dougch

U47 type mic build problems
« on: April 04, 2018, 12:57:55 PM »
I should have asked for advice before now but here goes.

I started a mic build with a FLEA 47 body I bought a few years ago. I had them ship it with a “FLEA Next” bell which has a Binder connector. I had an AMI BV8R transformer to use. I started off with the idea of using double 6028 tubes but later changed my mind and decided on the EF800. These are the problems I ran into and how i dealt with them, perhaps wrongly so. I welcome advice and comments.

Wiring. I started with 22 awg teflon insulated wire but found the wire bundle would not pass through the hole in the transformer deck. Not wanting enlarge the hole and risk damage, I left the ground wire at 22 awg and used 26 awg for the rest. The 26 wire was pvc insulation. Bad?

When I wired the connector, I used Max’s MK-U47 build as a guide, pins 1,2 audio pins 3,4 and 6 to ground pin 5 105v.on my 7 pin Binder. When I changed my mind about tubes, I used pin 7 for the heater with 22 awg wire. Strange configuration?

I am concerned that i used pvc insulated wire in the high impedance section. Should I start over and use teflon wire throughout? Should I rewire the connector to be more like Dan’s D-EF47?

Should I/can I clean the acrylic high impedance deck (for lack of a better word)? Will alcohol damage the part?

Thanks for any advice.

Doug



Delta Sigma

Re: U47 type mic build problems
« Reply #1 on: April 04, 2018, 01:28:29 PM »
What pins you use should only matter to you. Make sure you use the larger conductors in your cable for 0V and heater.

When you say pins 3, 4 and 6 are ground, do you mean 0V, cable shield or both? You should designate one pin as your shield termination to allow you to install a ground lift switch in your power supply.

26AWG is a good size for mics with the exception of heater and 0V. Stranded Teflon is a better bet than PVC though, especially if you bundle them together.

Keep alcohol away from acrylic (P2P builds), polystyrene caps and keep it far away from your capsule. I think anything else i fair game. Isopropyl alcohol is the easiest way to clean flux from HiZ, epecially before polystyrene caps and capsule are installed.

dmp

Re: U47 type mic build problems
« Reply #2 on: April 04, 2018, 02:26:38 PM »

When I wired the connector, I used Max’s MK-U47 build as a guide, pins 1,2 audio pins 3,4 and 6 to ground pin 5 105v.on my 7 pin Binder. When I changed my mind about tubes, I used pin 7 for the heater with 22 awg wire. Strange configuration?

I am concerned that i used pvc insulated wire in the high impedance section. Should I start over and use teflon wire throughout? Should I rewire the connector to be more like Dan’s D-EF47?

Should I/can I clean the acrylic high impedance deck (for lack of a better word)? Will alcohol damage the part?
At this point I'd say get it working and see how it is. If it has noise problems then you will have to address them through cleaning or rewiring.
For AWG, the tube mic cable Redco sells is a good reference. The TGS-M7 has 20 AWG for heaters, 26 AWG for the rest. While the Mogami has 22 AWG for the heaters. High AWG, thinner wire, will have a little more temperature rise but until it gets significant, it will not cut the power being delivered to the tube. So long story short, you're fine with 22 awg on the heaters I think.
Teflon is really nice from a installation standpoint, but I'm not sure you will see much performance difference in application. Again I'd try it and see before re doing it.

When you say pins 3, 4 and 6 are ground, do you mean 0V, cable shield or both? You should designate one pin as your shield termination to allow you to install a ground lift switch in your power supply.
Can you be a little clearer on what you mean with this? I don't think you want to 'lift' the shield from ground. The mic shield and mic body should not be lifted from the chassis ground as a safety consideration. Neumann schematic tie the 0v and shield together in the mic.

dougch

Re: U47 type mic build problems
« Reply #3 on: April 04, 2018, 06:16:26 PM »
Thank you for your comments. On the mic, pins 3,4 and 6 are tied together at the connector. One pin is wired to the mic body and one of the other of the three to 0V in the mic circuit.

I haven't built the power supply yet though I have the D-EF47 PSU board which I will use for the. I have all the parts but no case yet. I also have not wired any cables. In the D-EF47 thread it is a little confusing. On page four it shows wiring from the psu. There is a connection for shield and another for 0V. I will have to get my head around that.


 

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