[Amps] 12V IM3

[Joe Subich, W4TV]

On 2015-05-22 2:23 PM, Mike Waters wrote:
> What bugs me is that most (if not all) SS amps that are overdriven
> generate considerably more IMD than most tube amps would if
> overdriven by the same amount.

First, tube amplifiers (except for the sweep tube variants) have more
headroom than a SS amp.  Second, the tube amplifier saturates slowly
while the gain curve of solid state devices have a very pronounced
"knee".

> I can pick out an overdriven SS amp almost as soon as I hear the
> signal. And when we tune 3 kHz away and the signal is still fairly
> strong, there's almost zero doubt that there's an SS amp behind that
> trash.

If you want an easier way to tell ... just tune to the "opposite"
sideband or look at the other side of the suppressed carrier on a
panadapter/spectrum analyzer.  Overdrive/IMD causes the suppressed
sideband to be regenerated.  That was one of the reasons analog (NTSC)
TV transmitters required predistortion - to keep the signal in the
channel.

 > ... The band will not get better until the signals get clean.

The signals will not get clean until ARRL Labs stops being a lap dog
of the manufacturers.  They need to be very loud in condemning  rigs
with IMD worse than -36dBc (-42dB PEP), the rigs with really terrible
transmitted phase noise, rigs with "wide" (> 2.7 KHz) transmit audio
response, those with user menus that allow CW rise time to be set in
such a way as to guarantee terrible key clicks and those with FSK
that is two to three times wider than necessary.

73,

   ... Joe, W4TV


On 2015-05-22 2:23 PM, Mike Waters wrote:
> Well stated. However, at the risk of sounding pessimistic, I think that's
> wishful thinking.
>
> What bugs me is that most (if not all) SS amps that are overdriven generate
> considerably more IMD than most tube amps would if overdriven by the same
> amount.
>
> I can pick out an overdriven SS amp almost as soon as I hear the signal.
> And when we tune 3 kHz away and the signal is still fairly strong, there's
> almost zero doubt that there's an SS amp behind that trash.
>
> I think most people on this list know exactly what I mean. But I don't
> think there's a single thing we can do about it.
>
> 73, Mike
> www.w0btu.com
>
> ... The band will not get better until the signals get clean.
>>
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