[AMRadio] AM Usage with Linear AMPS

Gary Schafer garyschafer at comcast.net
Tue Nov 23 11:05:39 EST 2004

Geoff/W5OMR wrote:
>>>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,
> -Geoff/W5OMR
When you reduce the drive for AM operation you want to reduce the OUTPUT 
to 1/4 power, not the input power. The input power will be somewhere 
around 1/2 of full power when properly setup for AM.

Gary  K4FMX

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