09-27-2011, 10:27 PM
Hi,
According to RCA receiving tube manual RC-13, 1937, the load impedance of a 41 tube operating at a plate voltage of about 180 volts, where your 40-190 is at, should ideally be about 9,000 ohms. A push-pull 41 operation would be about twice that or about 18k ohms. A Hammond universal type push-pull center tapped primary audio output transformer gives you taps to select the pimary and secondary impedances you wish to match up to. See the Hammond catalog on the Radio Daze site. A Hammond 125A at 3 watts or a 125B at 5 watts should work for you. The 40-190 is rated at 2 watts audio output power per the schematics. You can measure the speaker voice coil DC resistance with a ohmeter to determine it's approximate impedance with the secondary of the audio output transformer disconnected. Multiply the ohmeter reading by 1.25 to get the impedance. This is then the secondary impedance you wish to match. Use the universal impedance chart supplied to match to the pimary and secondary impedances ideally needed for good match. I personally would go with the 125B for current handling headroom.
Good luck, John
According to RCA receiving tube manual RC-13, 1937, the load impedance of a 41 tube operating at a plate voltage of about 180 volts, where your 40-190 is at, should ideally be about 9,000 ohms. A push-pull 41 operation would be about twice that or about 18k ohms. A Hammond universal type push-pull center tapped primary audio output transformer gives you taps to select the pimary and secondary impedances you wish to match up to. See the Hammond catalog on the Radio Daze site. A Hammond 125A at 3 watts or a 125B at 5 watts should work for you. The 40-190 is rated at 2 watts audio output power per the schematics. You can measure the speaker voice coil DC resistance with a ohmeter to determine it's approximate impedance with the secondary of the audio output transformer disconnected. Multiply the ohmeter reading by 1.25 to get the impedance. This is then the secondary impedance you wish to match. Use the universal impedance chart supplied to match to the pimary and secondary impedances ideally needed for good match. I personally would go with the 125B for current handling headroom.
Good luck, John
