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.

Check out these features

Remember this was more than 40 years ago!

  1. Frequency meter dial accuracy (also available on earlier 75A-series receivers)
  1. Overload resistant - An excellent front-end design with 17 dB less gain than the 75A-3, combined with proper gain distribution, excellent filters and an effective AVC system produced a receiver capable of handling very strong QRM signals immediately adjacent to a weak desired station. The focus was not on greatest possible sensitivity but on the best overall compromise between sensitivity and dynamic range. In this area, too, the 75A-4 was way ahead of its peers.
  1. Mechanical filters - The ultimate in selectivity at the time. It took dozens of i.f. transformers at 455 KHz, a string of i.f amplifiers and incredibly critical alignment to match the selectivity of a small, self-contained mechanical filter. Other receivers added an extra mixer and moved the final i.f. frequency down to the 50 KHz range. Still, it took at least 8 hi-Q tuned circuits to approach mechanical filter performance. Unfortunately, nearly all quality receivers used only four tuned circuits (i.e., Drake R-4 series up to the C-series, SX-115, etc. The Hallicrafters SX-88 was a notable 8 tuned-circuit exception.) Mechanical filters were introduced in the 75A-3 receiver.
  2. Passband Tuning - Collins engineers created the most effective QRM fighting tool and it's still featured in nearly every receiver today. Combined with its excellent notch filter, the 75A-4 had an incredible arsenal of QRM fighting weapons. Amazingly, Collins was able to achieve passband tuning mechanically. This eliminated the need for additional mixer stages which could adversely affect the performance of the receiver. The combination of steep sided mechanical filters and clean oscillators (no spurious mixing products to destroy the effectiveness of the filters so common in modern rigs) produced a still impressive passband tuning system.
  3. Tracking first i.f. strip - The 1.5 - 2.5 MHz first i.f. strip is gang-tuned with the PTO. Yet, the PTO is still silky smooth and i.f. tracking in nearly perfect. (also a feature of earlier 75A-series receivers)
  4. Q-multiplier notch filter - It was far more effective than crystal notch filters in that it did not distort the passband shape badly and the notch depth was consistent and didn't need to be optimized with a front panel "notch depth" control.
  5. Product Detector - This incredible piece of circuitry moved SSB out from the annoying, distorted "Donald duck sound" sound to what, even to this day, is impressive sounding SSB and CW audio.

  6. The 75A-4 embodied everything needed for reliable SSB communications -

  7. Stability
  8. Selectivity
  9. Linearity
  10. Passband Tuning
  11. Product Detector
  12. SSB and CW Delayed AVC

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.

Jim's 75A-4

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 range.

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.
Still more 75A-4 Fun!
A 75A-4 from the inside out. Photographs and test specs of a 75A-4 I sold on the internet. An interesting article.