Originally published in Radio
and Hobbies in Australia, April 1943, the Tiny Tim 2 is one of the simplest
mains operated receivers. A regenerative detector feeding an output pentode
provides good speaker volume on most receivable stations.
A trap for those not familiar with valve circuits and Australian circuit diagrams of the period; the terminals marked "SPKR" are for feeding into the output transformer (7000 ohms for the EL3NG), not directly into the voice coil.
The 6J7 (modern equivalent
is 6AU6) functions as a regenerative grid leak detector with the feedback
winding fed from the screen grid, rather than the plate or cathode as seen
on other circuits. In effect, the screen is functioning as a plate as far
as the feedback winding is concerned, with the detected audio taken from
the plate in the usual way. The coil is a commercially made Reinartz type
marketed in Australia by firms such as RCS radio, or it can be made to
the following specifications:
Coils are wound on a 1.25" (32mm) former.
The receiver was intended
to have the coils on plug in formers for changing to the other bands. Data
is given for reception up to 30Mc/s. Although I made my receiver for MW
only, I used a plug in coil former because I had the genuine type on hand.
The detected and amplified audio then feeds into an EL3N output valve. For those outside Australasia and Europe, this is a high gain pentode of Philips design. I has roughly twice the gain of a 6V6. Although it's pin compatible, the bias is less and the load impedance is 7K instead of 5K. Power output is also a fraction less. The 9 pin equivalent is 6M5 (EL80) which was used in thousands upon thousands of Australian radios. Due to the high gain, a grid stopper resistor is often required. This, and an associated condenser are shown dotted in the circuit, to be added if required. Note that this circuit was one of many published by R&H during the war years when components were difficult to get, hence the simplicity and being designed around standard kinds of components.
Coil clearly visible in the bottom lefthand corner. Between it and the 6V6 output valve is the trimmer to set the dial calibrations correctly. The two aerial terminals are on the left, with the speaker terminals on the right.
I also had an RCS dial and
single gang tuning condenser as per the original design, so these were
pressed into service.
Essentially, my version of the Radio & Hobbies design is the same, except I've used a 6V6 instead of the EL3N. (The cathode bias resistor was of course changed to 250R). Also, as I'm not using an electrodynamic speaker, a filter choke
filters the high tension instead of a field coil. And, due to length of my outdoor aerial with its resultant absorbtion effects, I've added an extra aerial terminal which puts a 470pF condenser in series with the aerial.
In this pic, the grid leak resistor and condenser are visible connected to the 6J7 top cap.
Under chassis view. Speaker transformer is mounted in the centre. The knot in the mains cable is politcally incorrect but is correct for the period, is simple, and works. Note the simplicity of the circuit.
Performance is excellent.
Even though I'm using a 6V6 instead of the EL3N, the receiver easily provides
Regeneration is particularly smooth with absolutely no backlash. I've never liked the method of regeneration that involves a variable capacitor in series with the feedback winding. Not only is there backlash, but it affects the tuning. Besides, the 100pF value usually specified isn't what I'd describe as a common component. It's more expensive than a potentiometer too.
email me: cablehack at yahoo dot com