I'll reply to two posts at once!
Tio Pedro wrote:
Still mulling over some RX design changes for a HBR receiver project that I
am considering building. It is going to be an updated version of the HBR
receiver projects that W6TC, Ted Crosby, penned for QST in the late 50s and
early 60s.
Interesting! But be aware that the W6TC design has its flaws.
The first is that there are a lot of stages before you get to the
selectivity, which limits the dynamic range. The second is that the
tunable LO operates at a high frequency on the upper bands and uses
plug-in coils, which limits the thermal and mechanical stability as
well as having different calibration on each band.
I've decided to go with 6EH7 semi-remote pentodes for the RF and IF stages
(overload resistant), meaning my filament current requirements are going out
of sight.
6EH7 is a good tube. The receiver part of my rig (google my call) uses
one in the RF stage and one in the first IF, with a 7360 mixer.
Some must-read QST articles a
W1DX's article about "what's wrong with our present receivers",
January, 1957
The Squires-Sanders article about using the 7360 as a receiver front-
end mixer, September 1963
The "Miser's Dream" rx with 7360 mixer and other neat ideas, by W1DX,
May 1965
W1KLK's "Experimental Receiver for 75 meter DX", February 1972
An article about the 7360 product detector for the HBR, December 1960
A 6 meter SSB transmitter using the 6JH8 as a high-level balanced
modulator, October 1963
The RSGB Handbooks of the 1960s are the best for receiver ideas using
the 7360, IMHO.
I know of one design using the 6JH8 or its cousins the 6ME8 and 6HW8.
(The 6AR8 is an early tube that is reportedly microphonic).:
http://g4oep.atspace.com/retro/retro.htm
Worth a look!
4.7 amps so far, so I want to cut back on the number of stages.
One word: Don't. Use a bigger power transformer, or an auxiliary
heater transformer instead. The rx you describe is a big project, why
compromise it? With a separate heater transformer having its own power
switch, you could put the critical oscillator heaters on it and let
them run semi-continuously.
To cut my tube count and heater energy, I'm looking for opinions on
using
6JH8s with self excited injection. The 2nd mixer requires 1515kc injection,
and I'm considering using the 6JH8 as a self-excited crystal oscillator to
eliminate using a separate oscillator stage. Original plan was either a 6BH6
oscillator, or 6U8 combined osc/buffer.
The 6BH6 draws only 0.15 A of heater current. IMHO that small saving
is not worth the compromise.
Self-exciting the 6JH8 means the signal will go into the deflectors
rather than the grid. That's the opposite of usual receiver-mixer
practice, where the oscillator feeds the deflectors. I don't know what
the noise figure will be, but I do know that all the beam-deflection
receiver mixers I've seen put the signal into the grid for high gain
and low noise.
The only places I've seen the signal on the deflectors and the
oscillator on the grid is in high-signal-level applications like
transmitting balanced modulators and mixers, and receiving product
detectors, where noise figure isn't really an issue and low gain is
OK.
Second branstorm is to use a self-excited 6JH8 for the Product Detector and
85kc BFO, eliminating a separate tube and buffer stage that I had originally
planned on using (another 6U8). Will osc. pulling be a problem? Any other
drawbacks?
I don't know if you really need a buffer stage on the BFO. A 6BH6 will
do the job. See above about where the signal goes.
First mixer will be a 6ES8 Pullen, with a 6U8 used for the tunable LO and
buffer isolation.
How stable an oscillator is acceptable?
AGC will be via a 12AU7 plate detector, 12AU7 infinite
impedance AM detector, 12AX7 Q-multiplier, 12AU7 first AFA and S-meter amp,
and a 6AK6 AFA. First IF is 1600kc using cascaded modified ARC-5 IF
transformers. 2nd IF is 85kc, using cascaded R-23 command RX IFTs.
FWIW, I found some good material on this subject in the
March 1960 QST, pps 33 - 38.
Straight out of the RCA Transmitting Tube manual. (RCA considered the
7360 to be a transmitting tube!)
Another question: I've been told that the 6JH8 works best
with 0 volts on the deflector electrodes. However, it was
common practice with the 7360 to use a voltage divider
between the plate and deflector to provide negative feedback
(improved linearity) while biasing and balancing the
deflection electrodes.
I'm not sure about the negative-feedback part but I do know the idea
was balance. And the 7360 deflectors were supposed to be biased a
couple dozen volts positive.
I'm thinking of using the same scheme
on the 6JH8, except having the low end of the voltage
dividers going to a negative bias source. That would allow
setting the electrode voltages to 0 volts, or slightly negative
or positive--whatever works best--while still keeping the
advantages of having negative feedback from the plate.
Is this worth doing?
Only way to know for sure is to try. See the G4OEP rx page.
73 es GL de Jim, N2EY