Gerard's Homebrew EF80

35
Item nr.

Homebrew EF80 Table top

A homebrew radio of my own design and construction.

**Data for Homebrew EF80**
Production	The Netherlands, 1996. Price was 15 guilders.
Bands	MW.
Tubes	EF80 (1st RF), EF80 (2nd RF), EF80 (det.), EF80 (1st AF), 2xEF80 (output), AZ1 (rect.).
Cabinet	Wood (former loudspeaker cabinet). Size 36x28x17cm.
Power	AC.
Documents	schema.

The Design

A friend moving out of town cleared his attic and gave me a box of 50 electron tubes, among which there were 18 of type EF80. This inspired a radio-building project, aimed at constructing a radio containing EF80 as the only tube. As the EF80 does not make a good rectifier, I included one other tube (the AZ1). I didn't have to economise on tubes so I planned for seven of them.

Besides the tube line-up, I aimed for two more design criteria. Second, I wanted to do myself as much as possible: so I chose for handwound coils. Third, I wanted to use recycled parts as much as possible to allow many scrapped apparatus to donate some organs to the project. This explains for the cabinet (scrapped loudspeaker box) and the power cord (three-prone from refridgerator).

It was a real headache to design the thing (Henk Tattje was a great help in reviewing the schematic), but designing and constructing your own radio is a lot of fun. This thrill when it plays for the first time!

The Schematics

I used two radio-frequent amplification stages (V1 and V2), followed by a plain and simple diode detector. Coupling between antenna and each of these stages is through a transformer, of which the secondary is tuned (there is a three-gang tuning capacitor). The diode detector is, of course, an EF80 penthode, V3, but simulating a diode with all electrodes except the kathode connected together. Because there is no Automatic Gain Control (the EF80 is straight), there is a manually adjustable attenuator P1 between the antenna and the first tube.

The audio-frequent pre-amplifier is an EF80, and the output stage is planned for two EF80s connected in parallel to increase output power. The power supply is fairly standard but noteworthily it uses 50 year old electrolytics from a Philips radio, that still had their original capacity and almost no leakage.

Obtained	1/1996 from Built it myself..
Condition	8.
Disposed	Sold 9/2000.

This Object

With the three abovementioned disign criteria in mind, it would almost be a miracle if the performance were particularly good. For winding the coils I cut a ferrite rod in three (almost equal) pieces, wound them with ordinary construction wire and put them in the original aluminium cans of the scrap chassis I used. It took me several evenings to actually build the chassis as shown here:

It was very difficult to keep the radio from oscillating. I had to bend a few wires in the HF section, and the LF part always oscillates if I place EF80s in both output tube sockets. So I use the set with only one output tube.

The radio plays and receives about 20-30 stations in the evening. Selectivity is rather poor, especially on the lower part of the MW band. This performance results from the handwound coils. Their Q is lower than that of factory coils, and it is difficult to match their inductance precisely so that the synchrony between the three tuned circuits is difficult to achieve.

After all, the performance is hardly better than what can be achieved with a single tube, but to build this thing was a lot of fun. I later gave it away to make room for my next homebrew radio.

Part of Gerard's Radio Corner.
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