[Premium-Rx] Recievers for MW and 160 meters.

Fri Feb 11 16:24:50 EST 2005

There was a highly modified 1947 Hammarlund Super Pro 400 that came
through here a few years ago.  The condition was terrible (sub basement
of chicken house storage?) so I did not try to make it run.  It used a
6DJ8 frame grid 1st RF amp, and 7360 sheet beam mixers.  I can't
remember what the IF amps were, but they were miniatures.  Audio was a
6F6 driving a pair of 6F6s, all triode connected.  Hammarlund used front
panel controlled variable coupling IF transformer that allowed very wide
to moderately tight filtering.  The HFO was a voltage stabilized
miniature triode.  For MW dxing it should have been pretty impressive.

Barry 

________________________________

From: premium-rx-bounces at ml.skirrow.org
[mailto:premium-rx-bounces at ml.skirrow.org] On Behalf Of w3jn
Sent: Thursday, February 10, 2005 6:23 PM
To: premium-rx at ml.skirrow.org
Subject: Re: [Premium-Rx] Recievers for MW and 160 meters.

Brian, good points.  Tight RF selectivity is ALWAYS good.  The reason
the NF and dynamic range isn't that great on the HRO is due to its
prehistoric 6K7 RF tubes, and the abysmal 6A8 (6B8?) mixer.  A popular
modification of the day was to replace the 6K7 or 6K7 with a 6AK5,
thereby improving the noise figure by some 15-20 dB.

One of the best mixers ever is the 7360 dual sheet-beam tube.  The
Squires-Sanders SSR-1 used this tube as a mixer - and it didn't even
need a RF tube to achieve relatively good .2uV for 10 db S+n/n ratio,
but you could blast a couple volt signal 10 kC from a 10 uV signal and
the weaker sig wasn't even affected.  I built a homebrew RX using this
tube as a first mixer, directly driving 1.4 MC xtal filters pirated out
of a Racal 6217, and its performance really isn't matched by anything
else I've used.  Any of the DSP receivers I've used really don't hold a
candle to some of the late tube designs.

73 John

	----- Original Message ----- 

	From: Brian D. Comer <mailto:bcomer at cox.net>  

	To: premium-rx at ml.skirrow.org 

	Sent: Thursday, February 10, 2005 11:12 AM

	Subject: [Premium-Rx] Recievers for MW and 160 meters.

	I realize that most of us always want to have a receiver that
does everything but it seams to me that if you want the best performance
at these frequencies the high IF HF receivers are far from optimum.

	The problems in building narrow band filters at frequencies
greater than 30 MHz are much greater than at lower frequencies. Phase
noise increases by the square of the frequency so using an LO that is
about 40 times what is needed makes no sense. The diplexers that have
had all the discussion are not realizable at 40 MHz with narrow band
filters if one looks at close in stuff. The only passive ways around
this that I know of are complex and expensive. Following the mixer with
a broad band amplifier is the most common solution to this problem. 

	The point made by Micheal G8MOB that some of the older receivers
using a similar architecture to the HRO AR88 etc. may be a better choice
is a good one. However the HRO that I have has an IIP3 of 9 dBm and a
noise figure 15 dB worse than that of my Orion with the RF amplifier
turn off. This results in about a 30db lower dynamic range than that of
the Orion.  I am certain that a receiver made with this architecture
using today's components would make the best High IF HF receiver look
very bad.

	Allowing for the fact that a lot of the noise at these
frequencies is aggravated by highly selective filters I believe that the
best approach today for these bands would be the use of constant delay
filters for some pre-selection and direct DSP. 

	I think we have a bit of a dilemma that our interest in radios
is somewhat based doing things the hard way. We like to use these bands
more for their challenges than their advantages.  We find it hard to
part with the feeling of the tuning knob that had to turn a four gang
tuning capacitor and hence needed some very nice anti backlash gears and
a flywheel. Now we have to put up with a shaft encoder that runs so free
that a friction device has to be added to make it useable.  In the case
of the Orion this has been added to the knob and is probably partially
responsible for the wobble.  Gone is the analog feel as we now have
steps to deal with as a result of poor shaft encoder resolution. 

	At this time I have an eddystone EA12  and an Orion on my desk.
Most of my listing is on 160, still trying to learn the code after 40
years! If I want to just play listening the EA12 feels nice to tune and
is great fun. When it comes to being serious the Orion stays on
frequency, is easier to tune, has better adjustable selectivity, is
orders of magnitude better in handling QRM and is about 50% computer.

	 The Orion's main receiver has no roofing filters in the normal
sense, it simply has crystal filter IF selectivity at 9 MHz and DSP

	73 Brian KF6C G3ZVC

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