[Boatanchors] Comarison?

Fri Dec 18 18:11:54 EST 2009

Well, if you are building it yorself and without a machine/model shop handy 
a few corners have to be cut.

So this is what Im suggesting based upon what Ive been working on for 3 
years.:

High intercept point pentode RF with an aggressive AGC curve, 6GM6 or 6EH7. 
Dont bother switching it out. Use a switchable attenuator which may be 
necesary for your 5kw neighbor.

Dual triode mixer in a Pullen circuit. 6ES8 preferred as its a very unique 
variable mu tube.

Crystal control front end. Heathkit rig xtals are dirt cheap and then you 
can afford the other bands. 12AT7 oscillator/buffer.

You now have an absolutely bullet proof front end, NF around 4-5dB even with 
a double tuned manually controlled preselector. Saves trying to find the 
right components to get it all to track. I hate drive belts, gears and all 
that mechanical crap. Beam deflection mixers are vintage history and the 
only good one is getting real pricey.

This leads us to a potential weak link of a 500 kc wide IF using a pair of 
Heath bandpass filters to sharpen the skirts. More cheap parts. However low 
(minimum required) gain and hi Q tuneable preselector will keep overload 
away.

Heath doesnt believe in impedance matching so the mixer will drive a cathode 
follower preceeding those LC filters.

Next a low gain IF to support the Lamb style noise blanker and then to drive 
the Heath 3395 kc xtal filters. More cheap stuff. This might be a good job 
for a 6JH8 as a 6BA7 is overload prone. A dual control tube is needed here.

A stage of 3395 IF stage followed by another 6ES8 Pullen fed by a (cheap) 
Heath LMO and a buffer stage. Add a RIT with a varactor.

That dumps to 455 or 500 kc where you have many choices of some serious 
filters. 500 is harder to find but usually cheap. I have a 4 filter 8 pole 
module from a Racal cuz it was cheap. AM can bypass the filter and be gain 
matched.

A few more IF's to pick up the gain that was minimized earlier. Im a 
stickler for gain distribution. Put it after mixers and filters.

Now we add passband tuning, and T Notch filter to that IF.

Infinite impedance AM detector, 6ES8 Pullen SSB/CW detector, a real tuneable 
BFO.

Now it gets complicated. Two or 3 loop AGC using IF and audio derived 
control. At least 3-4 choices of attack and delay.

S Meter amp of course.

The lowest distortion audio that you can build ending up with PP 6V6's at a 
minum. Single ended audio sucks. Audio shaping for AM and a sharp filter if 
you really need something more than 250-400 cycle xtal filters.

Bells and whistles:  Output for a digital readout. I prefer a National PW 
dial for a 0-500 direct readout.  Panadaptor output. Outputs/inputs to/from 
a matching transmitter.

No need for a RF gain control, run that stage at its best performing point 
and use the attenuator if needed.

All filaments on regulated DC. B+ 175V maximum and regulated. PS and audio 
amp in seperate enclosure(s).

This should all be pretty much modular allowing easy changes and repairs. Im 
using SMA connectors and mini Teflon coax for RF.

And the best for last, absolutely no rotary switches in any signal path, its 
all done with miniature dip relays. No more compromises, cross coupling, 
etc. Now you can have a nice user friendly panel layout for right or 
lefthanders.

Comments?

Carl
KM1H

----- Original Message ----- 
From: "Ian Wilson" <ianmwilson73 at gmail.com>
To: "Boatanchors" <boatanchors at mailman.qth.net>
Sent: Friday, December 18, 2009 4:11 PM
Subject: Re: [Boatanchors] Comarison?

> This is an interesting thread from more than one perspective. Suppose you
> were going to design a tube receiver today (let's just say CW/SSB/AM and
> the HF bands only), you would have (at least) the following design
> choices to make:
>  - RF amplifier or not (possibly switchable in/out of circuit)
>  - choice of first mixer stage
>  - single conversion with heterodyne VFO, dual conversion
>    with tunable 1st IF, or something else
>  - tuning rate, size of band segments (if relevant)
>  - 1st (and others) IF frequency
>  - how do you get the required selectivity and where do you place it
>  - AGC: IF-drived/AF-derived/none/other
>  - additional processing (notch/noise rejection/...)
>  - other features (breakin/hifi audio/transceiver operation/...)
>
> It would be great to know what set of features from actual receivers
> if combined (obviously, this may not be possible) would produce a
> receiver that would be generally felt to be outstanding.
>
> My set of choices, not related to particular receivers:
>  - low-gain pentode RF amplifier/attenuator, can be bypassed
>  - beam deflection mixer
>  - dual conversion, tunable first IF (not the best choice for highest
>    front end performance but constant tuning rate, VFO stability, etc,
>    are appealing)
>  - 1st IF around 5MHz, second IF 455kHz (filters, etc, are cheap)
>  - 500kHz segments
>  - 455kHz SSB filter / audio CW filter
>  - audio-derived AGC
>  - simple noise blanker. Notch filter
>
> 73, ian K3IMW
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