• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

To ground or not to ground the OPT secondary, that is the question

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
The McIntosh MC75's OT has the following secondary connections: COM-4-8-16 and a separate winding for the feedback which has one end of the winding internally grounded. There is the option to install/remove a link to ground/float the secondary. Using the link the COM-tap is grounded.
Now iff it works, grounding the 4Ohm tap and using the com/8Ohm would seem like a nice idea. But does it really work that way? Do you get a balanced signal between com and 8Ohm them? What about the 16Ohm tap? Will it really be symmetric to 4OHM=GND? What happens to the primary impedance, does it see what's going on on the secondary in this case?

Keep in mind that the MC outputs are really wound COM-4-9-16...even though they are labeled for 8 ohms...

Chris
 
Hi everyone, sorry to revive this old thread but I did not see the following talked about: What about using a bleeder resistor to drain any B+ charge that is coupled to the secondary, while still leaving the secondary without direct connection to ground?

MxduQon.png
 
For Hi Fi use: What about a short from the primary to the secondary. How much transient current can the multiple uF B+ capacitor apply to that (perhaps too small of a wattage rating) "safety" resistor. Will it burn out?

I recommend grounding one of the secondary leads: Common, 4, 8, or 16 taps. As noted earlier, some ground the 4 Ohm tap, and so can use cathode negative feedback on a push pull amp. Single ended amps can have the Common lead grounded, and can still use cathode negative feedback.

Just remember that if you have Ground on the one hand, do not put the other hand to anything that has a voltage that is referenced to Ground.

Safety first, or your music hearing days may go away when you are electrocuted.

Guitar Amps present other grounding problems, bad venues, Microphones, Instruments, etc. I can not make bad situations safe. Just beware of your particular equipment and surroundings on the day you perform.
 
Last edited:
swich401 - some output transformers are designed for, and have performance measured using, a particular test connection setup - which typically grounds the OPT core and a particular tap of the secondary - as that provides a defined set of stray capacitance levels for the windings.

In a safety related sense, resistively earthed power supplies are typically accompanied with a voltage sensing relay across the resistor, to shut down the equipment if a fault to earth occurs - ie. one fault can occur without possible damage.
 
Having an output transformer secondary grounded to the internal ground of the amplifier common (like the B+ return, and the common connection of the input connector) is one thing.

But then we still have 2-wire AC power systems, and we have 3-wire AC power systems.

Some of the 3-wire AC power systems Do connect the 3-wire 'Earth' ground to the amp B+ return and input connector common return.

Some of the 3-wire AC power systems do Not connect the 3-wire 'Earth' ground to the amp B+ return and input connector common return.

"Grounds are Commonly Misunderstood" . . . me
 
Folks,

In my current amp design, the secondary of the output transformer (Edcor CXSE25-8-5k) is floating. This isn't a problem per se, but I'm not a fan of allowing the secondary to charge up to B+ through the inter-winding cap between the primary and secondary.

Should the secondary be grounded at the black speaker terminal or should I leave it floating? If it should be grounded, I take that it should be to the chassis ground?

~Tom

If the OPT develops a short from primary to secondary, I would much rather that the amp blows a fuse than kills the user. Therefore, I ground the OPT to the chassis along with the mains protective earth connection.

Tom

This message brought to you by eight years of careful consideration ;)
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.