[russell_w_b]

I would always use a 3k3 resistor on a Trimphone. I plugged in my (un-resistored) 1975 722 Mk:2 and the bell on my N1071 (Type 59A bell with 3K3 resistor in series) fell silent when it was meant to ring.

So I did a few tests... When I put the Trimphone on my ringing generator supply (nominal 100V unloaded) it drew nearly 59mA and pulled the voltage down to 47.3V! This gives it an impedance (the Trimphone ringer is near as dammit resistive) of 800 Ohms(ish): a very high REN! Not a problem with series bells on the old system.

To complicate matters, the impedance follows an inverse curve and varies from 1962 Ohms driven with 5mA to 800 Ohms driven with 59mA.

The unmodified Trimphone was shunting the 59A bell and 3k3 resistor combo and sinking the combined ringer impedance to about 710 Ohms: only the Trimphone sounded, and most of the ringing voltage was being dropped across the 1u8 cap in the master LJU.

With a 3k3 resistor in the Trimphone and an unmodified 59A bell movement, the combined impedance will be around 1800 Ohms Z. With a 3k3 resistor in the Trimphone and an 3k3 resistor 59A bell movement, the combined impedance will be around 2390 Ohms Z. Both Trimphone and Type 59A bell movement instrument will ring.

The good news is that the Trimphone volume is hardly affected. The sounder is cleverly designed to point downwards and utilise the gap between the base of the telephone and the surface upon which the telephone is mounted to resonate the audio tone produced. (I've exploited this on Morse oscillators in the past by mounting a flat plate a centimetre or co above the hole behind which a rocking-armature 4T inset receiver is mounted).

The current drawn by the Trimphone is unaffected by the volume / off control.