My U47 408A (D47) Build

[pnutz]

Bump @poctop

 

[rock soderstrom]

Your substitute resistor should work. It has 2M ohm more resistance, which will result in a slightly lower cut-off frequency (probably not measurable for you, it's really a homeopathic difference and not audible at all). Just to put this in context for you, there are suggestions for building U47 clones that use 1G ohm at this point, but I would not recommend this.

So, as long as the resistor fits in terms of space, everything is fine. The power doesn't matter at this point either, your new resistor can withstand a little more power, that's all.

 

[pnutz]

Your substitute resistor should work. It has 2M ohm more resistance, which will result in a slightly lower cut-off frequency (probably not measurable for you, it's really a homeopathic difference and not audible at all). Just to put this in context for you, there are suggestions for building U47 clones that use 1G ohm at this point, but I would not recommend this.

So, as long as the resistor fits in terms of space, everything is fine. The power doesn't matter at this point either, your new resistor can withstand a little more power, that's all.

Sweet, thanks @rock soderstrom

 

[geekmacdaddy]

Hey Folks
What is the voltage supposed to be coming out of the transfo into the board? (yellow and black wire readings) I have about 26v AC
Thank you
Jeff

 

[HerbertR]

Hello, 26 VAC is not correct. Disconnect the transformer from the circuit board screw connections and measure it again “unloaded”. If you live in the US, the primary windings of your Triad 230 110 transformer should be connected in parallel. This will give you about 250V AC at no load on the secondary windings connected in series on the black and yellow wires (red and orange wires are shorted). Test this before connecting the yellow and black cables to the board.

(But: Personally, I would prefer to operate the power supply "only" with 110 V AC secondary, which means the two secondary windings are also connected in parallel - this means that much less heat is emitted via the large series resistors and the potentiometer. There is more than enough voltage for smoothing (approx. 160 V DC after the bridge diode rectifier - this means that approx. only 55 V DC has to drop over the Choke and the Drop Resistors - compared to over 200 V!!! DC to drop - you will have about 310 VDC after the Bridge rectifier with the secondary side connected in series like in your case... And down to necessary 105 V DC for the Mic..), Of course, you also have to reduce the sum of the large drop resistors on the PCB)

 

[geekmacdaddy]

I appreciate the help and clarification, Herbert.Thanks you.

 

[geekmacdaddy]

Is there a schematic for the D47 Power supply?