[AMRadio] Mike


Jim candela jcandela at prodigy.net
Wed Mar 16 23:01:00 EST 2005


John,

    I been busy trying to reassemble my shack. Getting 2 tons of junk in a
1/2 ton of space is a challenge! I need to get a ladder to the attic for my
collection of tubes, capacitors, etc. I still got a 10 X 20 rented storage
room that is nearly 50% full. Might be time to trim the fat...


   Anyway the grounded plate pentode making a triode has been used before. I
seem to recall it being used with a 6AU6, and used as a low noise hi mu
triode where the signal levels are low, and s/n ratio is important. Maybe
somebody else has a better idea about this. I think I once read about this
on a VT audio reflector.

Regards,
Jim



-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net]On Behalf Of John Coleman, ARS
WA5BXO
Sent: Tuesday, March 15, 2005 9:28 PM
To: 'Discussion of AM Radio'
Subject: RE: [AMRadio] Mike


Jim,
 	That is an interesting circuit with the plate grounded and
output taken from the screen grid as an anode.  I don't quite see the
advantage other than fewer parts.  The gain seems to be about 10 db less
if I read it right.   Perhaps the screen to grid capacitance is much
lower than an equivalent triode.  As I see it a triode especially one
with a lot of plate to grid capacitance is going to represent a large
amount of "miller effect capacitance" to the crystal cartridge, similar
to what you and I experimented with by placing the 1000pf caps from
plate to grid of the 12AX7 input.  This must have been equivalent to
placing a .01 from grid to ground except it did provide some gain and
special control.  I would guess that a 10 meg grid leak resistor and 1
to 2 pf of plate to grid capacitance and a 12AX7 with a gain of about 40
would be about 100 pf to ground.  I not sure how the math works on all
that, but I do know that the higher the input resistance, or the high
the gain is, the more "miller effect" is present. This could be a
desirable thing for limiting high freq response while boosting the low
freq response do to a greater RC time constant from the crystal
representation the Bob described in another message.  On the other hand
a pentode connection with less plate to grid capacitance and miller
effect would not attenuate the high frequency components and the full
amplification of those signals would be realized with less phase
distortion, not that anyone cares much about phase distortion at 10 -
15KCs.

John Coleman
WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of Jim candela
Sent: Tuesday, March 15, 2005 9:33 AM
To: Discussion of AM Radio
Subject: RE: [AMRadio] Mike



Steve,

     I just was reading about this in my "Radiotron Designers Handbook"
where a simple preamp using a 6SJ7 was used with a crystal microphone.
In
short you can use a higher grid resistance to change the frequency
response
(more lows), but the text describes the limitations involving reverse
grid
current, dc operating point shift, signal to noise ratio, dynamic range,
and
so on. There also is some alternative circuitry shown to give you some
ideas
if you want to do some extensive modifications. I find it interesting
that
they show the 6SJ7 in triode mode using two different techniques.

    I scanned in several pages, and have posted them on the net at the
following link. The text is sideways, and you can turn image 90 degrees
CCW
with your Acrobat reader. Hope this helps.

http://pages.prodigy.net/jcandela/Preamps/Mic_Preamps.pdf


Regards,
Jim
WD5JKO

-----Original Message-----
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


Thanks Guys:

This mike is for a vintage 1948 AM rack rig homebrew by W2PLY who moved
into
Senior Residence.
The speech amplifier/modulator schematic is right from August 1947 QST.

I got the rack and the 5 sections directly from the flat bed trailer at
the
estate sale...and now just about finished doing what very little fixin
required.

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 understand there plenty of gain after the 6SJ7...and one of the 6SN7's
has
a "tone control" potentiometer in the grid.

Personally, my voice is a little higher from center in pitch..and I'd
prefer
more lower than higher freq. response...which is generally a problem due
to
low freq. rolloff in the mike curves...that's why I'm eager to try this
'tone control'.

I guess the first question is: what impedance number is required for a
6SJ7
grid?

Your generous help is always appreciated.

Regards,
Steve
WA2TAK
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