>I'm looking for a practical balanced-modulator (tube-type) for a 7 MHz
>direct-conversion receiver I want to build with vacuum tubes (now,
>don't ask "why tubes", I just wanna... and don't know why). I figure
>with a balanced modulator at the front end, I can keep the BFO energy
>out of the antenna, and maybe even get a few db gain. Maybe a dual
Sorry to take so long to reply, but better late than never.
Two QST articles are "must reads" on this subject.
The first is the article by White in May, 1961. He built a direct conversion
80/40 receiver with five tubes: 6SK7 RF amp, two 6SB7 in the balanced detector,
a 6U8 audio amp, and a 6C4 local oscillator. His reason for using a balanced
detector was to eliminate any signal reception by direct rectification of the
signal. (Note: The 6BA6 is essentially an improved miniature version of the
6SK7, and the 6BA7 is a miniature version of the 6SB7).
The second article is by DeMaw and Wilson and appeared in July, 1973. They
describe a complete 80 meter QRP transceiver using tubes. The product detector
is a single ended 6GX6 circuit. Apparently they had no trouble with direct
signal rectification. This design should be easily adaptable to 40 meters.
Leakage of the LO signal into the antenna is not a major concern unless a
detector circuit which has poor isolation is used. If an RF amplifier is used
ahead of the detector, there is no reason to be concerned at all.
The most important concern is that the detector operate in true "product
detector" fashion. What is meant is this: The only output from the detector
should be the result of beating (heterodyning) between the received signals and
the local oscillator. Direct signal rectification should not produce any
output. The reason for this requirement is simple: It's not practical to have
much RF selectivity in front of the detector. So if a detector which rectifies
the signal is used, you'll hear dozens of signals at once.But if a true product
detector is used, only signals which are within a few kHz of the LO frequency
will produce audio output.
The White receiver, based on an article by Villard, achieves true product
detection by means of a balanced circuit that causes rectification products to
cancel out. Other product detectors avoid the problem by operating only on the
Other considerations are low noise, reasonable gain, and available parts.
Some have suggested using the balanced modulator circuits used in SSB
transmitters. While the basic circuits may be adaptable, it must be remembered
that a balanced modulator for an SSB transmitter is not designed with low noise
figure in mind.
The 7360 tube was widely used in such circuits, but it can be expensive and
hard to get. The 7360 was considered to be a transmitting tube, and was only
made by RCA (I think). There are other beam deflection tubes, however, such as
the 6ME8 and the 6JH8, which are often not recognized for what they really are.
They were designed as demodulators in color TV sets, and are actually newer
designs than the 7360. By the way, if anybody has practical data on the use of
either tube as a receiver mixer, please let us know!
My recommendation would be to start with either the White circuit or the
DeMaw/Wilson one, depending on what parts you have available.
You don't have to explain why you want to use tubes. Those who know why, don't
need an explanation.
73 de Jim, N2EY