Question: Tom, how do I connect and drive my AM modulator tubes with an EXTERNAL hi-fi audio amplifier?
There's not much to it.
Find a hi-fi 20 watt or greater tube or solid state audio amplifier. Also, locate a hi-fi 8 ohm to 2-3K center tapped audio output transformer, 20 watts or greater. The 2-3K winding can be most anything around that range, not critical.
I use here a 55 watt UTC Linear Standard "LS" series audio output transformer to drive my triode connected 813 grids. But, there are many transformers around that were at one time sitting in tube audio amplifiers as the output transformer coupling from the push-pull audio power tubes to the speaker- perfect for the job!
Here's the connection hookup:
Take the 8 ohm output of the hi-fi audio amplifier and connect it to an 8 ohm winding of the audio output transformer. Two leads, floating above ground.
Then take the two 2-3K (or whatever value) high impedance winding leads and connect them to the grids of the modulator tubes. Take the center tap of this same winding and connect it to the same bias source that the original modulators used. This may even be ground in some rigs.
Drive this amplifier from the "auxillary" (aux) input with your normal line level audio, preferably using a mike, preamp and equalizer.
Reverse the winding connections at either the 8 ohm or 2K positions to reverse your audio phase to align the maximum positive peak polarity with your voice. Switching the modulator plate cap wires does the same thing. (Do only one spot, your choice). Experiment...a scope or an on-the-air report can usually determine the proper phase for you.
Transformer Protection and Audio Amplifier Load:
I have a 5K / 20 watt power resistor across the 2K winding so that there is always a constant load there. Winding damage can occur from high audio voltages when no load is present. Also, this steady load for the audio amplifier to work into is desireable for somewhat less distortion.
Real simple and you'l love the sound, OM!
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