[AMRadio] AM Usage with Linear AMPS

Gary Schafer garyschafer at comcast.net
Mon Nov 22 22:23:06 EST 2004


What he is saying is: you need to have the 811A tuned up at 200 watts 
output on cw or pep for a 50 watt carrier out. (pep is 4 times carrier)

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. 
Normal ssb or cw operation tune-up. input power will be 333 watts. The 
tube will be dissipating 133 watts in heat at full carrier.

You then reduce the drive until you have 50 watts out (1/4 the carrier 
of full output). You can not retune at this point or it will not handle 
the peak envelope power of 4 times carrier. At 50 watts out the 
efficiency drops to around 30%. So that means to get the 50 watts out 
the tube has to be running around 150 watts input. 150 watts minus the 
50 watts output leaves 100 watts to dissipate in heat in the plate as 
long as the carrier is on.

It doesn't get any better either. With modulation the dissipation does 
not decrease even though the efficiency increases at the peak power 
levels. The carrier power is still there 100% of the time at 30% 
efficiency. The audio is in the form of separate side bands that is 
additional power that the tube has to handle.

I guarantee you that an 811A will run with a red plate under these 
conditions. If it is not red then it is not tuned up properly for this 
mode of operation at the 50 watt carrier out level.

Gary  K4FMX

Brian Carling wrote:
> On 22 Nov 2004 at 20:23, W7QHO at aol.com wrote:
>>Going to shorten it's life considerably. 
> Theoretically maybe. Have you tried it. How can you know 
> for sure? Theory says a bumble bee can't fly. My 811A tubes 
> never burned up.
> Where did you get your 30% number from? Who transmits 
> their AM with carrier only!? I can't see how a 50 watt carrier input
> will cause an 811A to DISSIPATE 100 watts!
> Where are you getting the additional 50?
>>To properly adjust a linear amp for AM it is necessary to tune the thing up for the peak power 
>>level, i.e., 200 watts for a 50W carrier. The linear will be about 60% efficient at this output level. 
>>Then WITHOUT CHANGING THE TUNING adjust the output from the AM exciter to provide for 
>>50W out of the linear with no modulation. The problem here is that the linear will now only be 
>>running about 30% efficiency under carrier-only conditions and the 811 would be dissipating 
>>something like 100+ watts. The 811A, of course, is only rated at 65 watts.
>>Dennis D. W7QHO
>>Glendale, CA
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