[HBR] HBR-11/2000. Comments?

[N2EY at aol.com]

In a message dated 7/13/06 12:21:13 AM Eastern Daylight Time, Tuchueh7 at cs.com 
writes:


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

Congratulations!

The following devil's advocate comments are IMHO and YMMV.

I have begun to think about 
> 
> layout and have
> breadboarded some circuitry. 

Please take your time and do a *lot* of sketching and mocking-up with 
carboard and pencil.

In this first letter I will discuss the tube 
> 
> line-up. One major break is instead of plug-in coils, toroid coils and reed 
> relays. Here's the dope:
> 
> 1) Double tuned critically coupled preselector

Coupled how?

The reason I ask is that the coupling method has a lot to do with how the 
circuit responds far from resonance. This can be important from the standpoint of 
IF and image rejection.


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

Same as in Electric Radio. 

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.


> 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.

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

6) 6xxx xtal 2nd local oscillator
> 
> 7) Two Miller 710 IF transformers (Can you believe I found 5 of 'em for 
> free!!!! thanks to
>        some great generous folk from the Lake Erie Boat Anchor group)
> 

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.

> 9) One Miller 710 IF xformer
> 10) 6EH7 IF amplifier (lots of shielding)
> 

The gain from two 6EH7s will be incredible. Also the problems with stability. 
Been there!

11) One Miller 710 IF amplifier
> 
> 12) 6BN6 product detector
> 

Shock mount it - that tube is microphonic too.

13) 6xxx xtal BFO (98.5 kc. and 101.5 kc) maybe
> 
> 14) 6xxx infinite impedance detector for AM
> 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?

> 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. 


> 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.


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

Voltage regulation for the oscillators?

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. 

- Will there be variable selectivity?

- What tuning capacitor?

- Consider making the chassis oversize from what you think you need. 

73 de Jim, N2EY