[Boatanchors] Re: Hii FI amp as speech amp

[Drew P.]

Barrie wrote:

"I know it's possible to use a Hi Fi amplifier to drive the modulation in the BC-610, but Googling for half a day has not yielded
results."

As Roger mentioned, you can use an external amplifier as a driver for the BC-610 modulator tubes without worrying too much about the amps' output impedance so long as you have a suitable transformer to somewhat match the amp's output to the modulator tube grids.  You need to develop sufficient voltage swing at the modulator grids.  Impedance is either near infinity (class AB1) or varies over the course of the cycle of the voltage (class AB2 or Class B).  It would be well to ballast the secondary of the chosen transformer with fixed resistance to provide a more stable value of load impedance.  This will reduce waveform distortion caused by the varying load presented by the grids.

A selection procedure might be approached thusly:

1. Determine necessary voltage swing at modulator grids.  Determine output voltage swing of amp to be used as driver based on amp's output impedance and power rating.

2. Select transformer of approximately suitable turns (voltage) ratio with sufficient power rating.  This will be influenced by how hard you want to run the hifi amp and how much excess voltage capability at the tube grids you would like to have, and of course, what transformer is available.

3. Select resistors so that the impedance as reflected through the transformer to the amplifier is a somewhat reasonable match to the amp's output.  This assumes that your hifi amp has considerably greater power than necessary to drive the tube grids.  Resistors should be rated to handle the hifi amp's full power.

You might find that a push-pull output transformer connected backwards would be suitable. If your amp is stereo, you could commandeer the transformer in the unused channel.  A 60 cycle power transformer might work.  How about a vibrator power supply transformer from an old mobile FM transceiver or HF AM marine radio?  You could also connect 2 transformers so as to create a center tap connection (need to consider phasing).

Drew