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