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I'm finally getting into my first Model Wurlitzer Electric Piano Model 140, and I'm trying to figure out if the output transistor bias is right--and how to tweak it if it isn't. I've got too many unknown variables at this point, and I feel like I need advice from someone with more experience with older transistor output circuits.
First, the original output transistors have already been replaced by someone with SP1271/652085 with 1983 date codes. With information on transistors used by Wurlitzer so hard to find, it's difficult for me to tell how close these are to the original TA-2047/650429, especially in terms of gain. The replacements are germanium, which I can tell via the Vbe.
The static bias across the 0.33 Ohm resistors is 54mV, the same on both. Doing the math, that means that the idle current is around 164 milliamps. I was surprised to find a figure that high, so I checked my measurements and math multiple times. It's much higher than the 200A, for example.
Since the pitiful amount of information available on the 140 doesn't specify what you should read across the 0.33 resistors, I'm flying a bit blind. I wanted to see, first, how the supply voltages were affected by new power supply capacitors, but the supply voltages, even with new power supply caps, stayed slightly low (+/-18V). I'm assuming that this is due to voltage drop across the selenium rectifier and overall series resistance in the power supply. I could replace the selenium bridge with silicon diodes,of course, but, if I do that, I know that the supply voltages will go up and that I'll no longer have the internal resistance of the selenium rectifier to put the brakes on the output stage. I don't want to raise the voltages on an output stage that's already running hot--if it is.
On the schematic I'm attaching, R38 and R41 are 220 Ohms. R40 and R43 are 2.7 Ohms. R39 and R42 are 0.33 Ohms.
If re-biasing, which resistor value is the most important one to tweak? 220 or 2.7?
Options:
1) Replace output transistors with more appropriate ones from an aftermarket supplier and keep original bias resistor values (220/2.7)
2) Re-bias the output stage with different resistor values and, perhaps, replace the selenium rectifier at the same time.
3) Replace output transistors, go to a silicon bridge, and re-bias.
140 Output stage.pdf
First, the original output transistors have already been replaced by someone with SP1271/652085 with 1983 date codes. With information on transistors used by Wurlitzer so hard to find, it's difficult for me to tell how close these are to the original TA-2047/650429, especially in terms of gain. The replacements are germanium, which I can tell via the Vbe.
The static bias across the 0.33 Ohm resistors is 54mV, the same on both. Doing the math, that means that the idle current is around 164 milliamps. I was surprised to find a figure that high, so I checked my measurements and math multiple times. It's much higher than the 200A, for example.
Since the pitiful amount of information available on the 140 doesn't specify what you should read across the 0.33 resistors, I'm flying a bit blind. I wanted to see, first, how the supply voltages were affected by new power supply capacitors, but the supply voltages, even with new power supply caps, stayed slightly low (+/-18V). I'm assuming that this is due to voltage drop across the selenium rectifier and overall series resistance in the power supply. I could replace the selenium bridge with silicon diodes,of course, but, if I do that, I know that the supply voltages will go up and that I'll no longer have the internal resistance of the selenium rectifier to put the brakes on the output stage. I don't want to raise the voltages on an output stage that's already running hot--if it is.
On the schematic I'm attaching, R38 and R41 are 220 Ohms. R40 and R43 are 2.7 Ohms. R39 and R42 are 0.33 Ohms.
If re-biasing, which resistor value is the most important one to tweak? 220 or 2.7?
Options:
1) Replace output transistors with more appropriate ones from an aftermarket supplier and keep original bias resistor values (220/2.7)
2) Re-bias the output stage with different resistor values and, perhaps, replace the selenium rectifier at the same time.
3) Replace output transistors, go to a silicon bridge, and re-bias.
140 Output stage.pdf