Upon opening up the hood and checking transformer date codes, ive dated it to late 71/early 72. Big ol cap to ground off the bias pot gives this away as an AA270 circuit.
Looks like a power tube socket burned up in the past, and was replaced - i spotted a large burn mark on the outside of the chassis near the socket. This seems to confirm it.
First thing I did was completely redo the heater wiring. I used black/red a la marshall so i would know exactly if my heater wires were in phase. I noticed the artificial center tap resistors were replaced - also 3 of the preamp tubes had shorts in them. I think the guy kept popping center tap resistors because of the shorted tubes. I replaced them with some 5 percent carbon films that matched dead on to each other.
Other than those two things, I couldn't spot any other alterations other than a replaced grid leak on channel 1. You can imagine how relieved I felt to see a basically unadulterated circuit.....Now its my turn to do some hacking and chopping
I changed the bias circuit to blackface type. I swapped in two precision metal film 220Ks to the power tubes for this. They were spot on 220K match, so I figured that was important. I also replaced the 1.5K's on the tube sockets with dale 1% jobbies I had laying around. My thought is to get the bias circuit as equal and precise as possible.
Next up was cutting the death cap (for grins, i checked continuity - the cap had failed and was shorting. Glad i didnt fire it up!) and new three prong cord. I also cut the .002pf cap on the reverb input out, as my AB763 schematic didnt show one (i plan on basically blackfacing this thing)
electrolytics are all original. I am going whole hog and im just going to use Spragues or F&Ts.
Other than replacing the electrolytics, I think im going to leave the brown turds and ceramic disks alone until after I can fire it up. Ive read that people replace the ceramic disks along with the electrolytics - any weight to that? Im going to swap in some 3W 470 ohm metal oxides on the power tube sockets as well.
Speakers were the original type with the square magnets - are those CTS or Oxford? It had been reconed a few years ago (someone penned the date on the inside of the cones.
All in all, im super excited to be restoring this thing! I thought I would do my part and post my thought process a bit. Pics should be attached.
