[HBR] Another Project -- 1MHBT -- Part 2

[waltah at earthlink.net]

I have done some measurements on the '1 month HBR' with the 
goal of confirming that an SSB transmitter along the same lines 
could meet reasonable standards of spectral purity.  This receiver 
has 8 tuned circuits at 85 kcs (four 85 kcs command IFTs).

Measuring at 2 kcs on both sidebands, the unwanted sideband (4 
kcs away) is down 60 db.   Peaking at about 1.5 kcs, it's over 40 
db down on the other side.   With shaping of the audio (rapid cut-off 
below 500 cps or so) that should be good enough.  The set will not 
have much in the way of lows, but that's the price of admission, as 
well as the most efficient way to operate a low power transmitter of 
any sort.

I'll see if I can find five 85 kcs IFTs and enough space to mount 
them all.   

The image for the 85kcs IF is 170 kcs away; just over 10% of the 
1665 kcs 1st IF which has four tuned circuits -- two 1415 kcs 
'command' IFTs, retuned.    That image is down >50 db.   

That's a bit trickier because I'm considering conversion from 85 kcs 
to a tunable 2nd IF of perhaps 2.1-2.6 Mcs and then a crystal-
controlled conversion to the channel frequency.   Since the 2nd IF 
would be higher than in the RX and I'll only have two capacitor 
gangs to tune it the TX situation will be less favorable.  The Q, 
however, will be higher -- mix 6 toroids (or possibly mix 2) instead 
of litz wire on an iron powder core.

Calculating from the Q side, Q's for T68-6 (yellow) toroids at 2 Mcs 
run a bit above 200.  That's ~32 db down at 10% off resonance per 
tuned circuit or >60 db for two.  That should be good enough.  

Another issue would be feedthrough of the harmonics of the 85 kcs 
signal when the tunable IF is on a multiple.  Considering the high 
multiple, a balanced mixer should take care of that.  

Tentative tube lineup:

6U8 mic amp

6JH8 beam deflection 85 kcs osc. and balanced modulator.   

2 x 85 kcs IFTs

4EH7 1st IF (85 kcs) and speech processing (RF clipper)
(Not much speech processing -- maybe 10 db?)

1 x 85 kcs IFT

4EH7 2nd IF (85 kcs) ALC controlled

2 x 85 kcs IFT

6JH8 2185-2685 kcs osc/balanced mixer to 2100-2600 kcs 

Two tuned circuits track oscillator.  Oscillator and IF circuits tuned 
by 150 mmf/section 3-gang command set tuning cap.  

4EH7 bandswitched crystal oscillator -- may be possible to 
combine oscillator with next mixer.

6JH8 balanced mixer: on 80M, 6100 kcs - 2100 to 2600 kcs -> 4 
Mcs to 3.5 Mcs.   

12HL7 driver -- input and output ganged.   Or maybe something 
with a bit less gain with a 450 ma filament -- maybe another 4EH7, 
since except for circuit losses, no power is required. 

2 x 22JF6 push-pull PA

Total 10 tubes, series string (450 ma) filaments.   Unfortunately 
there's no beam deflection tube with a 450 ma equivalent, so 
balancing resistors will have to be used.     

I don't know about the 85 kcs osc.   Probably just a command 85 
kcs BFO coil with a switched trimmer cap to get the other 
sideband.  Crystals are available for these frequencies but with the 
small tuning range of the 85 kcs IFTs, probably not a good idea.  

I have found a copy of SSB for the Radio Amateur, 1st ed. so that 
just leaves the 2nd ed. and I'll be fully equipped to study obsolete 
SSB technique.   Well, almost ... none of those books mentioned 
the 'third method.'   Not that I could understand it if they did.

Walt 
KJ4KV