A Microphone Pre-Amplifier for the T1154
3 minute read
Self-contained microphone pre-amplifier
Not all models of the T1154 were capable of producing Amplitude Modulation (R/T on the Master Switch). The A and E models did not have this facility. In the airborne installation, the crew Intercom amplifier type A1134 was normally connected to the Microphone Input (+&-) on the front of the T1154. To cater for this, the link inside the transmitter had to be set for a ‘dynamic’ microphone as opposed to ‘carbon’ (see below). I tried a vintage carbon microphone and found the level of modulation to be pitiful. In fairness to the transmitter, this may well have been due to the carbon granules clumping together over time and thus reducing the output from the microphone. If I was going to use the transmitter in the manner that it would normally be used, an external microphone amplifier should really be used. A1134s can be found, but they are very rare. The amplifier was powered from a 120V HT battery and a 2V battery. Although the valves can still be found, there are two complex multi-winding audio transforms inside, and if one of these went faulty there would be little chance of repairing it unless you were very lucky.
I could have easily made a suitable pre-amp that would have fitted in a match-box. But I hit on the idea of a more fitting design using valves. It would need to be self-contained with it’s own PSU.
mic selector plate
microphone pre-amplifier schematic
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 from 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.
Note the ancient variable resistor with the original R1155 knob!
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