[AMRadio] AM Usage with Linear AMPS

Brian Carling bcarling at cfl.rr.com
Mon Nov 22 22:44:23 EST 2004

On 22 Nov 2004 at 22:23, Gary Schafer wrote:

> Brian,
> 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)

AH ha - I thought we were talking INPUT. 
For 50 watts per tube to this Quad 811A amplifier, then
I don't know why you would only get 30% efficiency but let's say 
you did. 50 watts out = about 166 watts input.
That would mean 116 watts of dissipation per tube.
I agree - it's excessive. Just not sure why it is 30% efficient.

I was basing it on 50 watts INPUT per tube.

> 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.

> 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.

Hmmm where did I get 166 watts?
> 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.

But not dissipate, I guess...
> 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.

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 
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.

Maybe it was more than 30% efficient... that part still mystifies me!
Sometimes I am dense though.

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