[AMRadio] AM Usage with Linear AMPS

Geoff/W5OMR w5omr at w5omr.shacknet.nu
Tue Nov 23 07:15:36 EST 2004

> > When you tune up you tune it for max output at the 200 watt level with 
> > full drive. At that tune setting the tube will be around 60% efficient. 
> Assuming Class C about 60 to 70% yes.

No.  Class C is typically 70 to 75%.  60 to 65% is more likely for linear
(AB1/AB2) operation.
> I have run a P&H Linear amplifier at these levels and even the 
> 837s didn't glow red. But it may have been a little below 200 watts 
> carrier.  I only had it for a few weeks so it would not be a fair test.
> Thanks for explaining it in more detail for me.
> I just remember QST articles where a single 811A was run at 200 
> watts input as a linear amplifier or CW final and it did fine.

The QST articles were probably talking about SSB or CW out of a 
single 811A at the 200w area.  More than likely, it AM wasnn't 
considered in that test (*noting that QST is a publication of the ARRL, 
who would for a l-o-n-g time would NOT recognize AM as a valid mode 
of operation.)
> Maybe it was more than 30% efficient... that part still mystifies me!
> Sometimes I am dense though.

Plate Voltage (Ep) * Plate Current (Ip) = DC Input.

So, your quad 811A amp is running 1500v @ 400mA, you're running
600w DC input to the final.  150W per tube.  That's to the max.  
When you tune the amp up for AM operation, leaving the amp at that 
-exact- tuning range, reduce the drive until you see only 150w, and 
probably a little less, to handle any asymmetry/transients in your voice*.  

150W, modulated to 100% is around 600wPEP, but we're still talking 
DC INPUT, when you look at the 150W.  So, 150 * 60% = around 90w
of carrier output, which would be around 360W PEP output.

Your mileage may vary, depending on Ep and Ip.  If you just -have- to 
run a linear amplifier, instead of building up a plate modulated rig, at a 
MUCH higher efficiency rating, then build something with BIG tubes in it,
that would handle the dissipation.  like a pair of (or 3) 450TH's.  
450W of plate dissipation, each.  That'd bring you back up to the 1.5kW
PEP level.

That just might be a nice project... I could make a quad 450TL amp, and 
drive it with my Kenwood TS-680S I use mobile. 
(what a waste of a good tube!)

73 = Best Regards,

(*See: http://www.qsl.net/wa5bxo/asyam/aam3.html for more info on 
asymmetric voice patterns, and a few hams ideas for working WITH the 
1500wPEP Output nonsensical FCC ruling.)

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