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why we need Load resistor?

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
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Hey Duke, I'm a bit confused. You didn't specify exactly where I'm "a little wrong"? Ha,... if I am, it won't be the first time in my life, and most assuredly it won't be my last! :p That said I need specifics. Uh,... without being nitpicky! :D

Chris
 

duke37

Jan 9, 2011
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Hi Chris,
I can manage to be a lot wrong, it happens often !

In the case of a valve amplifier, the output impedance of the valve will be very high so if this is matched to the speaker, the anode voltage will vary a lot but there will be little change in current.
The power output will be maximised at this volume control setting but will be much lower than that possible. The voltage/current (load) line is set to get the right swing of voltage and current. The load line resistance is roughly the maximum voltage/maximum current.

The maximum power is when the source and the load impeances are the same but there are limitations which normally predominate. In a radio receiver, each stage may be matched to get the maximum gain but this cannot be done when significant power is involved. Then, the load impedance is higher than the source impedance to get most of the power delivered to the load.

Sorry about rambling on, I have not woken up properly yet so I may be wrong !

Duke
 

CDRIVE

Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Joined
May 8, 2012
Messages
4,960
Hi Chris,
I can manage to be a lot wrong, it happens often !


Sorry about rambling on, I have not woken up properly yet so I may be wrong !

Duke
Hi Duke,
I don't think it's a question of right or wrong. My post in question didn't address inter-stage power transfer of valves or solid state. It dealt only with the PA stage and what it sees looking out into the output port and what the load sees looking back into the output port. It was more of an observation of the difference between solid state vs vacuum tube design as it applies to the PA stage and power transfer to the load. My main focus was the output transformer (employed in valve designs) as one of many possible factors that may have contributed to the belief that the Maximum Power Theorem should be a goal that's always satisfied. I threw RF power amps into the mix (whether solid state or valve) because of the importance of impedance matching when dealing with transmission lines, as well as milking maximum power transfer.

BTW, I'm aware of the fail-safes like fold-back circuitry employed over the last 30 years or so but they didn't commonly exist much beyond that time.

Chris
 
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