[Johnson] Viking 1 "gotchas" ?

[Richard Knoppow]

----- Original Message ----- 
From: <tbs50a at aol.com>
To: <bcarling at cfl.rr.com>; <burnsguitar at yahoo.com>; 
<patrick at ae1pt.com>
Cc: <Johnson at mailman.qth.net>
Sent: Friday, February 22, 2013 7:53 AM
Subject: Re: [Johnson] Viking 1 "gotchas" ?


> Hi Brian and Bry,
> One the V1 a few thinks come to mind. Bry mentioned the 
> final toob.Oringinally it used a 4D32 but at one time they 
> where very expensive is Johnson came up with a mod to use 
> a 829B.The output is a little less abt 85wt vs 120wts. The 
> 4D32 was abt $100 vs $10 for the 829B. In the 1990's a 
> bunch of the 4D32's hit the surplus market at $20.\
> Always make sure that you have grid drive to the final 
> before you apply the HV. Originally the V1 didn't have a 
> clamp toob so you can wasted the final.
> Three things come to mind on the electronics.The voltage 
> divider resistor R13? 15K 50watts Usually open..When open 
> you won't have screen grid voltage on the mod & final 
> Clean the meter switch really well. And pull out the 
> roller inductor and clean it too.
> Bry, On your D-104 I read somewhere that in the cartige 
> there is a diaphragm with a pin in the center of it. That 
> pin has a wax seal on it. Use heat to reflow the wax on to 
> the pin. That should fix your low fq problem. Long time no 
> chat OM.Have been in bed all week with a bum back. If you 
> could say a few words in prayer that would be wonderful!!
> GL&GB
> Terry N3GTE
>

     Most crystal microphones and headphones, at least the 
ones with diaphragms (there is another kind) drive the 
diaphragm with a pin. I would be very careful of using heat 
because heat can destroy the crystal element.  They are 
sensitive to heat and moisture.  One of the features of the 
Astatic crystal mics was their method of sealing the element 
to prevent moisture damage.
     The D-104 does not have a lot of low end; this one may 
be normal. I would compare it to another to be sure.  The 
D-104 was designed for voice communication purposes and has 
a steeply rising response curve with a fairly sharp peak at 
about 4 khz. It can sound metallic or tinny.  Also, the load 
impedance has a very strong effect on the response of a 
crystal mic; the element looks electrically like a capacitor 
so any parallel resistance makes it a high pass filter. 
Crystal mics want to work into a very high impedance, the 
closer to infinite the better, but a megohm or more anyway. 
Many mic pre-amps present too low an impedance causing the 
low end to roll off. Since the D-104 does not have a lot of 
low end to begin with it becomes very tinny.  Putting a 100K 
resistor in series with it may help.  Also cable capacitance 
is important; excessive capacitance rolls of the high end 
and reduces the output level. Short cables of 
low-capacitance co-ax are necessary.
    You can probably do better with another kind of 
microphone. Crystal mics were popular because they were 
relatively inexpensive for the quality and because they put 
out very high levels. The latter was of importance when 
electronic amplification was expensive.  The construction is 
simple compared to a moving coil or ribbon dynamic mic but 
the usual conical metal diaphragm limits the quality.
    Long ago "sound cell" type crystal mics were available 
where the crystal element was directly exposed to the sound 
waves. These had quite high fidelity and extended 
high-frequency response but very low level.  Brush 
Development and some other companies made sound-cell mics 
for measurement purposes and occasional use in sound 
recording. They are essentially obsolete now.
    FWIW, I have two D-104's, both sound exactly alike and 
both sound tinny to my no longer very good hearing. 
Currently, I am using an Electro-Voice 664, which gets good 
reports.  There are newer mics with similar characteristics.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com