[TMC] TMC and AM?

[John Vendely]

Hi Eugene,

TMC made a plate modulated, 1W AM/CW/MCW exciter, the Model GPE-1, which 
will work directly with your PMO, and is ideal for driving any of the 
TMC linear power amps.  You can get 125W of very good AM with your 
PAL-500 this way.  This is what TMC originally supplied to customers who 
still used AM, and I think this is your best bet.

Among their sideband exciters, the TMC MMX-2 series solid state exciters 
are best suited for AM, as they have a separate AM modulator in addition 
to the ISB modulator.  However, relatively few were made compared to the 
SBE and SBG series, so they're kinda scarce.  And there are other 
serious difficulties with the MMX-2 because of their complicated direct 
analog synthesizer.  These things are invariably all screwed up when you 
get them, are very tricky to get working, and can have spur problems 
requiring fairly regular tweaking.  They weren't a terribly successful 
product line, but when they're working, they're very good exciters, with 
top notch audio quality on both SSB and AM.

Frankly, I've never quite understood all the current fuss over AM, and 
don't operate AM too often.  Despite all the obsessing over audio 
quality, AM usually sounds horrible, due to selective fading problems, 
particularly during even shallow carrier fades.  There-- I've said it, 
and now I'll descend from the soapbox.  I confess I do run AM 
occasionally, and then I use either the GPE-1A, MMXM-2, or an SBG-1A 
exciter with an external AM modulator.

The SBG-1A frequency synthesized exciters have essentially the same 
sideband modulator as the SBE, but in a separate chassis (Model CBE) 
together with the sideband monitor.  The CBE sends the 250 kc ISB signal 
to the "Frequency Amplifier" (Model CHG), which upconverts it to the 
operating frequency.  For AM, I just disconnect the CBE sideband drive 
to the CHG, and feed in my own high linearity 250 kc amplitude 
modulator.  The rest of the system operates in the normal fashion.

An old buddy of mine devised still another solution many years ago.  He 
added a small plate/screen modulator to the 6146 output stage of an 
SBE-3 exciter.  A small modulation transformer which he improvised was 
mounted inside on the rear apron of the exciter, and provided an 8 ohm 
input impedance for a small external audio amplifier.  Any small hi-fi 
amp that will deliver a good clean 5W will do fine.  He used one channel 
of a Dynaco SCA-35.   In retrospect, he considers the mod a debauchery, 
but it worked great.

I hope these suggestions prove useful.  Have fun.

73, John K9WT

On 3/2/2014 12:08 PM, W2HX wrote:
> So all this discussion about the SBE not being well suited for AM.  So I have a PAL-500 and would like to run AM through it. Can anyone recommend a TMC exciter that will do good AM and will work with the PAL? And something that has a VFO or can use my PMO-5. And more importantly, does anyone have such an animal for sale?
>
> thanks
>
> 73 Eugene W2HX
>
>
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