[AMRadio] Kenwood MC-50 mike with AM gear


Gary Schafer garyschafer at largeriver.net
Mon Jan 12 22:41:27 EST 2015


Hello John,

A little story about mikes. A long time ago I had a heath SB100 transceiver
that I had a dynamic mike hooked to. The mike had a transformer in it.
Everything worked fine.
I decided to hook up a D104 crystal mike to the rig. It had low and
distorted audio. I went to the radio store and got a new element for the
D104, same thing. Took the element back for another one and it was the same.
I finally swapped out the first audio tube and everything worked great.
Turns out that the first audio tube (6U8 I think) had grid leakage. With the
transformer mike it held the grid at ground (the grid was direct connected
to the mike jack thru a resistor) and the bias was correct. 
With the crystal mike it allowed the bias to drift up and put the tube into
saturation and killed the audio.

73
Gary  K4FMX

> -----Original Message-----
> From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of John
> Coleman
> Sent: Monday, January 12, 2015 3:33 PM
> To: 'John Neeley'; amradio at mailman.qth.net
> Subject: Re: [AMRadio] Kenwood MC-50 mike with AM gear
> 
> Hi John, A lot of the old rigs did not even have a coupling capacitor to
> the
> grid of the preamp stage.  This is of no consequence when using a
> crystal/ceramic microphone, but other mics may drain your grid bias on
> the
> preamp tube and cause a lot of distortion.  If the coupling cap is not
> there
> put one in. .01 to .05 is good.  Also a lot of the modern microphones
> have
> way to much bass response for the old rigs modulator and modulation
> XFMR. A
> coupling capacitor in series with the microphone cable and a resistor
> across
> the cable on the rig side of the coupling capacitor will reduce the low
> frequency response. Start with a .05 uf and 100K resistor. Decreasing
> the
> value of either will reduce low frequency output and vice versa.  Don't
> worry about matching impedance.  The 1 or 2 meg resistor at the preamp
> grid
> is there for what is known as contact bias. And it works well with the
> crystal/ceramic microphone as they actually  have their best response
> with
> no load or very high resistance.
> 
> Good Luck,
> John, WA5BXO
> 
> -----Original Message-----
> From: AMRadio [mailto:amradio-bounces at mailman.qth.net] On Behalf Of John
> Neeley
> Sent: Tuesday, January 06, 2015 11:25 AM
> To: amradio at mailman.qth.net
> Subject: [AMRadio] Kenwood MC-50 mike with AM gear
> 
> I have a Kenwood MC-50 mike and am wondering if I can use it with my
> older,
> tube type AM gear, like the HT-32a, Valiant or DX-100?  Otherwise if
> not,
> will need to get a old D-104, etc.  Any info would be greatly
> appreciated.
> 
> John W6ZKH
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