Mod Ideas/Design for DiY U47

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Melodeath00

Well-known member
Joined
Jun 7, 2012
Messages
416
This will be a giant post. Hopefully I can get some much needed help from you guys, though. You may have seen my original AMI/Tab-Funkenwerk U47 Build thread here: http://groupdiy.com/index.php?topic=52022.0. For the sake of clarity, and the fact that I am now deviating from (i.e. modding) the original design, I have decided to start a new thread. Let me try to lay it out before I start with the questions. I am still essentially a noobie, so I have lots of questions, and they may seem rather basic.

The schematic of my mic can be found here: http://www.tab-funkenwerk.com/sitebuildercontent/sitebuilderpictures/U47ArchutAltTubewithRemote.jpg
Some of the values are not quite right because my PSU is 120V, not 105V, but that is the basic layout of the circuit.

Goals
-Flexibility/versatility
-"True" cardioid
-Use more "vintage" values for resistors to discover sound difference

What I intend to do
-Install a relay in the mic that will allow me to switch out the rear diaphragm of the capsule from the circuit.
-Install a switch in/on the PSU that allows me to activate or deactivate that relay
-Install a switch in/on the PSU that allows me to "shunt" the rear diaphragm polarization voltage to ground
-Install a mod in the PSU to be able to adjust heater voltage. It is currently set at about 5.6V measured at tube.
-Install a mod in the mic to be able to adjust tube plate voltage
-Install a mod in the mic to be able to adjust cathode voltage
-Replace 1G resistors with more historic values

Questions
I did not design this PSU, so I have some questions that I think you guys will be able to help me with.
I want to be able to adjust the heater voltage. The heater is on the yellow wire on Pin 6 of the Binder, and you can see where it attached to the board. Is there somewhere where I could install a potentiometer to physically adjust the heater voltage? Or perhaps is that white phillips screw driver right next to the heater connection on the PCB a potentiometer for adjustment?

I plan to switch the 1G resistors to 60M, 100M, and 150M. These values coincide with what was used in the U47, U48, and M49. 150M is for the rear diaphragm, 100M for the backplate, and 60M for the "grid bias." Will this cause any problems inside the mic with the supplied voltages from the PSU, or is it safe to try these values? I am not positive how these resistors work in the circuit, so it is hard for me to know if messing with their values will cause problems. The fact that we can use 1G tells me the PSU is not really supplying many amps, as we are not using these as dropping resistors. I noticed that the M49 uses a 100M then the 150M on the rear polarization, while the U48 uses a 150M and then another 150M. However, the U48 is tapping from the 105V B+, whereas the M49 has its own remote 120V polarization supply. Is this what accounts for the difference?

A relay can be used to disconnect the rear diaphragm from the mic amp. It is my understanding that I can use a relay that is powered by the tube heater voltage, or I can use a latching relay, which I have heard would be better for noise. With a 5V single pole Relay, it seems I could attach the tube heater supply to this relay, and "switch" 5V temporarily into the relay to activate it, and initiating true cardioid mode. Does this sound right? Is is simpler to go with the non-latching relay?

In either case, do I need to worry about the Relay drawing too many amps and causing a fluctuation in the tube heater? What sort of specs do I need to look for in the relay? Tube heater is anywhere from 4.8V to 6.3V if I can successfully get it variable as described earlier.

Does it matter if the C4 capacitor that blocks DC from the rear diaphragm to the front is "before" or "after" the relay? Some designs have the internal "cardioid only switch" disconnect the cap from the front diaphragm, but I have seen some designs where it's the other way, and the cap remains connected at all times to the front diaphragm, even when the rear gets disconnected by a relay or switch.

My current Gotham GAC-7 cable uses the thick blue wire for the heater return. This heater return is tied to Ground in the PSU on Pin 5 as you can see in the attached picture. If I use a non-latching Relay, I will need to use this blue wire in the cable for the Relay return, and connect the heater return to ground. Would I be better served using this thick blue Wire for Ground instead of the Relay return, or does it not matter much?

The Omni setting on the PSU is supposed to supply 0V to the rear diaphragm. However, this 0V does not have electrical continuity with ground, so I figured it would be best to install a switch that shunts the polar pattern to ground, so that I get a true 0V on the rear diaphragm. Can I just use a 2-way switch connected to green wire Pin 7 and the 0V-120V rotary switch to either pass the voltage from the rotary, or to pass it to ground? Is it safe to connect 120V to Ground through this switch if I happened to have the setting on Bi-Directional and I flipped the switch to shunt it? Do I need to runt he signal through a resistor before it hits ground?

I am having trouble figuring out how one calculates the value of R8 to get his/her desired plate voltage. Is it possible to replace R8 with a potentiometer to be able to have simple control over plate voltage? I ask because even though R4 and R8 are in series, it seems like people leave R4 at 100K, and change only R8.

R2 and R3 form a voltage divider for getting a positive voltage on the cathode. Can I replace R2 with a potentiometer to "dial in" my desired cathode bias/voltage? What sort of specs would that potentiometer need?

Every time I turn off the mic, there is a large transient in audio. This doesn't happen in my C12. Is there a way to "fix" this, or is it expected behavior? I don't particularly care, as long as it is not damaging the internal components of the mic.

Thanks a lot for your time! Any help is appreciated. I have been thinking about this for a long time, but unfortunately, I'm still very new to all of this and don't know enough yet to not ask for this kind of help.
 

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I too am suprized you have not had any responses, Melodeath00. I may be able to help with a few of your questions.
Yes, you can use a potentiometer in place of R2&R3. A pot is nothing more than a voltage divider. Be careful with power ratings! The pot (or trimpot pot) should have a decent wattage rating as you are dealing with FIL current. 1W+ should do it. The pot will have a scratchy sound when turned though. This should be of little concern as the pot is not turned while using/recording, if you will.
Trying to power the relay off of the FIL supply may result in a voltage sag when the relayis activated. You would still have to use a remote voltage to get around the voltage drop. Let me re-read your original post to see if I can answer (or better answer) some more of your questions.
I am curious as to what the difference in tone will be with lower value resistors as opposed to using the 1Gresistors.
 

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