A special 75A-4
My Favorite "Classic" Receiver .....
and probably one of the best receivers ever made.
Collins pushed the state of the art with the 75A-4. If you were around in 1955 when these receivers were introduced to the market, you had your choice of a great many receivers. When the 75A-4 was introduced, all of the then current receivers were relegated into two classes of radios: fair and poor. In a single stroke of generous, Art Collins and his engineering team from the "Green Room" in Cedar Rapids established the standard to which all future receiver aspired. Some came close. Bob Drake had some inspired moments with his famous R-4 line. Even Collins couldn't produce a sequel. The very popular S-Line and KWM-2 line was always in the 75A-4's shadow. Even the incredibly expensive (by yesterday's and today's standards) 75S3-C doesn't nudge the A-4 from preeminence. The major feature of the S-Lines was their ability to lock the transmitter and receiver together for transceiver operation.
Of course, only a few could afford this receiver in 1955. It wasn't cheap, but it was the best.
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Remember this was more than 40 years ago!
Interestingly, the 75A-4 is most famous for its SSB virtues. However, all of the ingredients which made the A-4 the best SSB receiver of its time and perhaps all time, also produced a CW receiver of similar caliber.
Click on photo to enlarge. Modification discriptions are included.
|This is a 75A-4 with a serial number in the
mid 5000's. Only about 6000 were built. Beyond it's very high serial number,
there are many other unusual features of this 75A-4. First, it is a W3HM
restoration. That means that it not only looks good, but there have been many
changes to it circuitry. In this particular receiver, cascaded mechanical
filters provide exceptional selectivity. The usual AVC, product detector and
audio mods popularized by W7CMS are included. Since I purchased this A-4, I
have added two additional mechanical filters. As received, the filter
configuration included a 2.1 KHz mechanical filter wired under the chassis. A
3.1 KHz mechanical filter replaces an i.f. transformer between the first and
second i.f. stages. I installed a standard 3.1 KHz filter in an unoccupied
filter socket along with a 1.5 KHz mechanical filter in the remaining empty
filter socket. Obviously, with 3.1, 2.1, and 1.5 KHz filters, I have dedicated
this receiver to SSB. You might think the 1.5 KHz filter would be too narrow
for SSB, but when the passband tuning is adjusted properly, the narrow 1.5 KHz
filter is very useful when QRM is severe. The use of two 3.1 KHz mechanical
filters in cascade produces an excellent response curve for use with stations
producing quality audio. The combined filters provide a - 6 dB bandwidth of
about 2.8 KHz and -60 dB skirts slightly over 4 KHz wide. The audio quality of
this modified A-4 is excellent with that filter combination. When the going
gets a bit rougher, then a 2.1 KHz filter is switched in. This filter is still
followed by the 3.1 KHz filter which still improves skirt selectivity and
increases the ultimate rejection from -60 dB to well beyond -80 dB. It's
actually better than that, but my test equipment won't measure beyond this
I have a modification that I have installed in several 75A-4 receivers. It's a special, active 2.2 KHz ceramic filter (with excellent specs) inserted in the circuit just before the Q-multiplier. I use this new filter in combination with a standard 2.1 KHz mechanical filter. The results are spectacular. You can't imagine how well the passband system in the A-4 works when you have really narrow skirted filters installed. I've also added this mod to my 75S-3C with the same excellent results. In case you're wondering, the mod is reversible in only minutes. I've also modified one 75A-4 by using a standard 3.1 KHz mechanical filter backed up with a 3 KHz ceramic filter. That setup is even better than the double 3.1 KHz mechanical filters used in the 75A-4 presented here. Before I modified the A-4 with the 2.1 KHz mechanical and 2.2 KHz ceramic filter, it had a 6 KHz mechanical filter backed up with a 6 KHz active ceramic filter. I also left the 3.1 KHz filter in place so that I could select the least effected sideband when QRM was present. Since I'm not into AM, so the 2 KHz filters remain installed.
Other fairly standard modifications to this 75A-4 include a switch in RF amplifier tubes from a 6DC6 to a 6GM6. The first mixer was changed from a 6BA7 to a 6DJ8 and the second mixer is now a 6EW8 (was a 6BA7). This improves sensitivity tremendously without sacrificing dynamic range. There is an even better first mixer mod which uses a 6ES8, twin remote-cutoff triodes with a very low noise characteristic combined with tremendous overload and cross mod immunity. I'm going to install that mod in one of my clunker A-4's.
Operationally, this 75A-4 has a sensitivity of about .2 uv on most bands. The PTO linearity is within 100 Hz from band-end to band-end. Stability is such that if allowed to warm up for about half an hour and then set on frequency, it will stay there for the rest of the evening. It only moves about 100 Hz during warm-up. Unlike many of the 75A-4 receivers I've had, the backlash in the passband system is less than 50 Hz. Tuning dial backlash is imperceptible.
The 75A-4 and this one in particular is a wonderful receiver. It looks like a real radio and it feels like a real radio. When it comes to pulling signals out of the QRM, few receivers can surpass it. It will definitely give modern, quality transceivers a run for their money under all but the most difficult band conditions. And..... sometimes the A-4 wins even then.
|A 75A-4 from the inside out. Photographs and test specs of a 75A-4 I sold on the internet. An interesting article.|