If you get the Greenbacks maybe try installing them in a cross pattern in both cabs instead of one cab of Greenbacks and the other T75’s. This way if you ever play a gig where you only need one cab you still have the mix of speakers.
Please explain your theory of damaging the output transformer due to speakers being wired out of phase. There’s no magnetic or electrical coupling between the speakers. The acoustic cancellation would definitely occur but not silence as you said. The speakers unless possibly being dual voice coil and wired out of phase would be the only possible chance of any interaction that could cause problems with the output transformer. The individual speakers are going to move independently of each other and have no electricity effect to each other. In fact in a transformer that has multiple windings you can wire windings in series, phase correct to increase voltage and in series phase reversed to reduce voltage and no harm occurs. An example being taking the 5 volt secondary winding and series connecting it to the primary 115 volt phase correct. This will create a 120 volt primary and lower secondary voltage. Same scenario, hook the 5 volt secondary in series phase inverted to the 125 volt primary winding and it will bring the primary winding effectively to 120 volts vs 125 volts to increase the secondary voltage. I’m doing this with Hammond power transformers right now to get 120 volt primary from their 200 series power transformers because the primary is wound with a 115 tap on a 125 volt primary and my mains voltage is 121 volts. 125 winding makes the 6.3 low and 115 tap makes 6.3 ( and everything else) high. I don’t use the 5 volt winding because of solid state rectifier and this solution is perfect.
It’s not the same as a direct short. Imagine hooking up two electric motors in parallel but on positive to positive negative to negative and the other positive to negative (the motor wire terminals) and negative to positive. We’ll use DC in this example and it will represent the positive flow of AC current. One motor will turn clockwise and the other will turn counterclockwise. They don’t represent a short circuit but they represent a load but their output is opposite of each other. With the speaker example and an AC waveform the correctly phased speaker cone excursion is correct and it’s cone travels outward away from the magnet structure and the reverse polarity speaker cone excursion travels inward toward the magnetic structure. This is the reason we have sound cancellation because the air displacement of one speaker is absorbed by the opposite displacement of the other speaker. The motor structure of a speaker has the same impedance whether it’s checked phase correct or reversed and checked phase reversed. The impedance’s are still present exactly as they would be connected in the circuit but the orientation of their output is opposite each other. If they were housed in the same sealed cabinet the speakers could be observed vibrating but sound output would be minimal because the internal cabinet pressure would be 0 and outside of the cabinet nearly no sound.
I apologize for the question a little ... naive ... but, at this point what would be the possible technical solution to solve this problem and ..... do not change / sell a 1960A cab (or in a 1960A style cab)??? All in order to make a Classic Stack 1960A + 1960B as efficient and with the best volume as possible?
Just make sure all the speakers are wired in correct polarity. It’s easy to check. You use a 9 volt battery to energize the cabinet or each individual speaker with the back off the cabinet. Using a flashlight, observe the speakers. When you hook the battery positive to the speaker positive (tip) and battery negative to speaker negative (ring) all the speaker cones should extend.
