The audio output from the R1155 is designed for high-Z headphones. The noise in a
Lancaster bomber (Halifax or Stirling) was unbelievable! The operator had to wear
headphones, a loudspeaker would have been totally impractical. However, here is a
really simple beam-tetrode design to accommodate a loudspeaker. I used a Russian
6P1P which has characteristics similar to a 6V6 or 6AQ5.
Note that in the PSU, the -ve side of the 220V is NOT connected to the chassis. This
is because in the R1155, the chassis actually ‘floats’ to allow a -ve bias of about
30V to be produced. The 39R resistor is chosen to match the characteristic of the
valve rectifier, in this case an EZ81. I didn’t have a suitable smoothing choke,
and if I did, I doubt I would have had room for it, so I simply put a 300R resistor
between the two 33uF capacitors. Quite by chance, the on-load voltage is 217V which
is precisely the output of the dynamotor. S3 and the 3K3 resistor serve to provide
a ‘Standby’ facility. A jack socket is provided for high-Z headphones. This is actually
necessary when operating the T1154 in CW or MCW mode since the side-tone is fed to
the R1155 audio stage and is too loud for me via the loudspeaker!