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@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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