Hi Japes; we're doing good now. First off, the term 'center tap' usually refers to the primary side of the output transformer (confusingly, to the secondary side of the power transformer) but the OT center tap is shown in red below. It's the source of the B+ for the 6V6 plates and screens from the choke and ultimately the rectifier. I now understand what you meant; the 'center' or 'middle' tap on the secondary side; where the speakers are connected. For discussion purposes, we'll call that the middle or (+) tap.
Okay; you are getting sound from the middle and black taps because the standby switch is shunting or grounding the weak signal to the chassis and the black (-) wire that you say is connected to the chassis is picking it up.
The standby function is shown in green; in either of the first two switch positions - Off and On/Standby, the signal, which sees that .05 cap as a short, bypasses the resistor and goes to ground ... where your grounded black wire is picking it up again. In the second switch position - On/Standby - the inbound 110V AC power is connected to the primary side of the power transformer and everybody fires up; 5 volt rectifier heater (at the top of the power transformer drawing), 6.3 volt filament heater (at the bottom of the drawing), and the secondary AC in the middle with its center tap shown grounded.
In the third switch position - 'On', the signal is no longer grounded out (the green wire effectively stops at a dead end), the amp is on, and you hear nothing because the black (-) isn't connected to the speaker, it's connected to the chassis effectively grounding the signal out again. The fourth switch position is the other polarity position; if humming in position three, the idea was to bump the switch over to position four to kill the hum .... without having to turn the amp off, pull the plug, turn it over, plug it back in, and turn the amp back on.
The 'left'/magenta tap is connected to the middle/blue/(+) via the nfb resistor. So long as the black wire is grounded to the chassis, there won't be any sound in either switch postions 3 or 4. The left terminal is where the yellow/white wire should be connected and it's other end on the tone expander switch.
But but but but but ... before you go any further with this amp, you have to get a 3-wire cord on it and that .0? cap, shown on the left side of the dotted box as connected to ground, has to go. You also have to get a new power switch; you don't need all the contact points and they get old and ratty. 'Standby' allowed the player to unplug without nasty noise; after all, the signal path in Standby is grounded so whatever junk is happening at the input jacks isn't amplified. With a new Off/On switch, if you want to unplug, just turn the amp down. If you want to test your amp using the middle (+) and black (-) terminals, that's one thing; if you're going to do rehab work on it and want to use it, the next step is the switch and cord ... nothing else.
12AX7: if that tube has a diamond and the name Telefunken etched on it and it is in good condition, it might be worth $40-$50 or more. No point in letting it stay in the amp at some degree of risk when you can stick a sacrificial Sovtek there; we're testing, rehabbing, not starring at Lincoln Center .... yet .... and no, can't upload file of my GA30's tone ... am not yet enabled and, for the level of effort to get there .... I will promise you though that if you get it running including cord, switch, and power supply caps, you won't be disappointed. Besides, it won't sound like mine; it's full of 6EU7s and, not to embarass, the quality of my amp's output and rectifier tubes can't be exceeded. :wink: CJ