At some time in the past tube amplifiers enjoyed an effect dubbed
the 3/2 law. It had something to do with causing higher order IMD products
to increase slower than the mathematics predicted. The net result was that
they went down faster than the IM order predicted.
Unfortunately this doesn't happen with transistors, so that actually the
spurs crop up very quickly and unpleasantly.
Here is an extract from John Harper's very good paper:
"The plate curves follow quite closely a 3/2 power law, in which the
current increases as a function of the 3/2 power of either the grid or the
plate voltage. This is especially true at high currents and in the lower
(closer to zero) range of grid voltage. While this is not linear, it is
closer to linearity than any solid-state amplifying device, which is the
main reason why tube amplifiers are nowadays often considered (and certainly
by most of you who will be reading this) to be sonically superior. At low
currents, especially as the grid becomes more negative, the 3/2 power law no
longer applies and the plate curves "tuck under" very noticeably. The curves
are not equally-spaced, even at high currents, but rather get closer
together as current increases. All of this is causes extra non-linearity and
distortion. This is more true for some tubes than for others, and later on
we will look at some of the reasons."
You can't beat physics!!
Alex 4Z5KS
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Roger (K8RI)
Sent: Thursday, October 10, 2013 2:33 AM
To: amps@contesting.com
Subject: Re: [Amps] Regulations
On 10/9/2013 6:35 PM, peter chadwick wrote:
> Roger,
>
> It just so happens that at 1345 UTC this Saturday, I'm giving a
> presentation at the RSGB National
Convention entitled 'Spreading the Sewage' - we didn't think that 'Spreading
the S**t' was quite appropriate! I need to make sure that I have at least 2
if not 3 pints of beer available for refreshment during the 45 minutes -
although the other year, someone wrote to the RSGB General Manager
afterwards to complain about me drinking beer during the presentation and
livening it up with
a dirty joke!
>
> I have analysed the QST reviews of 123 transmitter/transceivers since
> 1973 for this, and the
frightening thing is the way that while tube rigs in general had negligible
7th, 9th, 11th order IMDs, the solid state rigs do - and then some. Both the
FTDX5000 and the FT1000D running in Class A are good.........but, I gather
get hot.
My 5000MP only gets warm if kept at 75W. Of course in class A reducing
power will cause more dissipation and thus more heat.
>
> As they say in Yorkshire "Tha don't get owt fer nowt".
Yah know, I actually understood that. <:-))
Now many years ago I knew a rather cute gal from near London...At times she
sounded like a foreign language.
>
> The results make me glad I still run a tube PA transceiver!
It's also one of the reasons I'm interested in QRO via tube.
I see lots of "claims" about the IM out of some of these new SS amps.
They don't appear simple and they are expensive! <:-))
What I don't understand is all three of the major manufacturers have rigs
with receivers that have sensitivity and dynamic range far in excess of
being useful on the low bands
What good does 120 db dynamic range, sharo selectivity, and sensitivity at
least 3 times greater than the noise floor do when you are on 160, 75, or 40
and the guy 5 up is 15 KHz wide? His signal is in your pass band.
I think it was on the FTDX5000 group a few months back. There was a ham
complaining about that specific situation. He couldn't understand why his
state of the art receiver was hearing this 20 over 9 signal 5 up when he was
trying to copy a weak signal.
Many tried, but I think he still blamed the receiver and that situation will
continue to exist until they clean up the transmitters. Being as we went
through several decades of relatively inexpensive bipolar rigs I don't see
that happening any time soon.
Seems like I read the new 990S from Kenwood is in the low 40s for IM3.
Of course you probably realized this is one of my "hot buttons" long
ago.<:-))
Good luck on the talk.
73,
Roger (K8RI)
>
> 73
>
> Peter G3RZP
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>
>
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