[Amps] A tale of two IMs What happens?

Wed Apr 14 16:24:08 PDT 2010

Thanks Jim,

For some strange reason it took two days for this to show up in my "inbox"

Remarks interspersed below.

73

Roger (K8RI)

Jim Thomson wrote:
> Date: Sun, 11 Apr 2010 14:45:24 -0400
> From: Roger <sub1 at rogerhalstead.com>
> Subject: [Amps] A  tale of two IMs What happens?
>
>  "No one has touched the question on how reducing power without returning 
> affects IM I now have another one.
>
> Given that most of todays transceivers have an IM or roughly -35db "so 
> I've been told",
>
> ###  You can look em ALL up on qst's extended lab reports. Any given
> xcvr, will have imd figures, that vary all over the map..depending on
> band... with the worse case usually  being 12m  for some reason. The
> QST results only depict the worse case imd.  The extended lab  results
> will  show all  9 x bands.. one at a time.  In the case of the kenwood 870,
> they also did it 3 x times.. just for 20m.  IMD  was -30db pep [-24db below one tone]
> at 100 w pep.. which improved a whole bunch, when power dropped  down to 85W.
> Then improved a whole bunch more, when tested at 50 w.  -30db pep is what you 
> get from a drake T4XC  with sweep tubes, from 1977 era.  And in order to pull that
> off, idle current was 70 ma @ 700 vdc = 49 watts.   Max 2x tubes diss is only 52 watts.
> So they had to idle em at 99% of their diss ratings.. just to get the lousy imd resuts they do. 
>
>
>
>
>  and we put a amp behind it that ALSO has an IM of 
> -35db, what is the resultant IM?
>
> ## I researched this out yrs ago... and the consensus was , at best it would be the
> same... and at the very worse, it could degrade imd  by 6db max.  In most cases,
> it appears to degrade by 1-2 db.   
>
>
>
>
>   What if the amp has an IM of -55db?
> Do they add, subtract, or go with the lowest number? 
>
> ##  they go to the lowest number.  A  -30 db pep imd  xcvr, driving a -57db pep
> 3CX-3000A7  will result in  -30db imd  at the very best...and  -28db  at the very
> worse.   That's  dealt with ad nauseum in ..." SSB systems and circuits" .  The same
> classic book also explains why the 2 x tone imd test is flawed. The IMD3.. and IMD5
> will  appear to be  lower than what it really is.  The  IMD7-9-11-13  products  will tend to
> cancel out the IMD3  product, leaving bogus results in any 2 x tone tests.    The test 
> is flawed anyway, since everything is in a steady state condx.  Neither the B+.. nor the
> bias vdc, screen vdc, etc, etc, is varying at all.   It's not a dynamic test.  Also, by carefully
> varying the SPACING  of the 2 x tones,  you can hit a ..'sweet spot' ... and get better
> imd results.  A steady state 2 x tone imd test.. won't factor in ALC overshoots, overdriving
> input audio circuits,  overdriving everything else down stream from the mic jack etc. 
>
> ##  If you use the ALC cable  from the xcvr to the amp.. and the  xcvr's ALC  has a different time
> constant than the linear's  ALC,  you just lost it right there.   
>
>
>
>  IOW it is the amps 
> job to "faithfully" reproduce the input signal, but that really only 
> happens when running class A, if the user is lucky.
>
> ## Correct.  So if you really want -42 db pep imd  from the linear... the xcvr
> had better be -42db pep imd as well.  Most of the yaesu class A  xcvr's  will
> far exceed -42db pep.. by a whole bunch.... so then you end up with the
> amplifier being the limiting factor.  What they need to do with these Class A 
> xcvr's is to incorporate either EBS in em [ with some type of TRI-state bias] 
> so they don't burn up... OR use some kind of 'sliding bias' scheme... like 
> Krell does in their Class A  stereo amps.   My yaesu MK-V   sucks 10A @ 30 vdc
> when in class A =300w diss !   That's  10A.. whether you apply audio drive or not! 
> When driven with a  1 khz sine wave, the mk-v puts out 75w.  300-75= 225 w diss.
> So the xcvr runs HOTTER, when keyed, but no audio applied. 
>
> ## They need adjustable bias too.  No point in having -63db pep imd, in Class A...
> when the amp is only good for -38db.   They need to have their bias tweaked, so the
> xcvr imd is the same.. or slightly better than the amp..... and incorporate sliding bias,
> or tri/quad state EBS  bias schemes. 
>
>
>
>
>
> Intuitively, "I would think" that the two figures would add, but if that 
> were the case the amp with -35db and the exciter with -35db would have a 
> pretty ratty signal. OTOH in the case of the -55db amp behind the -35 db 
> exciter does the amp "clean up" the exciter signal? Doesn't seem likely.
>
> ##  see above. The amp cleans up nothing.  In SSB systems and circuits,
> they hate GG triodes  driven by a high imd xcvr. ['ham stuff']  The writer's of the book
> [collins engineers]  prefer grid driven tetrodes.  Then the  drive power requirements
> are so low, that it's easy to  run the driver in Class A.  
>
> ## a buddy across town built a hb  4CX-1500B  3 yrs ago.. grid driven.  He can get an
> easy 1500w  out of it.. with just 10 watts of drive... and a 50 ohm globar resistor across
>  the input of the amp.  If a 200 ohm resistor was used.. and a 1:4 un-un... the drive power
> would be even lower still.  As is... 10 watts is obtainable from Class A.. from yaesu 
> xcvr's. 
>
>
> It'd be interesting to see the IM figures for today's exciters, (and 
> amplifiers) rather than than adds that just say "Provides amazingly low IM".
>
> ## Both the amps and all the xcvr's that have been tested in the arrl,
> have been written up in both the qst.. and esp the extended lab reports.
>   
I'll have to spend some time on their site and look them up.
> and lots of thin fins works better than fewer thick fins.  Try  sticking steel, AL, then CU
> into a grinder, and you will soon see [and feel] the super fast heat transfer of CU.
>   
Been there and done that.  OTOH it's not a good idea to stick either Al 
or Cu against a regular grinding wheel.
> later........ Jim   VE7RF
>
>
> I also see I need a refresher on working with logs.<:-))
>
> 73
>
> Roger (K8RI)
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