[HBR] Mixer questions and other tech questions..

[cerrogordo at iscweb.com]

Jay Helms responds  to Peter Bertini :

N750 disc ceramics work well for me, and I have never had any 
performance reason to use instead the older "dogbone" format in a W6TC 
HBR design.  Ted used ceramic discs in his later designs HBR11 and 
HBR13C as well as his HBR20 grand-daddy.  I bought various N750 disc 
ceramic combinations for him in a small retail radio parts store behind 
my office then on Colorado Blvd in Pasadena during the late 1960's.  The 
caps are, unfortunately, almost impossible to find today, just as are 
the parts stores.

You will need assorted small values 1 through 8 pfd plus 10 and 20 pfd 
as a large-value base.  Strive for a combination of one base cap plus 
one trim-out cap so they can both be fit into the "well" of the coil 
form, and cemented in place there.  My final sizing and performance 
below, after much experimenting,  for your guidance

80m   7.2 pfd   42 min/20min   2H5M still zero beat
40m   10 pfd    44 min/14min    4H7M still zero beat
20m   20 pfd   42 min/9min       2H3M still zero beat
15m   17.2 pfd 36 min/20min    2H3M still zero beat
10lm   15 pfd   29 min/19min     1H35M still zero beat

The first time given is to initial zero beat with cold coil/cold rx 
measured from initial power on, 42 min at 80m in the first example

The second time given is to initial zero beat with cold coil/hot rx, 20 
min at 80m  in the first example and and hot rx = 1 hour or more power on.

The hours + minutes "still zero beat" is the length of time my HBR20 
remained at dead zero beat after reaching thermal stability until I shut 
it off to go to work or go to bed.  It was still at zero beat at the 
instant of shut-down.  2 hours 5 minutes at 80 meters in the first 
example.  Ted and Alex Stewart both reported "still at zero beat" in 
excess of 12 hours, all using disc ceramic N750s.   

The large 15m N750 and the longer time to zero-beat is undoubtedly 
caused by the large size of the APC Ted specified.  If you can wind your 
coil close to the frequency (not an easy task) and substitute an APC50, 
your time will drop significantly and you will stay at zero beat longer.

These were all excellent figures for 1968, (better than most commercial 
receivers of the time) and aren't too bad for analog designs today.

Achieving those figures requires sound construction techniques 
throughout the HBR and best quality  APC variables in the L3 coil 
(preferably Hammarlund or National, nickel-plated brass plates at 50 pFd 
max).   It is my experience that the APC is the major source of thermal 
frequency drift - the smaller you can make it (and still find your 
frequency), the faster you reach thermal balance and zero beat.

Along with the N750s you will need enormous patience for the 
trial-and-error work to get results like these.  Mine required more test 
runs than I care to remember (or do again).  Ted's values (we worked 
together on the temperature stability thing) were not quite as good as 
mine, but his HBR20 receiver was in a very small cabinet running very 
hot whereas mine was in a full-size 19-in cabinet running 12-degrees 
cooler than his.  The larger air volume makes thermal mixing much 
faster, and the lower temperature lets you get to thermal balance faster.

If you find a source of N750s I would sure like to know about it.  I am 
experimenting with some WARC coils, and have exhausted my stock of N750's.

73 de w6hht 

Peter Bertini wrote:
> One other question!  Where in the world can one find  N750 ceramic
> dogbone caps for the LO coil temp. compensation?
> I see Surplus Sales has them, but the price is way too steep for what
> they are.  Are conventional N750 disc ceramics
> stable enough for this application?
>
> Pete
> ************************************
>
>