[Drake] Re: T4XC on AM

[Barry L. Ornitz]

Jon, K6JEK, asked:

> Anyone have any experience tweaking a T4XC on AM?  I would 
> like a little more resting carrier and actually a little 
> less peak since I'm driving a linear.  Seems like
> unbalancing the modulator a little more and just plain 
> turning down the gain might do it.

It sounds like you want the signal to appear more like a plate 
modulated AM rig and not the controlled carrier modulation 
that it really is.  Remember that the Drake output tubes are 
operated in Class AB1, not Class C.  Likewise the linear 
amplifier uses Class B or AB amplification.  What you are 
asking for is an increase in tube dissipation with no real 
increase in modulated AM output.  

The T4X is rated for 200 watts PEP AM input.  This is the 
equivalent of a 50 watt input Class C amplitude modulated 
stage as the peak power of a fully modulated carrier is 4 
times the carrier power. 

Driving the grids of the Drake finals harder will do a little 
to increase the carrier output, but it does a lot more to 
produce intermodulation distortion.  The primary control of 
the rig's output is the screen voltage on the finals which is 
modulated by the clamp tube driven by the audio stage.  [In my 
T-4X, I installed a switch in the screen supply line to the 
finals so I can disconnect the screen voltage and ground the 
screens instead.  This drops the output to around a watt which 
is what I needed to run a transmitting converter for two 
meters SSB.]

If you have an oscilloscope and a low distortion audio 
oscillator, you can play with the excitation and screen 
voltage levels to optimize the AM operation of the T-4X.  But 
it will never equal the fidelity of an optimized plate 
modulated transmitter.

        73,  Dr. Barry L. Ornitz     WA4VZQ     ornitz@tricon.net