[AMRadio] Resistive load on interstage audio transformers in

John Coleman jc at pctechref.com
Wed Mar 12 16:46:01 EST 2008

Hi Don:
I have never read any info regarding this but my experience has been that
there is a little better frequency response when a class "A" grid XFMR is
loaded slighted with a resistor but not necessarily the stated "Z" that the
manufacturer may say in specifications.  I generally end up with a much
higher resistance than that indicated by the Z ratio.   I believe the idea
is to prevent ringing and de-resonate the XFMR slightly.  The load resistor
will reduce the Q of the XFMR coil circuit.  Series resistors from the
secondary to the control grids also provide a de-resonating effect as well.
This is especially true if the tubes being driven are triodes.  There will
be certain amount of 90 degree feed back from the plate of the tube being
driven back to its control grid where the driver XFMR may resonate with it.
This is the same reason that we use neutralization circuits on RF
amplifiers.  There were even HIFI amplifiers designed in the past using all
XFMR coupling in push pull circuits and cross neutralization capacitors
where used to neutralize the effects of the above mentioned feed back and
peaked frequency response.

Consider however that the primary of the XFMR is connected to a device
producing the current change, which will have a back loading effect of its
own.  In tube circuitry this would be the plate resistance of the tube.  Not
to be confused with the recommended load impedance.  In triode drivers
(12AU7 for example) plate resistance is typically much lower than with
pentode drivers (6AU6 for example).  The triode will have less gain as a
rule but it will create back load for the XFMR to help prevent resonate
peaks and ringing.  The pentode can do the same thing with a little proper
inverse feed back but again, this will lower the gain. But be careful not to
build an oscillator accidentally. 

John, WA5BXO

PS: I had sent this earlier but with the wrong account so I apologize if
come through twice.  I am experimenting with GMAIL

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