Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on July 14, 2019, 12:56:13 am
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Hi guys, Got a Selmer Little Giant in for a service...
As per the schematic there is no grounding of the speraker.
https://el34world.com/charts/Schematics/files/Selmer/Selmer_littlegiant_mkiii.pdf
For any safety reason, should it be grounded?
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The speaker itself doesn't need to be grounded. The basket/frame and magnet are all electrically separate from the coil, and the coil wire is insulated, so there's not really any way you can get an electric shock from it not being grounded.
Grounding of (one side of) the OT secondary winding is important if you have a global NFB source from the secondary (like the MkI little giant has), because you want the same (ground) reference point for where the NFB is inserted into the amp's signal path as you do for the NFB source. (But in that case, one side of the secondary winding is grounded anyway, which is connected to the coil)
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Just ground it. I believe it is generally required. *IF* the OT insulation breaks-down, you have full B+ on the speaker terminals which are usually exposed. If the voice-coil has warped and rubbed, maybe on the speaker frame too. If the speaker winding returns to B-, an OT breakdown will quickly burn-up something and kill power, maybe before the user takes a fatal shock.
It was moderately common for a ground inside the OT to OT frame. Test each speaker lead to chassis.
Other than safety, NFB function, and occasional instability from a floating winding-- no, the speaker side does not HAVE to be grounded to work. In bigger audio the secondary is often floated. This can be more stable when both in and out lines run hundreds of feet together.