Hi,
New poster here looking for some advice on this Fender Studio Bass amp I'm restoring. If you're unfamiliar with the amp it's basically a Super Twin slightly reconfigured for bass. I'm having a problem with the heater filament system and have exhausted my knowledge, so I hope someone familiar with these amps could help.
My issue is that the heater filaments going to the 6 x 6l6 sockets have all the voltage on one side. IE on a given socket one heater pin will have the whole AC voltage of about 6.8vac and the other pin has 0vac.
I first thought the issue was in the hum balance circuit (which on late 70's Fender amps is separate from the bias balance, and is used to balance heater voltages). The pot was bad, but after fixing that circuit up it seems the balance only affects the pair of white wires from the power transformer which heat the preamp tubes I believe. Operating the pot I can match the voltages on those taps to like 3.6vac.
But the 6l6 heater filaments go to a separate terminal strip. One lead is connected to the green/yellow wire from the power transformer, and the other is connected to the green wire. But what's weird is that the terminal with the green tap is connected directly to ground, so no AC voltage flows to the heater filament and thus those pins. I pulled the green and green/yellow leads and measured their voltage and both are giving off 6.8vac, so it's not the transformer. Did the factory just miswire the amp back in the day? Should both heater filaments be connected to the green/yellow rather than one to green and ground?
Attached is the schematic, which shows the green tap going directly to ground. Thanks for any help/clarification you can provide!
Nate
New poster here looking for some advice on this Fender Studio Bass amp I'm restoring. If you're unfamiliar with the amp it's basically a Super Twin slightly reconfigured for bass. I'm having a problem with the heater filament system and have exhausted my knowledge, so I hope someone familiar with these amps could help.
My issue is that the heater filaments going to the 6 x 6l6 sockets have all the voltage on one side. IE on a given socket one heater pin will have the whole AC voltage of about 6.8vac and the other pin has 0vac.
I first thought the issue was in the hum balance circuit (which on late 70's Fender amps is separate from the bias balance, and is used to balance heater voltages). The pot was bad, but after fixing that circuit up it seems the balance only affects the pair of white wires from the power transformer which heat the preamp tubes I believe. Operating the pot I can match the voltages on those taps to like 3.6vac.
But the 6l6 heater filaments go to a separate terminal strip. One lead is connected to the green/yellow wire from the power transformer, and the other is connected to the green wire. But what's weird is that the terminal with the green tap is connected directly to ground, so no AC voltage flows to the heater filament and thus those pins. I pulled the green and green/yellow leads and measured their voltage and both are giving off 6.8vac, so it's not the transformer. Did the factory just miswire the amp back in the day? Should both heater filaments be connected to the green/yellow rather than one to green and ground?
Attached is the schematic, which shows the green tap going directly to ground. Thanks for any help/clarification you can provide!
Nate