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Couple of basic Qs on theory of operations.
1. Since OPT secondaries are not grounded, how does damping work?
2. How to calculate damping factor with OPTs.
The OPT secondary is most definitely grounded. The 0 Ohm or G tap on the OPT is ground, and should be tied to circuit ground.
The damping factor for a SET amp is typically low. Damping factor is a function of the output impedance of the amplifier (I think it is the ratio of the speaker+cable impedance over the amplifier output impedance). No feedback SET amps have a high output impedance. Feedback PP amps have a lower output impedance, and solid state amps have very low output impedance. Tube amps, particularily SETs, typically have a worst time with low impedance speakers (4 ohms), since this lowers the damping factor, whereas 16 ohm speakers are a much better match. But this is of little concern if not driving a bass driver (since it is indicative on the control on the driver), and should not be an issue at all with horn loaded midranges.
Do not worry about damping factor for your application and do not overthink the design. If you are concerned about specifications, no-feedback, Class A SET amplifiers suck. No one builds a SET amplifier to achieve high damping factor, low THD, or low S/N ratio. They build them for the sound that a minimalist circuit using very linear output tubes into an output transformer that is operating in the linear portion of the BH curve at very low power can provide.
David