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debk 29th December 2010 11:08 AM

Tubelab SE Output Transformer Grounding?
Should one lead from the secondary of the output transformer be grounded or should it be just directly connected to the speaker terminals? I have searched for this, and there seems to be some controversy as to which is best.



Ty_Bower 29th December 2010 12:09 PM

Amplifiers with no global feedback can leave the output transformer secondary floating. They will still work just fine. However, for safety reasons, you should ground the common (or zero ohm) end of the winding. Remember the primary winding has 300 volts DC on it. If that voltage should happen to get to the secondary (either through inter-winding capacitance or a short) and you touch the speaker wiring, you might get a nasty jolt.

tomchr 30th December 2010 04:02 PM

I think the grounding is built into the Tubelab SE board. Double-check the board and schematic.

For safety reasons, one side of the secondary should be grounded to chassis ground.


Tubelab_com 1st January 2011 12:10 AM


I think the grounding is built into the Tubelab SE board.
OPT secondary grounding is built into the Simple SE and the Simple P-P through the feedback path. Of course if the user choses not to connect the secondary to the PC board that connection must be made externally.

The Tubelab SE does not use global or any other type of feedback so the secondary of the OPT is not connected to the PC board. The OPT secondary should be grounded through the off board wiring. This is a serious safety issue. The voltages used it most tube equipment can be considered lethal and people have been killed by them.

I have seen an OPT with an internal short put 300 volts on the speaker leads. The amp in question functioned normally except for the HV on the speaker. It was a guitar amp and the user could have touched the guitar and the speaker jack at the same time frying him.

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