A question for the technically minded, will an Output Transformer designed for a deluxe reverb (in this case a Hammond 1760H) work in a Princeton Reverb Reissue? My primary concern is that the primary impedence might be wrong? Please advise.
The Princeton Reverb and Deluxe Reverb used approximately the same power supply voltage, and so had the same maximum output tube plate voltage swing available. From the equation Power = Voltage^2 / Impedance, if voltage stays constant and impedance is lowered, power is increased.
You can calculate peak output power from a peak voltage swing and the OT primary impedance. Since the amp is Class AB, use plate-to-plate impedance divided by 4, as this is what each side of the output stage will see during those peaks. Peak power (of a sine wave) = 2 * RMS power.
Assume peak plate voltage swing is 270v. Hammond's replacement transformers are 8.5kΩ for the Princeton Reverb and 6.6kΩ for the Deluxe Reverb. So that's 8.5kΩ / 4 = 2125Ω for the PR and 6.6kΩ / 4 = 1650Ω for the DR.
Princeton: Peak Power = 270v^2 / 2125Ω = 34.3w, and RMS Power = 34.3w / 2 = ~17.2 watts
Deluxe: Peak Power = 270v^2 / 1650Ω = 34.3w, and RMS Power = ~44.2w / 2 = ~22.1 watts
By changing just the OT primary impedance, the Deluxe Reverb can make more output power from the same tubes and the same supply voltage.
(Though changing OT primary impedance also changes the peak 6V6 plate current, and Fender switched to a long-tail inverter to better drive the 6V6's to those peaks.)
Thanks for that in depth explanation! I think the phase inverter in my prri has been modified too. Either way, having an OT with 6.6kohms will work? Thanks again, my RAM is full of accounting garbage so takes a bit for this kinda stuff to filter down.