First the usual amateur, stupid question, etc. apology.
Then the question - and if there isn´t a fixed rule, if it´s topology dependent or otherwise, than this pertains push pull tube amps. So I´ve built two, el84 push pull triode strapped no feedback amps. And on both I have connected the speaker minus to the star grounding point which is then grounded to the chassis. And the amps has given me both pride and joy, so they are working well, but recently in a video somebody said that you connect the output transformer minus (black) only to the speaker jack, not to ground/chassis. And as I really don´t trust myself I have to check and double check. Looking at schematics, some are marked with the ground/earth symbol, and some are not.
What´s right and wrong here, and have I done something potentially dangerous? Thankful for input.
Then the question - and if there isn´t a fixed rule, if it´s topology dependent or otherwise, than this pertains push pull tube amps. So I´ve built two, el84 push pull triode strapped no feedback amps. And on both I have connected the speaker minus to the star grounding point which is then grounded to the chassis. And the amps has given me both pride and joy, so they are working well, but recently in a video somebody said that you connect the output transformer minus (black) only to the speaker jack, not to ground/chassis. And as I really don´t trust myself I have to check and double check. Looking at schematics, some are marked with the ground/earth symbol, and some are not.
What´s right and wrong here, and have I done something potentially dangerous? Thankful for input.
You need to ensure that if the primary and secondary happen to short together that the amp output is never lethally live at hundreds of volts DC. This generally means the secondary must be earthed.
The need for good wideband coupling in an output transformer precludes the other way to ensure safety, namely using separate bobbins for primary and secondary.
The need for good wideband coupling in an output transformer precludes the other way to ensure safety, namely using separate bobbins for primary and secondary.
> in a video somebody said that you connect the output transformer minus (black) only to the speaker jack, not to ground/chassis.
Why should we listen to some un-named video?
Sure is a lot of crap in the internet.
Why should we listen to some un-named video?
Sure is a lot of crap in the internet.
Yeah, you want to earth it for safety. In a zero feedback amp, you don't have to run it back to your star ground, you can just tie to the chassis back by your speaker posts.
The only time I personally allow skipping this is when you have a parallel feed output stage where one end of the primary winding on the output transformer is already earthed. (Not that abnormal in fixed bias amplifiers and line preamps)
The only time I personally allow skipping this is when you have a parallel feed output stage where one end of the primary winding on the output transformer is already earthed. (Not that abnormal in fixed bias amplifiers and line preamps)
Remember that in the professional grade MC-3500/MI-350 amplifiers, most probably the finest tube amplifiers ever built, grounding the speaker windings is just an option, not mandatory at all.
Best regards!
Best regards!
Output transformers perform marginally better if the secondary is not grounded. If the transformers were either old, or of dubious origin i would definitely ground them.
bunkhokum:
There is no such thing as a "stupid question" when asking this great Forum for advise. 🙂
Joe.
There is no such thing as a "stupid question" when asking this great Forum for advise. 🙂
Joe.
Fully agreed to Joe, especially as can be seen that both options are advocated, and both without lack of arguments.
My personal options are: For sure I'll ground the speaker winding if a GNFB loop including the secondary is the goal, and I don't ground if either no GNFB or GNFB from a tertiary winding (as in the McIntosh MC-3500) has to be applied.
For security reasons the OT has to be designed with much more care if the speaker is not grounded.
Best regards!
My personal options are: For sure I'll ground the speaker winding if a GNFB loop including the secondary is the goal, and I don't ground if either no GNFB or GNFB from a tertiary winding (as in the McIntosh MC-3500) has to be applied.
For security reasons the OT has to be designed with much more care if the speaker is not grounded.
Best regards!
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I would wonder what the intention was for adding that in. Perhaps they had bridging in mind? Or lab use where you might want to provide your own ground reference? (based on the front mounted binding posts)