[HBR] HBR-11/2000. Comments?

[Walt Hutchens]

I'm going to piggyback on Jim's comments on Barney's HBR 
project.   Mileage may further vary ...

>> After spending the last couple years collecting parts (including 
>> the Eddystone 898 dial) I have decided to begin construction. 

> Congratulations!

YEP.   Just finding all the parts is an accomplishment.

>> 2) 6ES8 Pullen mixer (ala 75A4 mod on the Web)

> Replacing the RF stage/pentode mixer with the Pullen may result in
> loss of overall gain. I don't know exact numbers but you may want
> to do some calculations and measurements before putting holes in
> the aluminum. 

Overall gain, however, is not going to be a problem in this design.  
6EH7's have plenty to overcome the reduction in front end gain.   

A bigger issue is that Pullen mixer.   It's my impression that this is 
an excellent weak signal mixer but not a remarkable performer with 
strong signals.   (Jim, please correct if that's not so?)   Except 
possibly on 10M, low noise/weak signal performance (same thing) 
is not the criterion for HF receiver design.   Atmospheric noise will 
overcome any reasonable front end noise on the lower bands -- 
only a 6K8 or 6BE6 will be limited by tube noise.   I'd use a more 
conventional triode mixer circuit, operatin g the mixing tube at 
higher voltages to improve dynamic range -- in particular, 
'interference free dynamic range' -- the range of signals over which 
strong signals do not mix to produce spurious signals.  

The 'strong signal' dual triode mixer using a 12AT7 with the 
antenna signal on one grid, the oscillator on the other, cathodes 
joined and to ground through 470 ohms, and IF take from paralleled 
plates is an excellent first mixer for an HF set if used with high Q 
coils -- you need those anyway.   The 6J6 also works well, a 6DJ8 
would probably give you more gain.   Something around a couple 
volts p-p injection.

Or you could try a push-pull triode mixer -- signal in push-pull on 
the grids, LO in to the cathodes in parallel, IF signal out in push-
pull from the plates.   I realize the issue with your plans for 
bandswitching but that has its own issues.   

>> 3) 6DJ8 1st local oscillator, Vackar design

> I suggest two references before you finalize that idea:

> A two-part QST article called "VFO Stability - Recap and
> Postscript" about 1966. Although a transmit VFO is described, the
> issues discussed are right on for receivers. 

> A QST article called "Stability with Simplicity" by W2YM, about
> 1961, and a following article "An Easy To Build VFO" by W1ICP. A
> simple but very stable VFO circuit is described. I have used it
> with excellent results. 

The Vackar is a good oscillator.   There is no magic circuit that's 
more stable than the others -- the two issues are minimizing 
coupling to the tube, which calls for a high gain tube, and achieving 
a stable tank circuit.   The Vackar circuit gives you the easiest 
control of coupling.  The 6DJ8 has about as much gain as you're 
going to find.   

> 4) Pair of Miller 1731 link coupled coils (diode DBM in between for noise 
> blanking)

An interesting idea.  Though I hate to see solid state creep into an 
otherwise perfectly sound design -- HI.

>>  5) 6AR8 sheet beam single balanced mixer (like the 7360)

> The 6AR8 is reportedly microphonic. That may or may not be a
> problem. I don't recall any info about the 6AR8 as a receiving
> mixer. 

The 6AR8 was the first effort at a controlled-cost beam deflection 
tube.   The 6JH8 and 6ME8 are better tubes -- more stable and 
less microphonic.   Run the deflection plates a few volts negative if 
you want the greatest sensitivity for oscillator injection.

However I'm not sure a beam deflection mixer is worth the effort (or 
even a good idea) for a second mixer.   You don't need anything 
like that much gain and the sensitivity has been set in the first 
mixer.    

Most hams figure the more gain the better.   The truth is closer to 
the reverse: Use just enough gain to get over the noise of the 
following stage.   

>> 8) 6EH7 IF amplifier (lots of shielding)

> The 6EH7 is a really "hot" tube - lots of gain and low noise. The
> Southgate Type 7 uses two of them. Be warned that they can be
> difficult to stabilize, and at 100 kc you may be better off with a
> 6BA6. If you allow room for a 9 pin socket, but use a 7 pin in the
> space, you can convert later. 

One word -- shielding.  Oh, and good grounding and bypassing.   
Just do everything right from the start and you'll have no trouble.  
You do need a 100 ohm resistor right at the grid (1/8" lead) to kill 
VHF parasitics -- this is needed for any of the modern high-gain 
wideband tubes.   

This is a superior IF tube, not just because of high gain/low noise 
but more importantly for controlled intermodulation distortion.  I 
wouldn't use anything else without a good reason.

Feeding a 6BN6 these tubes will only be loafing -- they can handle 
tens of volts without distortion but you're limited to under one volt at 
the output of the 2nd IF.

>> 12) 6BN6 product detector

> Shock mount it - that tube is microphonic too.

A fine choice for a product detector.   And simpler than using a 
beam tube.

> 14) 6xxx infinite impedance detector for AM

Consider a plate detector -- except for large signal handling ability 
they have the same advantages as infinite impedance and give you 
a better match to the gain of a 6BN6 as a product detector.   

You'll notice that receivers that use the lineup you propose have an 
extra audio stage following the inf. impedance detector.   

Unless you copy an existing circuit exactly you'll have to tweek the 
plate detector to get really good linearity.  At 100 Kcs you'll need 
the 2.5 mH choke that most circuits show in the plate circuit.

You have to keep the signals down at the IF output anyhow 
because the 6BN6 will take less than a volt before limiting sets in.

>> 15) AGC will be solid state - there is a lot of potential for creative 
>>  design here

> Yes - but with all those bottles, why not hollow state?

Yeah ... why give up while you're ahead?   

It's hard to beat the AGC circuit of the Tempo ONE.  They use a 
plate detector with the cathode biased negative so the plate runs at 
ground level; signals pull the plate negative to generate the AGC 
signal.  You do have to have a bias supply of negative 50-100 volts. 

IF-derived AGC for SSB gives you the challenge of keeping the 
BFO out of the AGC.   If you don't want that challenge, then you 
have to go audio-derived which has a different set of issues.

>> 16) 5879 low noise pentode 1st AF with lots of negative feedback

> 5879 is an audiophile tube - may be hard to get at reasonable
> price. You don't need  or want a lot of audio gain. 

Any of the usual pentodes will do fine in this socket.   A com 
receiver does not require the audio dynamic range of a 100 watt 
high-fi amp (you don't need to hear the 3rd violin turning pages) and 
you have that AGC loop getting rid of dynamic range, so extreme 
low noise in the input stage is not worth the trouble.   

However you'll be surprised what using a low-distortion high-fi 
circuit for your audio amp will do for voice clarity.  If everything up 
to there is working correctly you WILL hear the difference.   That 
means using feedback with enough loop gain that the feedback 
eliminates distortion in your amplifier.   The high-fi amp circuits in 
the RCA tube manual are a good starting point.  You must cut off 
the amp frequency response much lower than for high-fi because 
you won't have a transformer that will handle much over a few Kcs 
without enough phase shift to lead to instability -- but for 
communications use you don't want lots of highs anyway.

Use a two stage amp with feedback from the voice coil winding to 
the cathode of the 1st stage.   Better yet, use push-pull output 
stage with a phase inverter and do the same.  

>> 17) 6CW5 power amplifier (lots of audio power is important)

> Good. You can always bias the tube more to cut down on output, but
> inadequate audio is hard to fix. 

The 6CW5 is overkill -- with a pentode 1st audio you've got plenty 
of loop gain even with a triode output stage and something like a 
12BH7 is plenty.   Push pull lets you go class AB so you save 
power while reducing the distortion so your feedback loop has less 
work to do.   

Exception: If you want to drive true high-fi speakers the efficiency is 
so low that you'll want more audio power.  In that case a pair of 
6CW5's wouldn't be out of line.   

Standard (modern) automotive 3" or 4" speakers are a good 
compromise between 'fi' and efficiency.   The One Month HBR uses 
a pair of 6AQ5's running at 150 volts and a 3" speaker and you can 
listen to that one in the front yard when it's in the back.  And that's 
with the volume just past the halfway point.   There's LOTS more 
available, though it might be a bit much for the speaker.   

The R8040A uses half of a 12AU7 as an audio output stage driving 
a 3" communications speaker.   Volume is ample for all but the 
noisiest locations.   

>> 18) 190V 200mA power supply, the iron is Triad hermetic 
>> isolation transformer  

Keeping the plate voltage down is an excellent idea.   The latest 
tubes don't need it (in fact ratings may be exceeded) and lower 
voltage means less dissipation with all the problems that go with 
that.  

> Voltage regulation for the oscillators?

Probably a good idea for the 1st oscillator since it operates well 
into the HF.   The crystal oscillators certainly shouldn't need it.

> Other comments:

> Reed-relay-switched toroids may not be the best for HF oscillator 
> stability. The original HBR design coils and sockets are a big 
> part of the stability. 

Bandswitching is a challenge.   It can be done, but I certainly 
wouldn't start there unless you have some receiver experience.      

Walt 
KJ4KV