wa5bxo at pctechref.com
Tue Mar 15 10:32:07 EST 2005
First of all there should never be an element of any tube that is
left open as far as DC is concerned, and typically, crystal microphones have
no measurable DC leakage so it is an open circuit to DC. If a Crystal
microphone is to be used on this circuit the tube would soon go to cut off
or thermal grid emission run away, depending on the tube condition. If the
tube has no cathode resistor for bias then I would recommend a few mega ohms
of grid leak resistor for contact bias and a coupling capacitor in case a
microphone such as a dynamic or some type of microphone that has a DC path
to ground is used. If the tube has a cathode resistor, then a lower value
of grid leak could be used as a load for the microphone. A 5 Meg
potentiometer is useful here with the wiper to the grid like a rheostat to
ground. Some microphones do better with a lower load resistor for damping
but crystal microphones will lose all low frequency response if loaded too
heavy with out equalization.
Matching impedances is confusing. If you match the source impedance
with the load impedance, maximum power will be transferred from source to
load. With microphones, we are not concerned in transferring power. It is
the response fidelity and ringing factors that we are concerned with. There
is a best load that is needed but it is hardly ever the impedance of the
source. See http://www.qsl.net/wa5bxo/power.html allow a lot of time for
the picture to finish loading.
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of StephenTetorka at cs.com
Sent: Tuesday, March 15, 2005 8:25 AM
To: amradio at mailman.qth.net
Subject: [AMRadio] Mike
The article cites a crystal mike; it feeds directly into the grid of a 6SJ7
with no grid leak resistor.
As I understand these things, the mike - what type of cartridge - needs to
put out 'so many' microvolts necessary for the tube into which it feeds.
And the impedance needs to 'match'.
I guess the first question is: what impedance number is required for a 6SJ7
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