacpete]

Yes the feedback winding is the only one that doesn't have 400v on it.

I agree the speaker, most likely wouldn't survive 400v but it appears to have done so.

Time to swap in another OT.

[g1]

What I'm saying is the 400V is only there with reference to ground. If there is 400V appearing at the 8R winding and also at the C winding then there is 0VDC across the speaker itself. It did not get 400 volts across it as the voltage was appearing at both ends of the speaker.

[pontiacpete]

Ah! thank you g-one, for clearing that up for me, that's what I didn't understand.