Twin Face biasing questions

Song Naga

Song Naga

Member
Hi all, I have a Twin Face happily sputtering away and am working on biasing it before I seal it up and take it from the workbench into the studio. I have 2N3904's in the Si side and some sweet P416B's on the Ge side, and they're successfully producing a nice fuzz effect now that I finally took time to read the schematic along side the datasheets for the transistors (and translated a little russian) and figured out which way to contort the transistors to work with the sockets on the PCB. I'm pretty sure they're installed properly based on the horrible squeals I was getting when they were NOT in properly. That in itself was an adventure! All the transistors are socketed in the event that I haven't contorted things right quite yet.

When I try to bias either the Si or the Ge circuits, I'm not seeing any difference in voltage when I adjust the trims. I'm sure I'm doing something wrong, please let me know if you know what it is!

I've been measuring with my multimeter's black lead on the negative power connection point on the input power supply. Maybe this is the wrong point to connect that to since there is a voltage inverter IC involved?

Measuring voltage between the negative connection point on the PCB and the emitter on NPN2 shows 0.71v regardless of what the trim pot is adjusted to.

Measuring voltage between the negative connection point on the PCB and the emitter on PNP2 shows 0.75v regardless of what the trim pot is adjusted to.

These values seem way off from the ideal 4.5v, any ideas on what I should do from here?

Here's a screengrab of the test points I'm using in my attempts to bias the circuit: blue line is showing 0.75v and the yellow line shows 0.71v no matter where I adjust the two trims on the PCB.



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Hi again, I'm back to looking at the PNP side of my Twin Face now that I know to measure the voltage at the collector rather than the emitter (thank you @leadfoot). Measuring voltage between GND and the collector of PNP2 while adjusting the 10k trim ranges from -8.89V and -8.83V, no where near the ideal -4.5V. I've triple-checked values on the board for resistors and caps and things seem good. I've subbed 22uF electro caps for 20uF electros cited in the BOM because 20uF caps don't seem to actually exist, and that is the only deviation.

The NPN side sounds great and its test point can easily be set to 4.5V between GND and the collector of NPN2 using the NPN trim.

The PNP side does produce sound and a fuzz effect but its signal level is low and sounds muddy compared to the NPN side.
 
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I read anywhere that the R4 resp. R8 are a bit too high with Russian Ge‘s. He recommended 68k in the spot. Anywhere lower than 7v would be okay.
I got two black 416 in my stash. They have the lowest hfe readings of all my BARTs. How is yours?
 
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hamerfan said:
I read anywhere that the R4 resp. R8 are a bit too high with Russian Ge‘s. He recommended 68k in the spot. Anywhere lower than 7v would be okay.
I got two black 416 in my stash. They have the lowest hfe readings of all my BARTs. How is yours?
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I swapped R4 for a 68k resistor and tested again, the voltage at the Collector of of PNP2 is now at 6.92V (down from 8.83), under the recommended 7V max, so we're in good shape as far as measurements go now! Thank you!

The PNP side of the pedal does now sound brighter and louder, but still not as bright and loud as the NPN side.

I've got a pair of P416B's in the NPN circuit at this time Q1 is HFE 80.0 and Q2 is HFE 59.1. Hopefully those are good numbers!
 
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