[Boatanchors] Hii FI amp as speech amp

[Flowertime01 at wmconnect.com]

Barrie,

Total power transfer from the audio amp to the power amp will determine the 
size of the transformer you need to use.

Audio transformers and 60 hertz transformers are not the same, that said a 
power transformer will work. A 400 hertz transformer will work better than 60 
hertz. But do not go off and buy one if you do not have one. 

Spend an hour reading on transformer ratios in a hand book or on line.

You want to reflect back on the value needed to drive the transmitter final 
to the audio amp 8 ohm output (or 16 or 32 or what ever the real value of you 
audio amp is)

So this come out as a turns ratio. Use the input to output voltage.
120 volts in 12 volts out = ratio of 10. An 800 ohm load would reflect as 80 
ohms.
120 volts in 6.3 volts out = ratio of 20. An 800 ohm load would reflect as 40 
ohms.

You could use a pair of 12 volts in series to go from 8 to 80 to 800.

The next thing is power 50 watts at 120 volts = 2.4 amps. So here you need a 
12.6 volt transformer that would put out 2.4 amps from a 120 volts. That is a 
hefty filament transformer. A pair would get you from 8 to 80 to 800 and 50 
watts of audio would only drive 50 watts of RF to 100 watts at 100 percent 
modulation (think peak to peak here).

In the audio line, transformers are called 70. volt line transformers. You 
need a big hefty 50 watt critter.

The other way around this problem (Heathkit DX100) was to run two tubes in 
series. Ground, cathode of audio modulator, plate of audio modulator, cathode of 
RF amp, plate of RF amp, power output plate tank circuit. Then you just need 
a couple watts to drive the grid of the modulator tube. The RF tube gets bias 
some where at 1/2 of the B+.  You also only need a couple watts to drive the 
RF amp.

Some times the DC power is then generated in split power supply as both a 
positive and negative voltage. This arrangement also helps with the filter 
capacitor arrangement. The modulator cathode is run at minus voltage. The modulator 
plate and RF amp are near zero volts. The RF amp plate is at B+ So the bias 
voltage on the RF amp is near zero volts.

Running a modulation transformer in the RF B+ or Cathode requires a 
transformer that will transfer 1/2 the power you expect to run in the RF final. Then 
you get to match the impedance's. Very doable 

Keep reading and good luck with the project.

Roger AI4NI
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