[HBR] HBR-11/2000. Comments?

[N2EY at aol.com]

In a message dated 7/14/06 1:52:24 AM Eastern Daylight Time, Tuchueh7 at cs.com 
writes:


> First of all, many thanks for the comments fellas.

You're welcome!

 My motto as a good Navy 
> 
> man is:
> "Fear God and dread nought" Also "A coward dies a thousand deaths, a brave 
> man dies but once".

Here are some of mine:

"If it was easy, everyone would do it"

"Experience varies directly as equipment ruined"

"Engineering is doing for a shilling what any fool could do for a pound"

"Scientists dream of doing great things
Engineers do them" (attributed to Wehrner Von Braun)

 Age here is 54, been messing with electronics since the 
> 
> early '60s.
> 

52 here, been building ham gear since 1966 or so. For an example of a 
receiver I built about 30 years ago, see the HBR website picture listing.

I have a background in field engineering; was privilieged to work in the last 

> 
> Apollo mission and development of Space Shuttle. Served in the Navy on a 
> frigate out of
> Yokosuka, Japan as an engineer and electronic material type. Worked in the 
> medical
> electronics/radiology field and lately have begun fiddling around with Ham 
> Radio. Got way too much junk here with vacuum tubes - you know the story.
> 

Yup.


> Let me tackle Jim's comments first:
> 1) Sketching of layout - Amen!!! Big weakness of mine, never understood what 
> 
> was meant by "measure twice and cut once"

Learned it the hard way. Now I measure three times.

> 2) Double tuned preselector - A tunable bandpass filter either top-coupled 
> with a carefully calculated cap (calculated for the mid-frequency) or bottom 
> 
> coupled with a mutual inductor - - need to consider this very carefully. 
> Compromise here will be decreased performance at either extreme of tuning 
> range. Will 
> breadboard an example and check it out.
> 

Variable tuning or bandpass? Stay C may be a problem with bandswitching if it 
is done at high Z points.


3) Inadequate RF gain - Maybe, but my thinking is low gain at the beginning 
> 
> simplifies
> the dynamic range problems. Gain distribution in receiver design is a major 
> consideration. A low noise low gain front end can be compensated with higher 
> 
> gain
> further into the receiver, yes?
> 

Yes. But the point is simply to make sure.

4) I chose a Vackar L.O. cause of the mystical belief of its superiority at 
> 
> higher freqs!
> 

There is no magic circuit.

I also like the fact that the tube is swamped out of the tuned circuit by 
> 
> large capacities; I intend to use polystyrene caps for these swamping 
> capacities. 
> I have the
> references you mentioned and will look into them. 
> 

Swamping C is possible with many circuits.

IMHO, stability is more dependent on good construction than the particular 
circuit used. 


5) 6AR8 was selected in order to preserve the limited quantities of 7360 for 
> 
> the classic gear. 

Aw heck, use 'em if ya got 'em. The Southgate Type 7 uses a 7360 mixer. Good 
but not magic. 

Also the dynamic range of this single balanced mixer 
> 
> configuration was
> considered.
> 


6) 6EH7 is a devil. I breadboarded a 6AR8 followed by one standard IF stage 
> 
> using some Meissner-looking 455 kc. cans. Peaking the IFs for max was 
> impossible.

That's not good.

 Never-the-less these two bottles gave me 70db of gain and great 
> 
> selectivity. 

One word: regeneration. 

In this context it's not a good word. 

This is on a sheet of green epoxy perfboard! I am thinking with a real metal 
> 
> groundplane and attention to detail its doable with the Miller 710s. Time 
> will tell.

Breadboard will tell better.



> 7) 6BN6 was chosen for its limiting capabilities. Got some rubber 
> shockmounts 
> for the socket.
> 8) I think that AGC circuits could stand a lot of improvement. I have yet to 
> 
> see one that can really track all of the mischief that the ionosphere 
> inflicts. I am thinking that
> a really sophisticated analog computer could servo all of that 6EH7 gain. 
> Need to resort to solid state opamps because of the sheer volume of 
> circuitry 
> here. Would like to advance the state of the art.
> 9) 5879 is engineered for audio. I want lots of audio gain to waste with 
> heavy negative
> feed back.

The result of that is a lot of bandwidth that isn't needed.

 The audio section is going to meet communication specs - low 
> 
> distortion,
> narrow bandpass for voice and lots of power. Would also like to keep the 
> heat 
> down.

How will you control the audio bandwidth? 


> 10) Solid state regulators for the oscillators plate supply. also regulated 
> 
> DC for the oscillators filaments.
> 11) Plug-in coils do simplify things greatly, but so do reed relays. They 
> are 
> a mature
> and reliable technology already used in the best radios.

The problem is whether they will do a good job switching tunable oscillator 
elements. 


 I also got a great 
> 
> big bag of them on the surplus market at a buck apiece. Toroids are small, 
> hi-Q, inherently shielding and I have found the the 2 and 6 mix to be 
> reasonably 
> stable, when in a constant temperature environment. Bandswitching is one 
> feature that I need for my
> operating style, so I will make an effort here.
> 

12) Variable selectivity - again, this is a communications, ham-band radio. 
> 
> Three kc.
> suits me just fine, and a Q-multiplier would be desireable. Also, IF 
> regeneration is a possibility.

IMHO, a receiver of this complexity deserves better. A Q mult is just a one 
pole filter, as is a regenerative stage. 

 No mechanical filters for this kid - after all this is 
> 
> an HBR. I would like to
> stay reasonably close to tradition


> 13) Tuning cap - three gang, I have a number of candidates here. I will 
> send 
> more
> details and pics later on down the log.
> 

Good! Must have very low torque requirement, and you need a good Oldham's co
upling from the 898.

14) Oversize chassis - Amen! Many of my projects have fallen by the wayside 
> 
> because of my failure to visualize or inability to modify my way out of a 
> blind 
> alley.
> 
Consider putting the power supply and maybe the last audio on a separate 
chassis, or in the speaker cabinet, both to save space and reduce heat.


73 de Jim, N2EY