The AA8V Twinplex Regenerative Receiver
by Greg Latta, AA8V

How To Use And Get The Most Out Of A Regenerative Receiver

Twinplex Receiver

Twinplex Receiver Pages:
 Main Page and Exterior Photos  How To Use A Regenerative Receiver
 Interior Photos of the Receiver  Receiver Schematic Diagram and Circuit Description
 L1 and L2 Coil Specifications  6SN7 Dual Triode
 Twinplex Receivers Built By Others  Modifications



How To Use And Get The Most Out Of A Regenerative Receiver:

There is nothing like the thrill of listening to stations and programs on a receiver that you have built. And there is nothing like the way that an analog radio tunes the bands compared to a modern, digital radio. The way an older analog radio tunes when scanning the bands is just plain better, at least in my opinion. I have several receivers in my shack, ranging from a modern TenTec DSP ham rig down to the Twinplex receiver. Despite the presence of other more modern receivers, I still like to use the Twinplex on many occasions.

The big advantages of the regenerative receiver, its simplicity and cost, are also its downfall. To get the most out of a regenerative receiver you have to know a few tricks, often use two (or three?) hands at the same time, and spend a little time practicing with it. Below I have tried to outline the most important things you need to know to get the most out of a regenerative receiver. I hope you find them useful.

Finding the Critical Spot on the Regeneration Control:
The first thing you must learn is to find the critical spot on the regeneration control. To do this, hook up your receiver and plug in a coil that covers a frequency somewhere in the 3 MHz to 6 MHz range. Set the antenna coupling to the middle of its range and then turn up the volume. Slowly turn up the regeneration control until you hear a gentle "plop" or an increase in the hiss in the headphones. The point where this occurs is the point where the detector starts oscillating and is known as the "critical point". Just above or below this point is where the receiver is most sensitive.

If you cannot get the detector to oscillate, try lessening the antenna coupling. (Unmesh the antenna coupling capacitor.) You may have to set this control to its minimum to get the detector to oscillate. If you still can't get the detector to oscillate, tune to a radically different frequency or use a different antenna. You may be on a "dead spot" where the antenna tends to absorb energy from the detector and prevent it from oscillating.

If you still can't find the critical spot and get the detector to oscillate, there may be a problem with your receiver. You may have a weak or defective tube, your batteries may be weak, or you may have the tuning coil or tickler coil wired backwards. You may also need to add turns to the tickler coil. It is up to you to figure out the problem if you can't identify the critical spot on the regeneration control.

Once you have found the critical spot, you will find that its location varies (sometimes greatly) with the antenna used, the setting of the antenna coupling capacitor, the frequency the receiver is tuned to, and the particular coil that you are using. This is one of the big disadvantages of the regenerative receiver.

Regeneration Control:
When tuning for stations you will usually want the regeneration control just below or just above the critical point. For AM and shortwave broadcast stations you will want it just below the critical point. For CW (Morse code) and SSB (Single Sideband) signals you will want it slightly past the critical point. As you tune into a station you will probably need to readjust the control for better reception. Too much regeneration can be just as bad as too little.

Receiving AM and Short Wave Broadcasting Stations:
For AM/SW stations advancing the control towards the critical point makes the receiver more sensitive and more selective. Higher selectivity means that you can separate stations better, but also means it is easier to overload the receiver. With strong AM/SW stations you will have to retard the control to prevent distortion. With weak stations you will have to advance the control to make them loud enough.

If you are having trouble separating two stations, try advancing the regeneration control to increase the selectivity. You will have to retune the receiver after advancing the regeneration control. If you still can't separate the stations, see the section below on adjusting the antenna coupling control.

In some cases, especially if a very strong station is next to a very weak one, it may be difficult, if not impossible, to separate the two. (This is another disadvantage of the regenerative receiver.)

General Comments on Receiving CW (Morse Code) and SSB (Single Sideband) Stations:
For CW and SSB stations, the regeneration control is used differently. For these stations you must have the regeneration set past the critical point, so that the detector is oscillating. Retarding the control back towards the critical point makes it more sensitive and more selective, but also increases the chance of overload. For strong CW and SSB stations you may need to advance the control beyond the critical point to prevent the receiver from "blocking". When the receiver blocks, the incoming signal controls the oscillation frequency, and you cannot decode the CW or SSB. Advancing the control makes the receiver oscillation stronger and prevents the received signal from taking over.

Receiving CW Stations:
For CW stations advance the regeneration control just past the critical point. For strong stations, blocking will occur at this point. For such stations, continue to advance the control until the blocking stops. Finally, tune the receiver to a comfortable pitch. Note that as you tune, the pitch will drop and then start to come back up again. The point where the pitch is lowest is called zero beat. You can tune on either side of zero beat. You may, however. find that interference worse on one side of zero beat than the other. Choose whichever side of zero beat works best.

For very strong CW stations, you may not be able to prevent the receiver from blocking by advancing the regeneration control. In this case, try loosening the antenna coupling (unmesh the antenna coupling capacitor.) You will have to retune the receiver as you do this, and then readjust the regeneration control. If this doesn't work, there isn't much you can do. This is another disadvantage of the regenerative receiver.

Receiving SSB Stations
For SSB (Single SideBand) signals, tuning is much more critical. If a SSB signal is tuned in with the regeneration control below the critical point, is will sound like a severely distorted signal. Advance the regeneration control past the critical point. With a very strong SSB signal you may need to advance the control well beyond the critical point to prevent blocking. Unlike CW signals, SSB signals will only tune correctly on one side of zero beat. If you try to tune a SSB signal on the wrong side of zero beat all you will get is gibberish. You must tune very slowly and delicately, first on one side and then the other of zero beat, until you find the place where the signal sounds correct. (But see my comment below on ham stations in the 160m, 80m, and 40m ham bands). For some receivers and on certain bands, the tuning rate may be so fast that you cannot successfully tune in the signal. For your first attempt at tuning in SSB, try tuning the 160m (1.8 MHz - 2.0 MHz) and 80m (3.5 MHz- 4 MHz) amateur radio radio bands in the evening after sunset.

Most amateur stations in the 160m, 80m, and 40m bands use lower sideband by convention. This means that you should tune in these stations on the higher frequency side of zero beat. In other words, start too high in frequency and then very carefully and delicately tune DOWN to the station. At some point the audio should clear up and you will be able to understand what is being said. Try this first with a medium or weak station. Stronger stations may block the receiver and make them very difficult if not impossible to tune in.

For very strong SSB stations, you may not be able to prevent the receiver from blocking by advancing the regeneration control. In this case, try loosening the antenna coupling (unmesh the antenna coupling capacitor.) You will have to retune the receiver as you do this, and then readjust the regeneration control. If this doesn't work, there isn't much you can do. This is another disadvantage of the regenerative receiver.

Bandset and Bandspread Controls:
The bandset control is the coarse tuning control. It is used to choose the portion of the band that you wish to tune, while the bandspread control is used for fine tuning. One way to scan a band is to fully mesh both controls. Then start to unmesh the bandset control slowly (tune up in frequency) until to hit a region you find interesting. Than back up the bandset control a bit and start to unmesh the bandspread control to tune the region of interest.

Another way is to start with both controls unmeshed and start to tune down in frequency (mesh the bandset control). When you hit something interesting, back up the bandset capacitor a bit and then start to mesh the bandspread control to tune the region of interest. After a little practice this will become second nature.

The Antenna Coupling Control:
The antenna coupling control is used to control how tightly the antenna is "connected" to the receiver. The more tightly the antenna is coupled (the more the antenna coupling capacitor plates are meshed) the stronger is the signal fed into the receiver. Unfortunately, tighter coupling also makes it harder for the receiver to achieve regeneration (you will have to advance the regeneration control as you tighten the antenna coupling). Additionally, tighter coupling makes it harder for the receiver to separate stations. (The selectivity becomes worse). Increasing the antenna coupling also lowers the frequency that the receiver is tuned to. If you are listening to a station while you adjust the antenna coupling, you will have to retune the station at the same time, so be prepared to use two hands.

If the a station you are listening to is too weak, and you have the regeneration control set the best you can, try increasing the antenna coupling. (Mesh the antenna coupling plates to increase the capacitance). You will have to retune the station as you do this. Then try readjusting the regeneration control. You may obtain a significant improvement in volume this way.

If a station is too strong and is overloading and/or blocking the receiver, try readjusting the regeneration control as discussed above. If that doesn't work, decrease the antenna coupling while keeping the station tuned. Then try readjusting the regeneration control. This may eliminate the overload and/or prevent the blocking.

If you cannot separate two stations, try decreasing the antenna coupling while you keep the station you want tuned. Then try readjusting the regeneration control. In this way you may gain enough selectivity to separate the two stations.

Volume Control:
The volume control sets the overall volume of the receiver. The regeneration control can also be used to adjust the volume of the receiver, but this usually has other, undesirable, side effects and it is preferable to use the volume control, rather than the regeneration control, to set the volume.

Interaction Among Controls:
If you have read the above, you will see that many of the controls in a regenerative receiver interact with one other, particularly the tuning, regeneration, and antenna coupling controls. This may require you to use two hands while operating the receiver. Adjusting the regeneration control with one hand while you tune the receiver with the other is a great way to get more performance out of your receiver. And using one hand one the regeneration control, a second on the tuning control, and a third (?) on the antenna coupling control will get even more performance out of your receiver, if such is possible....

Some Final Comments:
You can see why the regenerative receiver was eventually replaced by the superheterodyne receiver, which was essentially a "one handed" receiver. Superhets were more sensitive, easier to use, but much more expensive. They were also too complicated for the average person to build, unless built from a kit, and they ushered in the era where most receivers were commercially manufactured.

Virtually all commercially manufactured receivers these days are superhets, and though cheap, good performing receivers are readily available, you still can't beat the thrill and challenge of using a regenerative receiver you have built yourself.

73 and have fun,
Greg Latta AA8V


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