Homebrew regenerative receiver - UM58

This 2-valve regenerative receiver is a folly, actually. I decided to build it after I found back a few parts of a 1-valve regenerative receiver I constructed when I was 12 years old. That receiver never worked properly and I tore it apart. The UM58 receiver is an extended 2-valve version of that design. Building it was my way to take revenge on the past.

This is a regenerative receiver with an RF pre-amplifier and two tuned circuits. I have built it after a 1960 Amroh design "UN58" that appeared in the book "Jongens Radio" (Boys' Radio), published by De Muiderkring in the early sixties. I called my version "UM58" because all my projects have an "M" in their name :-). Amroh was a well-known radio trading company in The Netherlands in the 1930-ies until the 1960-ies. They sold radios, components and kits and published a lot of designs through their publishing company, De Muiderkring. If you can read Dutch, Robbie Hooft can tell you a lot about Amroh and the fond memories 1960's radio hobbyists have about it.

The circuit board and 402 coils I found in my junk box.

It is a rebuild and extension of a similar radio I built in my youth. That one had only one valve and one tuned circuit. Like the UN58, it used the famous Amroh "402" receiver coil for medium wave reception. My one valve radio never worked properly so I took it apart again. Most of the parts were discarded later on. Recently, I found back the pertinax mounting board with the larger holes for the tuning capacitor and the output valve socket cut in it.

Somehow, I had acquired a second "402" coil since the time, so I decided to build a two-circuit, two-valve radio after the UN58 design. This is an extension of the earlier radio, using the same mounting board, with the RF stage and extra tuning circuit added.

I wasn't able to get all the components from the original design. So I had to use a slightly larger tuning capacitor I happened to have on hand.

Construction drawing from "Jongens Radio".
Click on it to enlarge.

I did not have the Amroh transformers nor the loudspeaker. Neither did I have a selenium rectifier. In my junk box I found an output transformer and a small mains transformer from an old German colour TV. I used silicon rectifiers for the plate voltage. I found a small Philips loudspeaker that was still a bit too large as you can see on the photographs.

There is also a scan of the UN58 schematic here. The circuit contains two valves: an EF93 variable-mu penthode and an ECL86 triode-penthode combination. The EF93 is the RF pre-amplifier. It is also used to implement a peculiar way of volume control: by controlling the cathode bias of this valve, the gain of the RF stage is controlled. The triode section of the ECL86 is used as regenerative plate detector. The penthode section is used as the AF output stage.

Shrinery

Pieces of plywood...

...to form a home-made case.

I made a wooden case to look like a 1950-ies simple home-made case. The sides are plywood. The front edges of the top and bottom have rounded corners. There are small blocks of pine wood glued in the edges to reinforce the joints of the 4 parts.

Front plate, hammered finish.

After assemby, I varnished the case and it looks quite credible. I mean, credibly amateuristic :-).

The front plate I had to cut and drill from a piece of sheet aluminium, as I don't have the original Amroh Uniframe front plate. Then I bolted the loudspeaker baffle to the front plate, mounted the tuning capacitor and other controls, and mounted the circuit board on 4 posts, parallel to the front plate.

Assembly on front plate. Output transformer to be added.

My UM58 worked right away. I powered it up using my faithful HV power supply and let the plate voltage come up slowly, through a large resistor, to let the old HV electrolytics reform after their decades of rest. Then I turned on the heater supply and waited what was to happen. Noise came from the loudspeaker and at the turn of the tuning knob I was delighted at the familiar sound of stations coming in and out of tune. The sensitivity became better after trimming the input RF stage.

The sound quality is acceptable but no hi-fi, as you can expect from a somewhat crude radio like this. There is a lot of noise and funny whistles. Maybe there is added interference caused by the weak shielding between the stages because it is built on a Pertinax mounting board instead of a metal frame. But sensitivity and selectivity are quite good. I can get around 20 stations on MW using a 3 m indoor wire antenna in my attick.

And this is the final result... on the inside

On Regenerative Receivers

Regenerative receivers are a special breed of radio. They give you a lot of performance from only a few valves, but they have a temperament. Because there is so much to experiment and you can get quite interesting results with few components, radio amateurs and people with small budgets used to love them. But in the hands of a clumsy person they can be disappointing, which is why most commercial radios have been built as superhets since the late 30-ies. Today, it would probably even be illegal to sell a regenerative receiver, because they can cause a lot of radio interference (the notorious "Mexican Dog" howl).

There are still some people who build these "glowbugs". Take a look at the links page to find a few I found particularly interesting.