Glad to have you back, Tim! And working on my favorite Fender amp of all time to boot AB165! Just overhauled mine for the third time. I've owned my '65 since '96! Looks like the all 7 of the diodes on the bias board, as well as the bias electrolytic cap, have all been changed out. Stock, they would have been metal can diodes. They're cheap and should be replaced. Everything else looks bone stock. All coupling caps and resistors look to be stock. Transformers have sane date codes, and look correct. Transformer wires in my '65 and '64 (AA864) bassmen are both stock, and do not use cloth insulation wire. I generally leave the blob on the chassis. It's less invasive than heating that entire area of the chassis up, which basically requires full disassembly of the innards for deep cleaning (restoration that this amp clearly doesn't need). I would personally swap out 2 of the three clustered "brown-turd" film caps on the main board. These are the PI coupling caps to the power tubes. They should be replaced along with the electrolytics. Orange drop caps are expensive, but are the standard retrofit for the PI to PA coupling caps in an amp like this. Any DC leakage here can cause the amp to lose bias to the power tube grids. Both polypropylene and polystyrene film caps are moisture degradable, so much like electrolytics, they are born to die eventually here on Earth. The key to the AB165's sound is in the mixer stage, and all of the other NFB loops in the amp (2 SE, 1 diff pair). This many layers of foldback current cause so much destructive interference in the highs, that it mellows the amp out as the volume and distortion increase. This also causes it to distort chiefly in the mids and lower mids, but have note clarity as a result of the NFB phenomenon in chord harmonics. If you haven't had a chance to play one of these somewhere between 6 and 7 with a 4x12 of greenback-like speakers, you're in for a real treat, amigo.
Interesting vid. The plate voltage goes high, up to supply voltage, when the tube is in cut off so no current flows through the plate resistor. Plate voltage goes low as the current increases through the plate resistor as it approaches saturation.
Glad to have you back, Tim! And working on my favorite Fender amp of all time to boot AB165! Just overhauled mine for the third time. I've owned my '65 since '96!
Looks like the all 7 of the diodes on the bias board, as well as the bias electrolytic cap, have all been changed out. Stock, they would have been metal can diodes. They're cheap and should be replaced. Everything else looks bone stock. All coupling caps and resistors look to be stock. Transformers have sane date codes, and look correct. Transformer wires in my '65 and '64 (AA864) bassmen are both stock, and do not use cloth insulation wire. I generally leave the blob on the chassis. It's less invasive than heating that entire area of the chassis up, which basically requires full disassembly of the innards for deep cleaning (restoration that this amp clearly doesn't need).
I would personally swap out 2 of the three clustered "brown-turd" film caps on the main board. These are the PI coupling caps to the power tubes. They should be replaced along with the electrolytics. Orange drop caps are expensive, but are the standard retrofit for the PI to PA coupling caps in an amp like this. Any DC leakage here can cause the amp to lose bias to the power tube grids. Both polypropylene and polystyrene film caps are moisture degradable, so much like electrolytics, they are born to die eventually here on Earth.
The key to the AB165's sound is in the mixer stage, and all of the other NFB loops in the amp (2 SE, 1 diff pair). This many layers of foldback current cause so much destructive interference in the highs, that it mellows the amp out as the volume and distortion increase. This also causes it to distort chiefly in the mids and lower mids, but have note clarity as a result of the NFB phenomenon in chord harmonics. If you haven't had a chance to play one of these somewhere between 6 and 7 with a 4x12 of greenback-like speakers, you're in for a real treat, amigo.
Wow! Thanks, Jamie. Great info. I'm pretty sure I have some Orange Drops for the PI outputs. I'm glad to have someone more knowledgeable chiming in.
@@timkrauseelectronics9245 I wouldn't say I'm more knowledgeable, Tim. Just more familiar. I've had an ongoing love affair with my AB165 for 25 years!
Interesting vid. The plate voltage goes high, up to supply voltage, when the tube is in cut off so no current flows through the plate resistor. Plate voltage goes low as the current increases through the plate resistor as it approaches saturation.
looks like a 67