I have had older 1930s amps to work on, but this may be the oldest Fender which are my favorite.
This one had been sitting for decades and I was asked to go through it. I installed a 3 prong AC cord, removed the two .05-600 caps at the ground and standby switch. Cleaned pots and sockets, tested tubes, etc. Brought up slow. Had lots of noise at the preamp tubes , replaced V2 100K plate resistor, and solder a V2 wire that looked like it had never been soldered on. Tested, played for a while and all seemed good. Put back together.
Final check and had some noise again at preamp tubes, chopstick sockets/wiring/connections and getting some noise. Pulled all preamp tubes and tried a different set. Now a new issue, high current draw.
High current starts immediately as I slowly dial up the variac.
Only have high current when standby switch is ON. When OFF no issue and I have negative bias V at the power tubes. Same scenario with all tubes are pulled, or with no speaker load.
Seeing at 5881 tube
plate to standby switch 47 ohms
plate to standby switch 41 ohms
This amp has seen some prior work in the past (filter caps, lots of resistors, etc.). Do not know when. Seems like good work. Not sure why they left a 2 prong AC cord installed.
Also odd, when switching the standby switch there is a momentary pause, then a click sound as it activates on or off.
Has the selenium rectifier in the bias but I have only been asked to check it out and get make it quiet.
Would like to get the high current draw resolved first. Time for help, advice, opinions. Thank you, Mark
ps. I have double checked every wire at every tube pin for good connection and short to ground.
Using the attached schematic
Fender_bassman_5f6a.pdf
This one had been sitting for decades and I was asked to go through it. I installed a 3 prong AC cord, removed the two .05-600 caps at the ground and standby switch. Cleaned pots and sockets, tested tubes, etc. Brought up slow. Had lots of noise at the preamp tubes , replaced V2 100K plate resistor, and solder a V2 wire that looked like it had never been soldered on. Tested, played for a while and all seemed good. Put back together.
Final check and had some noise again at preamp tubes, chopstick sockets/wiring/connections and getting some noise. Pulled all preamp tubes and tried a different set. Now a new issue, high current draw.
High current starts immediately as I slowly dial up the variac.
Only have high current when standby switch is ON. When OFF no issue and I have negative bias V at the power tubes. Same scenario with all tubes are pulled, or with no speaker load.
Seeing at 5881 tube
plate to standby switch 47 ohms
plate to standby switch 41 ohms
This amp has seen some prior work in the past (filter caps, lots of resistors, etc.). Do not know when. Seems like good work. Not sure why they left a 2 prong AC cord installed.
Also odd, when switching the standby switch there is a momentary pause, then a click sound as it activates on or off.
Has the selenium rectifier in the bias but I have only been asked to check it out and get make it quiet.
Would like to get the high current draw resolved first. Time for help, advice, opinions. Thank you, Mark
ps. I have double checked every wire at every tube pin for good connection and short to ground.
Using the attached schematic
Fender_bassman_5f6a.pdf
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