• 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.

OT primary impedance for 6K6s

One old rule at the patent office:

If a new patent application presents a "new" product, and that "new" product idea is obvious to someone 'well versed in the art', the "new" product was not patentable.

Either the patent office no longer has people 'well versed in the art', or they have changed the rule.
 
> they have changed the rule.

Not explicitly. But if it is not drop-dead a copy-cat, the inclination is to demand the full patent fee, issue the paper, and let the lawyers argue it out when infringement is claimed.