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[K0ZN]

I should know the answer to this, but it seems to escape me....  That is:  In an HF linear amplifier designed and intended for full 1,500 watt legal limit operation, does the plate heat dissipation increase if the amp is operated at lower power levels?  i.e. Say you operate a "legal limit" amp at 500 watts output...does the tube(s) run as hot or hotter than at 1,500?
My thinking is that the design parameters in the tank circuit, operating voltages, etc. are optimized at the higher power level and once you "fall off the curve" at say 500 watts  efficiency suffers, thus more heat is dissipated at the lower power level due to inefficiency.   True or False ??  Wouldn't measuring the exhaust air temp be a good way to confirm or debunk this?

 Thanks & 73,  K0ZN

[K6BRN]

Jon:

No, the plate dissipation of a tube based linear RF amplifier run at 500 watts is not greater than when it is run at 1500 watts output - it is normally much less at 500 watts.

That said, the vast majority of linear RF amplifiers I've  worked with professionally, both tube and solid-state, are most power efficient at or near 100% output.  But in practice, all of them require some degree of "back-off" from full output to maintain signal quality.  The degree of back-off usually depends on the transmitted waveform type.  But this loss of efficiency does not usually mean that they dissipate more watts as heat, just that a larger percentage of the total power consumed by them is lost to heat.

Amateur amplifiers can be another matter, especially since many are not built to handle maximum output continuously anyway.  And SSB in particular has a very high peak to average power ratio, meaning that an SSB signal that reaches 1500 watts peak may only average (for example) 500 watts of RF power.  And if the amplifier really maxes out at 1500 watts peak power, it will be very close to becoming non-linear and could easily begin to generate significant distortion products/IMD, and should be "backed off".

It is common practice to tune Amateur tube amplifiers to resonance at reduced power to avoid component over-stress, with tuning touched up at full power, to ensure maximum output power is available on demand.

Brian - K6BRN

[KU3X]

Let's put this in the KISS mode.
Power in minus power out=plate dissippation.
Example:
2000 plate volts x .500 amps plate current = 1000 watts input power
If you get 600 watts output power that means you have to dissipate 400 watts.
Barry

[AE0Q]

If you tune up an amp at full power out (1500w) and then just reduce the drive to get less output, it will be less efficient and there will be more waste heat.  But most likely the tube(s) have excess plate dissipation and can handle it.  An example of this would be tuning up for AM use.

If the drive is reduced AND the amp retuned for less output, the efficiency will probably be very close to the same and there will be less waste heat.

Glenn AE0Q

[N3QE]

I should know the answer to this, but it seems to escape me....  That is:  In an HF linear amplifier designed and intended for full 1,500 watt legal limit operation, does the plate heat dissipation increase if the amp is operated at lower power levels?  i.e. Say you operate a "legal limit" amp at 500 watts output...does the tube(s) run as hot or hotter than at 1,500?
My thinking is that the design parameters in the tank circuit, operating voltages, etc. are optimized at the higher power level and once you "fall off the curve" at say 500 watts  efficiency suffers, thus more heat is dissipated at the lower power level due to inefficiency.   True or False ??  Wouldn't measuring the exhaust air temp be a good way to confirm or debunk this?

If you express watts dissipated on plate as a fraction or percentage of input power or output power, it certainly look like "efficiency suffers". We would usually express efficiency as a percent.

But I think that's just an artifact of showing efficiency as a percent.

[W8JX]

What everyone failed to mention though is that while tube amp efficiency can exceed 60% and it stays high with reduced drive power levels for reduced output, SS amps on other hand struggle to reach 50% and rated output and efficiency drops quickly as drive is reduced for lower amp output.

Also if you are thinking of getting a 1500 watt amp and that it will last a lot longer at half power or less think again. If it is based on a ceramic tubes like a 8877 or 3cx800 or similar tubes their life span is based on filament burn time not power out.

[KM1H]

My experience is that the 3CX800A7 or CPX version goes soft a lot faster than the 8877, Eimac or Chinese. In fact Ive rarely seen a weak 8877.
This is based upon servicing the number of brands that use(d) them since the 80's.

The reason behind that could be grid damage, cathode emission by design, or run beyond their output ratings.

[K6AER]

You have a certain amount of overhead  with any amplifier weither you are producing RF or not.  Filaments, screen bias and conductive current (class AB) before RF is applied. Obviously the higher the wattage output the less the opertional overhead will be as a percentage of total currently the amplifier is drawing. This is the cost of doing business.

[K0ZN]

Thanks everybody for the info/input; details/review was appreciated.  I was getting too caught up in the possible loss of efficiency.

I do "tune" (resonate) the amp at or slightly above the 500 watt level before I operate at that level. ....and yes, I am well aware that I am not "saving" the tubes, I just don't see the point in running 1,500 watts output to rag chew with someone 600 miles away on 40 M, etc.

Thanks & 73,     K0ZN

[KC4ZGP]

Pish posh mate, go solid state.

On 18.136MHz phone after 2200UTC.

Kraus