I'm sorry, but the 6414 is NOT "slightly lower Mu version of 12AT7 with similar specs". It has 2/3 the mu and 2/3 the plate resistance, takes 50% more heater current, is rated for no more than 2/3 the maximum voltage and 2/3 the maximum plate dissipation. The maximum of 200 plate volts is the killer, it is NOT suitable for use in the Paramour where the shunt regulator has 350v on the plate.

And before someone else points this out ... yes the 12AT7 is rated for 300 max volts. I think that's OK since the shunt reg does not see peaks above the regulated voltage - but 17% over is a far cry from 75% over voltage! And I want to drop the Paramount regulated voltage to 300 volts in the next revision.