A Microphone Pre-amplifier for the T1154 (/cont)

Absolutely no originality is claimed for the design which is a rip-off of a circuit in a well known publication. However I am proud in a way to say that apart from a few of the capacitors and two of the resistors, it is made entirely from components that I had in the ‘junk box’. That includes the case too!

I have had the mains transformer for close to 40 years and I think it came from TV signal-booster of early 1960s vintage. The audio transformer came form a scrap RA17. Curiously it is not the standard RA17 audio transformer. The 500K gain control was found in the R1155 that I recently re-built. Even the knob came from a scrap R1155. I think the 12AX7 was found in a box of valves that I was given and the Russian 6P1P Beam Tetrode was one of a job lot that I got on eBay.

Valves (or tubes) actually lend themselves to simple point-to-point construction. If it’s good enough for Racal (as in the RA17), then it’s good enough for me!.

The bridge rectifier consists of four 1N4007 diodes. There was no room for a bulky smoothing choke so I simply used a couple of 1K resistors in parallel between two 47uF capacitors. The entire circuit only draws 50mA maximum. Listening to the modulation on my RA3701, the audio has the typical limited bandwidth, no-bass sound that the 1154 is known for. There is no evidence of mains ripple on the resolved AM.

Note the ancient variable resistor with the original R1155 knob!