trobbins
Super Member
Fran, hi-fi OT's typically have primary winding self resonance out beyond 50kHz, and may have a sizeable shunt P-CT capacitance of circa 500pF due to interleaving. If you doubled the shunt capacitance by adding 500pF in parallel, then the simplistic resonance would reduce to about 70% (eg. ~35kHz).
Driving the 500pF primary shunt capacitance with pentode mode and about 30kohm plate resistance would give a simplistic low pass filter corner of about 11kHz. Doubling that shunt capacitance, by adding another 500pF, would lower the corner frequency to about 6kHz.
A not-so-hifi OT may have a somewhat lower shunt capacitance, and a lower self resonance.
Driving the 500pF primary shunt capacitance with pentode mode and about 30kohm plate resistance would give a simplistic low pass filter corner of about 11kHz. Doubling that shunt capacitance, by adding another 500pF, would lower the corner frequency to about 6kHz.
A not-so-hifi OT may have a somewhat lower shunt capacitance, and a lower self resonance.