[AMRadio] EQ matters


John Coleman wa5bxo2005 at pctechref.com
Tue May 24 10:33:26 EDT 2005


Hi Paul:
	Yep, the problem you're speaking of has plagued final Class C rig
using tetrodes for a long time.  The Tetrodes / Pentodes are easily
neutralized but the drawback is trying to balance the audio levels on the
plate and screen.  I did a lot of playing and research on this.  One of the
problems is the ratio of "percentage of modulation" on the screen grid.
Many rigs use a screen dropping resistor from the plate modulated source.
This means, that when the plate voltage doubles so does the screen, and when
the plate voltage hits zero so does the screen.  This is not good and causes
non linear modulation.  Here is an example which may not be 100% accurate
but it is a place to begin.

Static Plate voltage = 2000 VDC
Static Screen Voltage = 300 VDC
PTP audio on the plate = 4000 VPTP for 100% Modulation
PTP audio on the screen = 300 VPTP or about 50% not 600 VPTP

	You can achieve the reduction in audio by using a resistor (equal
value to the screen dropping resistor) and a very large capacitor (large
enough to pass all the audio frequencies) in series and then placing this
series network from the screen to ground.  This will keep the screen DC at
300 VDC but will load the audio voltage on the screen down by 50%.  It will
also reduce the effective impedance at the screen.  This reduction in screen
load impedance will help reduce the phase shifts of the higher audio
frequencies caused by the screen RF bypass capacitor.

	The audio on the screen must be in phase with the audio on the plate
at all modulating frequencies or it will detract from the modulation and
cause second harmonics in the audio of the higher frequencies.  This is
extremely difficult as you go beyond 3KHZ.

	Proper neutralization even if the tube is a pentode/tetrode is also
important in the reduction of phase modulation of the carrier and production
of phasing products that cause signals to be wider than is necessary.

	These are a number of reasons that you see triodes in the RF of most
HIFI broadcast equipment of yesteryear.

	I think you can see that it is not just the plate bypass that
reduces the frequency response. 

John, WA5BXO


-----Original Message-----
From: amradio-bounces at mailman.qth.net
[mailto:amradio-bounces at mailman.qth.net] On Behalf Of VJB
Sent: Tuesday, May 24, 2005 6:39 AM
To: amradio at mailman.qth.net
Subject: [AMRadio] EQ matters

Thanks for posting about the EQ. I swear the check's
in the mail for your nice comments there, P.J. And I
totally agree that there's a lot of satisfaction from
achieving a warm sound on transmit and receive.

John to your point on linear response, yes! and it
remains a problem in my T-368, which has a tremendous
amount of bypass capacitance in the RF tank, ruining
any high end response. I can boost the highs at the EQ
and it helps overcome that rolloff to some extent,
then it just gets dirty so I back it down. The
solution is to find the time to re-work the RF tank as
documented many places on the internet.

On the other hand, how many times have you heard
someone running a very peaky microphone on a rig that
already has a peak in the midrange?  Makes things
sound WORSE !

Paul/VJB
Annapolis


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