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AB165 to AA864

10/22/1998 1:35 AM
AB165 to AA864
I started some general maint. on my Blkface 67'Bassman. I have been trying to figure out what model it was. Sticker inside stated AA864 with the 2nd A crossed off with a B written over it. Now for the life of me I haven't been able to find a AB864 schem. Anyway had it apart to recap it and was looking at the bias supply and and realized it was a balance contol. With now this bit of info and a little detective work I have deduced it is a AB165. I changed the bias supply to AA864 but that was it. Will this amp really benefit from a full AA864 transformation. This amp didn't seem to break up much even full out thru celestions. Be nice to have a bit more gain without hacking up this amp too much. How would a project affect the value? I have plenty of amps, just lookin' for some building/modding experience. Opinions?
10/22/1998 7:19 AM
J Fletcher

Some of these have a feedback loop from the 6L6 plates to the grids. They're mounted on a terminal strip and not on the circuit board, if I remember correctly. I once repaired one of these amps that had an out of tune note accompanying every note played. After subbing in new filter caps, tubes, anything else I could think of, I removed the 220k feedback resistors...problem solved. Should give you a bit more gain too.
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10/22/1998 3:29 PM

J Fletcher: I had one that was like this too,it had some hum when using the 220k,I removed them too,and the hum went away. [Richie]
10/22/1998 11:16 PM
Andy Ruhl

The AB165 is the first silverface amp, but in blackface clothing. Go ahead and change it to AA864 specs from the coupling cap right before the phase inverter all the way to the power tubes. I leave the preamp the way it is, but I remove the caps across the plate loads. I've also played with the 470k interstage feedback resistor a bit. I've found that removing it can cause hum, and I didn't bother trying to figure out where it was coming from so I put it back in. I prefer the AB165 preamp after those caps are removed.  
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