I've been reading with great interest the excellent comments posted recently regarding 7360 balanced mixers. I've done quite a bit of work with the 7360 and its cousins and I can add a little information.
Beam-deflection tubes were designed beginning in the mid-1950s to be used as color television synchronous detectors. The first type to be introduced was the 6AR8, followed by the 6ME8 and the 6HW8 (I think this order is correct). These early tubes are subject to problems with microphonics and are not recommended for comm uses. The 6AR8 also had problems with loose internal elements and some TV sets actually had them mounted in rubber bushings! They were all built around the principle of beam-deflection control of plate current in addition to conventional grid control. The idea behind them was to switch the beam currents from one plate to the other at the LO rate, using the potential AC voltage between the deflecting electrodes. The reason they are called "balanced" is that the external bias voltages balance the beam currents internally-balance has nothing to do directly with external circuitry or RF balance. In fact, the 7360 can be used with one plate grounded for RF and still be "balanced". One of the main advantages to beam-deflection tubes is that electrons always move at high velocities away from the cathode. This eliminates the interaction between electron stream and grid structures as seen in conventional pentagrid mixers. Because of the balanced operation of the deflection electrodes, the influence of the deflecting-electrode voltage on the E-field near the cathode neutralizes capacitive coupling between the deflectors and the control grid, and hence internal coupling of signals. The result of this internal balance is an extremely linear plate-to-plate transfer characteristic. The balance pot seen in most of the published designs is included to balance internal deflector currents by establishing equal bias voltages on the deflecting electrodes. The 7360 was followed by the 6JH8 color demodulator which is much less expensive, and doesn't require DC bias on the deflectors. It is difficult to use because it requires about 300 volts on the plates and a lot of LO drive to get any mixing efficiency. However, it has an extremely linear transfer characteristic that approaches a straight horizontal line. There was also a type 7763 beam deflector, but it is no longer available and was mostly an experiment as an IF limiter for FM sets. Electrically, the 7360 is equivalent to a high-gain linear amplifier followed by a fast switch, which would seem to make an ideal mixer. Unfortunately, there are several problems, which combined with high cost, contributed to limited use. They have no rejection of either LO or RF signal at the IF output. They efficiently amplify these signals, and up to 15 db gain is seen of both signals at the IF output. This makes the use of high-Q output circuits mandatory. Even so, LO harmonics and any unwanted RF energy will freely to mix with the IF and produce a lot of spurious energy at the IF, which is difficult to remove by conventional IF filters. I tend to think that an RF preselector with just enough gain to overcome the 7360-mixer noise would be a requirement. Another disadvantage is that the conversion gain goes way down above about 1 mc, and can approach 22-db conversion loss at 9 mc. About 1964, JW Miller came out with two transformers (1740 and 1741) designed to be used as a 9-mc SSB generator with McCoy crystal filters. Although the circuit worked well enough with audio, when adapted for use as an RF mixer it has a conversion gain of about 1. Most of the published RF mixer circuits for the 7360 used IF frequencies at or below about 1 mc for this reason. Using balanced transformers at the deflectors (LO injection) and at the plates (IF output) makes about 1 db improvement in rejection of RF and LO at the IF. All the balanced coils do is provide some 2nd harmonic rejection of the LO and IF frequencies. You can't compare 7360 balanced mixers with modern-day balanced mixers because they are not the same terms. The deflection beam is very sensitive to modulation from external AC fields, and a good shield is mandatory. I've used IERC shields with good results. The great advantage to a 7360 is its near immunity to blocking. I've measured 100 db ratios with homemade RF mixers. They make EXCELLENT product detectors that have high conversion gain and low distortion. This is not surprising, since that's what they were designed to do. All of the published designs may be followed, I suggest the March 1960 QST article. One noted problem is a tendency for the product detector to act like a random synchronous detector at low-frequency beat notes and it will try to lock onto a nearby signal. The cure is to try another tube. DC on the filament may or may not help. Here are some references to balanced mixers: RCA Review, June 1960 SSB Exciter Circuits Using A New Beam-Deflection Mixer, QST March 1960 7360 Mixers in the 75A-4, QST July 1964 A New Approach to Front-End Design, Bill Squires, QST September 1963 The Single Sideband Sixer, QST October 1963 (6 meter rig using 6JH8's) Miser's Dream Receiver, By Goodman, QST May 1965, also in ARRL handbooks The W5OMX Communications Receiver, December (?) 1968 An Advanced General-Coverage Receiver, W6BD, QST November 1970 RSGB Handbook, 1968 RSGB Amateur Radio Techniques, 6th edition, 1978 73, Ray Osterwald, NØDMS ----- Original Message ----- From: "Brett Gazdzinski" <[EMAIL PROTECTED]> To: <amradio@mailman.qth.net> Sent: Thursday, November 06, 2003 7:42 AM Subject: RE: [AMRadio] FW: Homebrew receiver > Well, I never even seen a Squires Sanders receiver, let alone listened > to one! > > I cant recall offhand any other receiver that used a 7360 > as a mixer, but need to look in my vacuum tube receiver book, > it lists all the receivers with their tube lineups. > > The ARRL sure liked the 7360, they used it in a lot of their > receiver projects, in the 1967 handbook anyway. > > I suspect the cost of a 7360 was lower back then, not sure why > its so expensive now, must be very rare? > > On the lower bands, I am not sure getting a really quiet mixer is > important at all, but have not tried the other designs yet. > > I never realized just how noisy some receivers are till the > first homebrew was done and compared it to the R390A. > I guess you think its atmospheric noise, or just get used > to it, but I cant stand the R390a anymore. > > > The first homebrew receiver is VERY quiet, but is an odd > design, I hope the new one is fairly quiet with the design > I picked. > If its not quiet, maybe I will try to duplicate the design of the > first receiver using a 7 or 9 pin tube in place of the 12SA7. > > I am sure there is a tube to replace the 12SA7 in a miniature > type. > Maybe I should have planned it that way from the start, go with > what you know works well, but trying other things is part of > the fun... > It will be easy to change the tube type if the first design > does not work out (6AH6, cathode injected LO). > > Brett > N2DTS > > > > > > The Squires Sanders SS-1R and SS-IBS both used a pair of > > 7360s. I never had > > a 1R, but I never thought the IBS worked noticeably better > > than any other > > relatively high end radio with more conventional vacuum tube > > mixer circuitry like > > the NC400 or the 51J4. And the thing was harder to align > > correctly as well > > (maybe that's why I won't impressed = never got it right:) Scott > > > > _______________________________________________ > AMRadio mailing list > AMRadio@mailman.qth.net > http://mailman.qth.net/mailman/listinfo/amradio > >
