[Johnson] Viking 1 "gotchas" ?

[Patrick Thrush]

Great info Richard.  There are quite a few variables that one encounters
with the D-104 and other piezo mics.  For the most part, the sweet spot for
impedance is around 10M Ohms.  Anything from 1M up is closer suited to the
overall input impedances of the vacuum tube circuit.

A lot of the crystal elements have not aged very well.  Shock, moisture and
heat are well known destroyers--freezing temperatures can do the same.  Even
under the best of conditions the salt block has a tendency to bend and
create fractures.  These totally kill the low end response.  Rebuilders are
often surprised to find when a mic exhibits low output that there is a
ceramic element in place.  The overall performance of these elements is not
as good--but they are certainly much more hardy.  Golden screwdriver folks
can take a share of destruction also.  I have opened several heads to find
pinholes all over the aluminum diaphragm.  This was a chicken band folklore
"improvement' that essentially ruins the element.

Mine is dated 1972 (stamped on the cartridge).  No one likes the sound of it
on the SB-401, but it showed great performance with the Valiant.  Just
another reason to have lots of microphones!


73,
 
Patrick Thrush, AE1PT
Bath, NY  
http://www.qrz.com/db/AE1PT
42.333  -77.309   CQZ 5  FN12ih
SKCC #4473
Keuka Lake Amateur Radio Association
ARRL, ARRL VE



-----Original Message-----
From: Richard Knoppow [mailto:1oldlens1 at ix.netcom.com] 
Sent: Friday, February 22, 2013 1:25 PM
To: bcarling at cfl.rr.com; burnsguitar at yahoo.com; patrick at ae1pt.com;
tbs50a at aol.com
Cc: Johnson at mailman.qth.net
Subject: Re: [Johnson] Viking 1 "gotchas" ?


----- 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