[HBR] Another Receiver Project -- HBR-4, Part 3

[waltah at earthlink.net]

Someth'n is gonna go wrong -- it has been too easy so far.

Both the product detector for SSB/CW and the plate detector for 
AM seem to be working fine, plus all the IFs, the AGC, the S-meter 
and audio stages.  There's way too much audio gain, so that needs 
to be adjusted.   I think the gain of the SSB detector is high 
compared to the AM detector but that's best adjusted by listening 
to actual signals.  

Doing a product detector with a 6BN6 was ... interesting.   The 
tube description is 'gated beam discriminator' but when you dig into 
what that means, things get complicated.

Like the 7360 (etc.) electrons leaving the cathode are focused into 
a narrow beam.   However there is just one plate and no deflection 
arrangement.   Instead, there are two control grids, both with very 
sharp cutoff characteristics -- plate current goes from off to 
saturated over just three volts or so.  When supplied with a signal 
large enough to put the tube in limiting mode (as normal for an FM 
discriminator circuit) the plate current flows in rectangular pulses at 
the signal frequency.  

In discriminator use, G1 is the signal input, G3 (called the 
'quadrature grid') is connected to a tuned circuit adjusted to the IF 
center frequency.   G3 picks up some current from the beam and 
because 'beam on' corresponds to the maximum excitation current 
to the tuned circuit, the tuned circuit (and G3 voltage) *lags* the G1 
signal by 90% on average.  In this situation the beam is gated on 
and off by the combined action of two signals 90 degrees apart.   
The tuned circuit/G3 signal does not instantly follow changes of 
phase by the beam current so phase shifts of the input signal lead 
to pulse width modulation of the beam.   A capacitor in the plate 
circuit integrates the pulses to recover the modulation of the input 
signal.

When used as a product detector, G3 is excited with a BFO signal 
of 5 volts p-p or so.   A CW signal at a slightly different frequency 
(a phase difference changing at a constant rate) leads to sine wave 
modulation of the beam pulse width, recovered as a sine wave at 
the difference frequency in the plate circuit.   

For SSB detection, G1 must be operated in the linear range.   In 
this case the beam is *both* PWM and amplitude modulated and 
the modulation is again recovered at the plate integrating capacitor.

The serious CW man could push G1 into limiting mode and recover 
a pure CW note, stripped of nearly all background noise.  The 
advantage over audio limiting (sometimes so used) is that doing it 
at the IF avoids audio harmonics -- of course some CW ops like 
the complex sound of a near-square wave output. 

As an SSB detector the tube has a conversion gain of about 50 
and begins to limit at around 2 V p-p input.   Furthermore, to obtain 
proper (linear) operation you cannot change the gain or limiting 
range.   Plate and cathode resistors must be set according to the 
directions, the focusing electrode voltage must be as specified, or 
you will get distortion.   When properly adjusted, it produces the 
nicest sine waves I've seen from a detector, but it is not very 
flexible -- to get good results you have to feed it what it wants.   

The audio gain is way too high, the plate detector doesn't seem to 
have as much gain as the product detector -- it's a little hard to be 
sure because the percentage of modulation on AM affects results 
and I don't have a high percentage source handy.   With that stuff 
fixed it will be time to start on the oscillators.   A Butler crystal 
oscillator and a Colpitts VFO ... how hard can it be?   

We'll see!

Does everyone know about Frank's tube data sheet archive?   Over 
20,000 tube data sheets -- stuff you can't imagine! -- is available at:

http://home.wxs.nl/~frank.philipse/frank/frank.html

I was reading RCA tube handbooks about the time I finished with 
Dick and Jane -- RC-15, with the 12AT7 and 50C5 among 'recently 
added types' listed in the rear of the book.   But Franks' site is  
*wonderful* -- you get full manufacturer's data on tubes that are 
barely or even completely unmentioned in the little handbooks.   
Understanding the 6BN6 would have been impossible without those 
sheets, the transfer characteristics for the 12AT7 were very handy 
for checking the design of the plate detector.  

Walt 
KJ4KV