-----Original Message-----
From: Gary Schafer
Sent: Wednesday, August 07, 2013 8:36 PM
To: 'Jim Thomson' ; amps@contesting.com
Subject: RE: [Amps] NFB vs IMD question
Hi Jim,
According to Collins the IMD reduction is not quite equal to the amount of
feedback in an amp. In other words for a 10 db reduction of IMD you will
need a little more than 10 db of feedback.
73
Gary K4FMX
## Tnx. Now does that pertain to just tube amps only....or does it include
SS amps ? The arrl SS amp is now finished. IMD3 + IMD5 is -40db below
one tone. Then it drops off...and ends up at -60db for IMD15. SS amp
being operated at 200w pep out. Total idle current for the pair of
VRF-151Gs
was 800 ma. IMD got worse when idle was increased to 2A.
## A 6db pad was used between the K3 and the SS amp. The K3 was being
run at 30w pep out..which is its sweet spot for imd. Here are the k3 imd
results.
IMD3-11 varied from -52 to -55db....below one tone. IMD13-15 was -60db
below one tone.
## whats really needed is to use two separate RF sources + a hybrid
combiner, to
conduct the 2 tone test of the 250w SS amp. The SS amp cant be any better
than the k3
that is being used to drive it. Still, in normal operation, the SS amp
will be driven by the
K3. So from a systems point of view, what goes over the air is the combo
of the K3 plus
SS amp.
## What will be tried next is to increase the idle current > 2A... which I
doubt will improve things.
The tone spacing will also be varied, while watching the HP spectrum
analyzer. Funny thing is,
when the 800 ma idle current was being tweaked, the IMD3 + IMD5, etc, would
shift about.
At certain bias settings, the IMD3 would drop a bit, while the IMD5 would
increase etc.
## The resistors in both the base + the emitter can be tweaked to adjust
the NFB used. So
one, the other, or both can be tweaked for increased NFB. That will be
tried in the next few days.
## As is, -40db below one tone with 200w pep out for IMD3 + IMD5 is pretty
good.... vs the typ
200w pep out ham xcvr. My yaesu MK-V is typ -32db below one tone for IMD3.
The higher
order IMD products on the hb SS amp appear to be typ 6db lower than my yaesu
mk-V.
Stay tuned....... Jim VE7RF
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