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

[waltah at earthlink.net]

The detectors and audio section seem to be okay.   By bypassing 
most of the plate load for the product detector (SSB) and plate 
detector (AM) with a 10 mf cap I cut their gain way down, but the 
product detector more that the plate detector because the 6BN6 
has a much higher plate resistance.   That brought the audio to the 
volume control to the right range while roughly equalizing SSB and 
AM output.   Reducing the gain of the audio stages to 2 (vol. 
control to loudspeaker) ended overloading in the audio stages.   

A single 6J6 seems to make a perfectly adequate audio output 
stage at this low plate voltage (160) and for ordinary shack volume 
levels.   Considering the low gain requirement it might have been 
possible to skip the 1st audio and phase splitter entirely and drive 
the 6J6 directly from the volume control, allowing it to self-invert via 
the unbypassed common cathode resistor.  

The VFO seems to be working fine.   Second to the usual 
requirements for warm up and mechanical stability the goal there 
was to change the tank circuit as little as possible so as to not 
have to spend a day getting the tracking right.

I had intended a 6KT6 high gain pentode oscillator but had 
forgotten that pentodes aren't a good bet unless you need buffering 
or frequency multiplication from an ECO circuit.   So I settled on a 
6GK5 VHF RF amp triode.  With a transconductance of 15,000 
umhos it needs only about 1/3 the coupling to the tank that a 
12AT7 or similar tube would require, meaning better stability.   

The original circuit was a grounded base bypolar transisitor 
oscillator; the tank circuit portion mapped directly to a Colpitts 
circuit with the tube cathode bypassed with a 470 mmf cap and the 
grid bypassed to the cathode with 250 mmf.   Those values proved 
too large to allow oscillation; I got it working by changing them to 
270 and 180, respectively.   Plate voltage 50V, current 2 mA, 
output 1V p-p on the cathode.   

With the trimmer adjusted at the high end of the range (tunes 5 - 
5.5 Mcs) it tracks within about 1 kcs from end to end which is 
plenty good enough for now.   It drifts upward -- no surprise there 
because the tube shunt caps had been silver micas and I replaced 
them with NP0 ceramics.  This VFO assembly has adjustable 
temp compensation via a differential trimmer so it shouldn't be hard 
to get it close to spot on.  

Next -- the crystal oscillator.   Butler circuit, one crystal for each 
band, 14 Mcs above the high end of the band.   Must deliver output 
push-pull to the premixer stage.

Walt
KJ4KV