A Receiver Evolves
New 7MHz Valve Receiver
(mainly for AM work)
image to view full size
27 March 2006
chassis has been punched and drilled, but no components have been
8th April 2006
still has work to be done, particularly in the front-end which is not
tracking well. I have left this section until the last. However, most
sections are working and quite satisfactory. You will note a large hole
in the panel and this is for the digital counter dial which is under
construction at present.
22nd April 2006
completed. Receiver is running smoothly and has been used on air. All
front panel facilities have been wired in and most of the inevitable
bugs have been eliminated. The box underneath the chassis houses the
6CB6 RF amplifier, and a shield separates the grid circuit from the
plate circuit. This box eliminated an oscillating front-end and it was
one of those no win situations. If you put it in it would take a lot of
time and effort and it possibly would never have been needed, but if you
don't put it in you would end up with a cranky front end. So I had to
make it and "implant" into the rest of the wiring. Some of the wiring
still has to be cleaned up etc.
have any idea where I can buy white Decals to put on some lettering?
There used to be a product called (I think - it was 40 years ago!) "Teknicals".
It even came with a solvent that dissolved the clear plastic backing and
just left the letters embossed on the panel.
DESCRIPTION OF THE RECEIVER
* Tubes: 16
6AM5 - Audio output push pull
12AT7 - Audio amp and phase splitter
- BFO & BFO buffer
6J5 - S meter driver (Yes! an old
metal 6J5 from WW2)
6AK5 - 1500 KHz amplifier and wave shaper
- Modified Infinite Impedance Detector
(see "Why did my receiver have distortion?" below)
- Audio amplifier
9003 - IF amplifiers
- RF amplifier
6BE6 - Product detector for SSB
- Converter 1415KHz IF to 85KHz IF
6BA7 - HF pentagrid mixer
E180F or 6688 - HF conversion oscillator
Just one - 7MHz.
conversion - uses old Command Receiver IF's. (3 at 1415KHz and 2 at
85KHz). Past experience has shown that only two IF's at 85KHz are needed
for excellent selectivity for AM. Three IF's cause sideband cutting and
muffled sound and we don't want that!
* AM & SSB
* Push Pull
audio output (2x 6AM5) - for that lovely clear sound that valve
amplifiers can produce. (see "Why did my receiver have distortion?"
controls: Bass & Treble - boost & cut - these controls are about the
valve equivalent of Digital Signal Processing (as I see it!), but as far
as I am concerned, a whole lot more useful.
mechanical dial - uses a counter module kit to count VFO frequency then
subtract the 1415kHz IF. May not be as accurate as the synthesizers
used in modern solid state tranceivers, but this is not a modern solid
Incorporates a diode noise limiter.
WHY DID MY RECEIVER
receiver can reproduce excellent sound. That is one of the reasons that
I did not use solid state. However, the sound was not as I expected and
I must admit that fixing it was more of a task than I had bargained for.
6AL5 detector. This was the easiest fault to fix. The sound was very
mushy but responded well to a new tube. Still, there was distortion.
Something else was wrong.
Over-drive of audio amplifier. The oscilloscope showed up this one. The
drive into an audio stage exceeded the cathode bias voltage. A small
preset potentiometer was used to preset the audio level into the stage.
The sound was better but still distorted. More time needed with the
waveform on the plate of the same AF amplifier was very distorted. This
was caused by a poor selection of resistor value for the plate load.
With a HT voltage of only 170 volts, this resistor needed to reduce the
plate voltage to about 85 volts so that a wide AF swing could occur,
without getting to close to either limit. The value I had selected was
220K and the waveform was compressing badly on the lower swing. A change
to 100K fixed that one. I was getting depressed - I still had
often do you hear of hotting up front-ends? Why use a low gain tube when
there are plenty of higher gain tubes available and after all, why not
use the hottest that is available? This next distortion producer was
found unexpectedly. The tube I had selected for the project was a 6CB6 -
a TV IF amplifier. I was fiddling with the front-end and had plugged in
a 6AH6 - a tube with higher gain again. The result was higher S meter
readings and MORE DISTORTION. I plugged in a 6AK5 and although it had
lower gain than the 6CB6, the receiver sounded much better. So I plugged
in a low gain, remote cutoff 9003, all the way from
WW2. Gone was most of my distortion. Seemed the higher gain tubes were
overdriving the 6BA7 mixer and producing all manner of rubbish and
distortion products. Think of this next time you think of hotting up a
receiver with a "better" tube than the manufacturer had used. It may
work on VHF but on a band with huge signals high front end gain is not a
good idea. A good tune around 40M indicated that SSB sounded great, but
AM was still a bit rough especially on the strong commercial stations.
For me, I knew the receiver could work better but I was running out of
big break-through! I must say that right from the beginning I had a
doubt about using a diode detector. I knew that the infinite impedance
detector had excellent performance in the distortion figures, but I
wanted AGC and infinite impedance detectors don't normally provide AGC
voltage. Some research into an old book by RSGB "Amateur Radio
Techniques", printed in 1974 had a circuit of an infinite impedance
detector that produced AGC voltage, so I took out my 6AL5 and wired up a
6AK5 as per the circuit from the book. It worked! The only problem was
the production of much too much AGC voltage (about 25 volts of it!).
This was easily reduced to manageable levels. Gone was my distortion.
Here is the circuit of my detector as it now is:
more or less AGC voltege is required for your particular application,
this can be adjusted by selecting different values for Cx and Rx. (The
original circuit used Cx = 60pf and Rx = 47K.
Why use a
diode detector when this little circuit works so much better and will
even save on heater consumption?