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Author Topic: Mic for my AM station?  (Read 4462 times)
W5KG
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Posts: 73




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« on: April 26, 2013, 07:41:22 AM »

I am trying to "build" an AM station. I have a Hammarlund 145A and a Dx60. I was wondering what mic would be the best to use. I don't have Hi-Z mic and I do not like to use a hand mic.
So what do ya'll think would be a good one, but reasonable one to use?
Thanks
Don, W5KG
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K8AXW
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Posts: 3756




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« Reply #1 on: April 26, 2013, 08:09:34 AM »

Try an Astatic D-104.  They're still available but before you buy you should find out if the element is good.  Don't expect broadcast quality from the DX-60.
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W5KG
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« Reply #2 on: April 26, 2013, 08:52:34 AM »

Thanks for the input Allen. I had thought of a D104 and I have used them off and on for about 35yrs. And you can change the head on the things with 5 different elements and get a different sound from each one.
Some good and some that sounds bad.
I also know that the DX60 is not the greatest transmitter but I don't know if I would even like AM but maybe a better one at some point.
Don, W5KG
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WB2WIK
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Posts: 20574




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« Reply #3 on: April 26, 2013, 10:02:35 AM »

I built a DX-60A back in the mid-60s and used it on AM a fair amount for a while; the D-104 seems a good choice (it's what I used, also) because it has a light of "brightness" to it (high frequency response), and the DX-60 unmodified is a little bit "dull" sounding (too many lows) -- the D-104 "brightens it up" a bit and sounds pretty good.

Of course, there are modifications that can be made to almost anything including the DX-60 to make it sound a bit better than "stock."

Why not try the D-104 and see how it works?  The AM guys who are rabid AM enthusiasts (quite a lot of them!) are all into "modulation," and will give you some critical reports.
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K4KYV
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Posts: 40




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« Reply #4 on: April 26, 2013, 08:25:16 PM »

The D-104 can sound very good.  And so can the DX-60 with some modifications.  I have heard one or two DX-60s that had broadcast quality, and if the op had told me he was using a converted broadcast transmitter on the ham band, I would have believed him.

With the D-104, the load impedance that the crystal element works into must be at least 5 megohms (the value recommended by Astatic).  Mine works into a 20 megohm load.  If you are using the "power mic" version with the built-in pre amplifier in the base, I would recommend searching the web for a replacement pre-amp, or homebrew one yourself; the stock Astatic pre-amp does not fully accommodate the audio quality that the D-104 crystal is capable of.

Most older tube type ham rigs have a high impedance mic input, but the grid resistor of the mic pre-amp stage is usually much lower than 5 megohms, often between 1 megohm and as low as 50,000 ohms. I would recommend replacing the stock resistor with a 4.7 megohm one.  The purpose of the amplifier in the base of the mic stand is not to increase the output of the microphone (the bare crystal element has plenty of output), but to allow the high impedance crystal element to work with a CB transceiver or a modern ham rig, which almost inevitably has something like a 500-ohm mic input impedance, and the high impedance crystal element simply would not work directly into such a low-Z load.

In summary, if you are planning to use the D-104 with an older tube type rig, go into the mic and by-pass the amplifier in the base if it has one, and wire the crystal element directly to the shielded mic cord. If the grid resistor in the first audio stage is less than 5 megohms, replace it with a 4.7 megohm resistor.  If your rig is designed for a low impedance microphone such as a dynamic, search the web for a better pre-amplifier to replace the one in the base of the microphone.
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K2OWK
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Posts: 1055




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« Reply #5 on: April 26, 2013, 11:53:51 PM »

The D-104 works great on old or new transmitters. I use one on a FT-450AT with excellent audio reports. Now to the Heathkit DX-60. If I remember right it uses controlled carrier modulation (same as the DX-35 and 40). The modulation sounds decent if the signal is strong, but not so good if the signal is weak. The fix is to plate modulate the transmitter. I used a pair of 1625 tubes in a home brew plate modulator and got broadcast quality reports on am with my DX-35. Should work just as well on the DX-60.

73s

K2OWK
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W9GB
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Posts: 2616




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« Reply #6 on: April 27, 2013, 11:13:57 AM »

Since you have a Heathkit DX-60, I like the Electro-Voice 638!

The Electro-Voice 638 microphone head has NO PTT function !!
The Model #423A micropone stand was normally sold with the model 638
by Heathkit as the model: HDP-21A
http://www.heathkit.nu/heathkit_nu_HDP-21A.html

The Heathkit Desk Microphone HDP-21A came on the market in 1968. It was really an Electro-Voice 638.
It was the successor of the HDP-21 (Electro-Voice 605) that came on the market in 1962.
The HPD-21A was then replaced in 1976 by HDP-121 (Electro-Voice 619) and
 by the HDP-121A (Electro-Voice 620) in 1978. It is a Hi-Z dynamic microphone with crystal clear audio.

==
NOTE: The EV 638 was sold in TWO versions: Hi-Z (~ 20 Kohms) and Lo-Z (150 ohms)

The Hi-Z version shipped with 6 foot, 1 conductor, shielded cable with Amphenol MC1M connector (Switchcraft makes an equivalent today) at end.
The Lo-Z version shipped with 6 foot, 2 conductor, shielded cable with Amphenol MC2F connector at end.

Electro-Voice 638 Desk microphone, shown with 423A stand.
http://www.coutant.org/ev638/index.html

Electro-Voice 638 data sheet
http://www.coutant.org/ev638/ev638.pdf
==

w9gb
« Last Edit: April 27, 2013, 11:16:58 AM by W9GB » Logged
N4NYY
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Posts: 4748




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« Reply #7 on: April 28, 2013, 04:58:54 PM »

Unamplified D-104, or Shure 444 would work nicely. Maybe a Hi Z Turner, too. You can likely mod a Heil if need be, but I would start at the other mics.
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ZS5WC
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Posts: 410


WWW

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« Reply #8 on: April 30, 2013, 04:14:41 AM »

 :)Astatic JT-30   crisp and clear.
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W5KG
Member

Posts: 73




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« Reply #9 on: May 01, 2013, 07:26:39 AM »

Thanks to all for the input. I have an Electrovoice but it is low-z. So I have bought a D-104 and it should be here in a few days.
Again thanks to all
Don, W5KG
btw, I will do the mods after I get the DX60 going. So far all checks out but the 6146 is soft. I did buy one crystal (3890) so I could check it out. Nice tone on CW but I will see about AM after I get the mic.
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W8AAZ
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Posts: 335




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« Reply #10 on: May 06, 2013, 06:01:46 PM »

Well if you had a nice dynamic mic with an XLR balanced output, you could buy one of those little Shure inline transformers that are XLR input and quarter inch phone plug output with high impedance.(18$ or so?)  However beware that alot of dynamics that have cardiod patterns may boost bass too much, close talking. Like, the Shure 57/58 is an excellent voice mic until you get too close and the bottom end jumps up alot.  An Omnidirectional dynamic will not do that.  The whole world of crystal/ceramic vintage mics and even vintage hi Z dynamics is open to you, but beware that enough of the old mics have gotten weak or are dead to make testing or a no DOA guarantee important. And I have seen alot of old D 104 mics with pinholes perforating the aluminum diaphram.  CBers did it on claims that sounds better (!) and maybe just idiots or kids poked the rest.  Lets air/humidity reach the crystal and sometimes that ruins them. Or the diaphram comes unattached around it's perimeter and that has the same problems.
« Last Edit: May 06, 2013, 06:05:29 PM by W8AAZ » Logged
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