[HBR] Another HBR Project -- Chapter 5 -- Pictures

[hbr@mailman.qth.net]

John said:

> my laptop (at dial-up snail's pace ...

You mean they make them faster than that?   The only machine I 
use on line is a 100 megacycle laptop ...   In any case I don't see a 
way to make the pictures go any faster than loading them from the 
web addresses I gave.

> Also, thought of calling on you over across the Potomac and
> introducing myself. But didn't want to interrupt such a busy HBR
> pace. 

That address is several years out of date, and one of my things-I-
need-to-get-done is to correct it.  I'm about a hundred miles down I-
81 now, in the country.   For those not yet retired, I suggest the 
following plan:

1.  Become rich

2.  Retire.

Otherwise you'll wind up where I am, needing to work hard at doing 
chores you'd rather pay someone else to do so you'll have the money 
for a few ham activities.   I'm a lot more strapped for time now than I 
was in an average working year before retiring and as a practical 
matter, if I'm going to homebrew at all, that's about the only hamming 
there's gonna be.   I listen some, got to two hamfests this season.  
Maybe someday ...

Last night I hooked up the 1st oscillator and wound a coil.    No 
oscillation and after checking all the usual suspects, I reviewed the 
specs for the HBR coils (HBR web site to the rescue!) and realized 
that my feedback winding wasn't large enough.   However I wasn't 
sure that the command set coil forms would have enough space for 
the windings called for in the specs, so I switched from tickler 
feedback to a Hartley circuit.   That lets you tune the whole coil, 
meaning less total turns and has the added advantage that the circuit 
becomes a true electron coupled oscillator -- that is, that the screen 
grid (which serves as the plate for the oscillator) is at RF ground.   
Meaning that it serves to shield the oscillating elements from 
capacitive feedback from the mixer circuit.   

The disadvantage is that the cathode becomes 'hot' for RF.   There is 
no squirrely-er electrode in a tube than the cathode because it is 
coupled to the filament by what amounts to a capacitor shunted by a 
diode, both naturally of the worst (and time-varying) quality.   The 
effects are not large, but when you start looking for stability below 10 
cps/minute or so, things get very tough with a Hartley circuit.   You 
will always have a trace of 60 cps FM with a Hartley if you operate at 
a high enough frequency.  

It's a good idea to select the best tube from several.   A modern high 
gain tube with the tap moved down to the minimum that allows 
oscillation helps things.    That's a degree of optimization I probably 
won't attempt right now.

You can put chokes in the filament and connect one side directly to 
the cathode.  That gets rid of the squirrels but gives you the drift of 
the choke characteristics instead.   You think 300 ma might cause a 
bit of heating?   The usual such choke has a ferrite core; you think 
that stuff has a zero temp coefficent?   Another way is to bifilar wind 
the part of the coil below the tap and run the filament current through 
there.   That gets rid of the ferrite but your coil has to handle filament 
current without heating.   

Basically any ECO is a compromise design, useful when it's 
important to use a single tube as both oscillator and buffer, and 
particularly so when second harmonic output is desired.   

The Hartley ECO took off easily.  Drift ... wow!   Up a few kcs, back 
down a couple, up 1 kcs, down 2 ... endlessly.   Bad part 
somewhere ... wonder where?   I swapped the tube -- no change.   
Replaced the padder cap -- no change.   Replaced the shunt cap -- 
bingo!   Only 20 volts RF or so at that point, but enough to cause 
partial breakdown in a cap that is rated at 1 KV.   It hit the trash real 
quick and the drift went from all over the map to a few tens of CPS in 
a minute in lashed-together test mode.  Good enough for now.

20 volts of RF at the grid is surely too high a power level and I'll cut it 
down shortly.   I just guessed the various voltages and obviously I got 
them a little high.   There's a design trade off here: high levels of 
oscillator power lead to drift via heating not just of the tube but also of 
tank circuit components (the shunt cap that is always required in a 
ham-band-only set is a prime candidate, as is the coil) but lower 
levels with tighter coupling to the mixer mean you can pick up drift 
from there as well as get oscillator pulling with strong signals.    

The advice in the HBR articles to use silver mica caps in the 
oscillator circuit reflects ham practices and pocketbooks in the 
1950's:  if building today fixed shunt and padder caps as well as 
coupling caps should be NP0 ceramics if at all possible.   The shunt 
cap carries the most current and using two or three in parallel is a 
wise idea if you have the parts.  Also, use large caps -- the half inch 
diameter ones -- if you can find them.   Better cooling matters.   
Believe it or not, these changes will produce a noticible improvement.

Back to work.

Walt 
KJ4KV