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Old 3rd March 2013, 05:25 PM   #51
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Also i changed out the 1780 ohm resistor for a 1k since i will be using 130 volts.
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Old 3rd March 2013, 06:53 PM   #52
rmb is offline rmb  United States
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Chris: I need to look at my notes on the U47 mod. The 407A heater is 40 volts (sections in series using pins 1 and 9) with 50 ma heater current, but the heater is being operated at 36 volts for noise purposes...need to check other values and comments from my notes...The NG supply was a "brute force" supply with the heater and HT drawn off the 105 volt dc. If you are using 130 volts d.c. HT, you will need a 1880 ohm 10 watt (wirewound) resistor for the heater. You will burn open the heater using a 1K resistor!

This mod is not a direct "drop in"...there are some components involved. More later.
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Old 3rd March 2013, 06:57 PM   #53
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Ok will make sure i use that bigger resistor then, I am not sure ive got one in that value however, so i'll see what i can do. Also it should be noted that I have the input in the microphone for a heater already, i could easily up it to 20 volts.

I am not understanding the 34 volts on the plate however, please check out the redrawn schematic for feeding the capsule polarization voltage.
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Old 3rd March 2013, 07:03 PM   #54
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A new moded version
Attached Images
File Type: jpg moded u472A.jpg (101.9 KB, 78 views)

Last edited by btown2009; 3rd March 2013 at 07:07 PM.
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Old 5th March 2013, 01:58 PM   #55
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Default n60

in reference to the n60 .. i have a n60ea and pin 3 is not chasis ground but is a floating ground . in no way is it tied back to the chasis ... doing so caused a 60 hz hum also took out dc heater supply and replaced with a lm317 reg transistor that i built on a separate perf board. it is capable of up to 1amp of current . 5703 wants app.200ma of current to operate and will run a little hotter than a ac701"105 ma" . i have a c60 mic that i am replacing the ac701 with the 5703 tube ... not sure as of yet what the cathode resistor might need to be for 5703 tube anyone have any ideas. thanks
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Old 11th March 2013, 04:34 AM   #56
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I have never had luck with the 5703 tube, every time i attempted the mod i would get no sound. I never could figure out why though. The voltages were always correct, but for some reason it just never worked right.
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Old 11th March 2013, 03:17 PM   #57
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hey after checking with my mic cable attached it is connected to chasis ..i was like you i still had hum .both ends of the cable have to wired exactly the same'' my shield was broken right before where it entered the connector... did you replace the 8 mfd. 150 volt cap connected to low side of transformer.this cap checked good on checker but was bad . this caused me problems with lots of low end from the mic ..there is 5 wires in the mic power supply cable plus the sheild connect pin to pin and with pin 3 and 5 split the shield wire and connect this shield wire to pins 3-5 along with other wires... when i did this hum went away ..has to be something to do with this being an unbalanced line from the mic to the transformer in the ps.....i did have to replace my voltage regulator with a lm 317 .i'm real pleased with the 5703 tube ..i have neumann km54's and this little mic blowns them away . it's hard to beleive that a mic with this size capsule can make things sound bigger than they are.. hope this helps if i can help let me know
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Old 11th March 2013, 03:20 PM   #58
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. i didn't change the cathode resistor left everything the same .. with 5703 ..i liked the way the mic sounded so not planning on changing ..
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Old 11th March 2013, 05:30 PM   #59
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WEll Actually i've been playing around with a new mic body and using the 5703 in it, I have the transformer in the mic body as I have the room for it, I am also using a lm317 in the psu.
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Old 18th March 2013, 05:55 AM   #60
rmb is offline rmb  United States
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Good evening Chris..

Most of the 1 % metal film resistors are quiet by nature and individual testing may be a waste of time unless you encounter one that is undeniably bad. Again, you can use a test fixture connected to an amplifier and connect a resistor in circuit (i.e. FET gate or tube grid leak) and listen for noise between several identical value resistors.

The AU7A capsule in my modified mike needs reskinned. Diaphragm is kaput. Don't want to "break the bank" getting the capsule reskinned either. Also looking for some thin nickel or sputtered mylar....

I find with interest about the "purists" saying the U47 must have the VF14 to be useable....I would love to get hold of a dead U47 and experiment with various tubes....the 407A mod was done to keep a friend of mine's mic as close to genuine as possible where the mod could plug in like the AR47 kit....but if someone wants that "sound" of the metal VF14, I wonder how a 12SJ7 (triode connected) or another metal tube (6F5, 6J5, etc.) built into a U47 would compare in A/B tests against a VF14 U47....sooner or later, the VF14's and AC701's are going to be history and some preparation should begin now.....
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