[HBR] Yet Another HBR Project -- Chapter 1A

[hbr@mailman.qth.net]

After more than the usual number of hours of staring blankly at a 
bunch of parts, I have a concept for the oscillator.   

7"x7"x2" chassis holds the front end -- everything through the IF xtal 
filter.    5"x7"x2" chassis mounted on left of main, has the rest.   
14"x8" front panel with the oscillator at the right, 1" overhang both 
sides.   A 3"x4"x5" box at the very right of the osc. chassis and 
against the panel, 4" dimension vertical, 3" lateral, holds the tuning 
cap.   

I'm in the process of shortening the posts on the LM dial drive to 
reduce the empty space between the drive and the box/tuning cap; 
with the shorter posts I'm not sure that it will be necessary to provide 
any other support for the oscillator box and that both simplifies things 
and improves the heat flow situation.   Such a box would stay at air 
temperature, so it's only necessary to see that the air doesn't get too 
warm.

Coil mounts behind the cap in the box and the rear face holds the 
two oscillator tubes -- I still can't locate a low power miniature dual 
triode.   3A5 seems to be the smallest there is.  Heck, I can't even 
find a 1.4V x 50 ma miniature *single* triode, so push-pull triode-
connected 1U4's will be tested.  All the oscillator small parts also go 
inside the box.  

Only the oscillator will use battery tubes -- a reduction from 2 watts 
(12AT7 or similar) filament power to 140 mW is worth some effort, 
when the tubes must be located within a couple inches of the 
oscillator tank circuit!   And with typical oscillator plate input of 60 
volts, 2 ma = 120 mW, this amount of power is a big deal.   The 
issue for battery tubes is of course stability: The filament is one or 
two strands of wire with a coated surface and how well it stays put, I 
don't know.   The oscillator circuit I'll use will have about 50-100 mmf 
from each grid to ground but whether that will be enough to swamp 
any variations is to be determined.   Sort of a push-pull Colpitts 
circuit but with the cathode grounded.

Filament power will have to be well filtered to avoid hum -- those 
filaments do respond to low audio frequencies.

The premixer will be behind the osc. box on the chassis.   Left of 
there, the Hahnel oscillator (multiples of 1.75 Mcs) and the RF stage. 
The bandswitch and 5-gang tuning cap go under the chassis.   I look 
for this to be a little hairy since both the premixer output to the mixer 
and the push-pull RF stage are switched and all that is gang-tuned -- 
the parts are small, but so is the space and there are a bunch of 
parts.   I'll probably take a first crack with a plate on a junk chassis.   
  
I will do the complete layout on the front end chassis before starting 
the oscillator, so the size of that is subject to change.    I've got a 
17"x13" aluminum chassis -- anyone want to swap something a little 
smaller, say 12" x 10" for it?   

I do notice that Hammond has a 12" x 10" x 2" chassis.   Antique 
Radio Supply doesn't sell that one, but Allied does.

Walt
KJ4KV