When an amplifier is too big.

“Ridiculously Large Amplifiers”

By Matt Erickson, KK5DR

There have always been home-brewers in ham radio who build linear amplifiers that are far in excess of what is considered practical, and there probably always will be these types in the hobby.

Personally, I like to build amps that are large enough to operate at the current “legal limit” for US hams with a modest amount of headroom. None of the units I build take up more than 1 cubic yard of volume space. None of them dissipate more than a few hundred watts in the idle state (keyed, but without RF drive.)

All this being said, I think that those builders who insist on building ridiculously large amps in both physical size and RF power output, are doing something that is not entirely ethical and or somehow blurs the gray-line of legality. It is akin to owning a 1000HP street-legal hot rod while saying, “I would never go over the speed limit!” - all the while with a “knowing wink” and crossed fingers behind his back. The temptation to cross the line is far too great. Those who are not just tempted might blatantly break the rules. All because he feels “oppressed by the Man” or wishes to “rebel against the system”, whatever rationale he might use to quiet his conscience. All this would be a very weak argument in a court of law.

I would like to give the example of energy waste as just one argument for keeping your amp project to a reasonable size and output. The example is an amplifier running the 4CX10,000A, being run at rated plate voltage and having a plate PSU capable of delivering full plate current in a class AB1 grid driven configuration. (I do know several hams that have just such a beast.)

If the tube is operated at the industry standard idle current level (keyed without drive) of 20% of full plate current, it is still 800mA at 7.5kV for a dissipation of 6kW, with no signal applied. All this energy is converted directly to heat, which is dumped into the shack. At the same time the screen is operating at 250 Watts, of which no less than 50% is converted to heat output or another 125 Watts added to the heat output of the amp. The grid dissipation varies with the drive, so if there is no signal applied the only heat from the grid will be from the PSU regulators. Next, add the heat generated by the cathode heater, 562.5 Watts, which also is exhausted directly out the stack and into the shack. So, the total heat output of the idling amp (keyed on) is now up to 6,687.5 Watts, and no signal has even been transmitted yet! This much heat and more is generated if the amp is only driven to 1500 Watts of RF output. You can see that it is not very efficient; in fact, it is a gross waste of energy, not to mention the strain it would place on a 230Vac single-phase mains supply. To run 1500 Watts RF output from this amp, now the heat output will be at least 9,687.5 Watts that would require a 230Vac mains service of no less than 42A. Nothing less than a 50A dryer type service would operate this amp. Even the 50A service would be strained to the limits of safe operation.

All this just to run the “legal limit”,… yeah… right. Why is it that I have such a difficult time believing that last line?

I have not even mentioned the fact that these huge amps are extremely dangerous - more so than the much smaller but still deadly amps, like the SB-220 for example. The ultra-high plate and screen voltages required by these giant tubes can jump very large air gaps, so what WAS a safe distance in your relatively small SB-220 is no longer safe at all in the great beast. The amount of energy stored in the huge PSU is not merely capable of burning a hole in your finger like the SB-220; it can remove your arm from your body and burn it to charcoal. Here we have yet another reason not to “go big”.

The desire of the user might be entirely legitimate, but I think that there is a deep-seated wish to be a “Big Broadcaster”,. Perhaps he is over-compensating for some other psychological or emotional shortcoming. It could all just be a huge ego massage as well. I personally know of “Contester/DXer" stations that use amps of this “Mega-size”, running them to full output to “get them over the hump”. The “legal limit” is a total joke to them, when it comes to making that desired contact or busting that big pile-up. It is one of the “dirty little secrets” of "Contester/Dxer" operations in the ham hobby. There are many more secrets, but these are for some other article.

Keep your building project within practical reasoning. A good rule of thumb; for an amp that is to be used primarily on SSB, the total plate dissipation of the tube(s) should be no more than 1600 Watts. This gives a good amount of “headroom” for comfortable operations. For an amp that will run full time RTTY or some other high duty cycle mode, the total plate dissipation should be no more than 2000 Watts. This amp should operate comfortably and cool at the legal limit. Any more is a waste, in energy, money, and space.

Even though my amps are capable of exceeding the legal limit (not by much), I run 1kW to 1.1kW, rarely 1.5kW and NEVER EVER over it! I figure that if this level of RF power does not get the “job” done, no amount will.

Any ham who tells you that he can hear the difference between 1kW and 1.5kW (1.8 dB, or about one-third of an S-unit) on the air, is a deluded liar! The only person who notices is the guy on the transmitting end.

There is no legitimate reason to exceed the limit. Anyone who says otherwise is part of the problem.

The solution; If you want the “Big Signal”, get out there and work on your antenna system! Big antennas, high in the air, put out big signals with or without an amplifier connected to them.

73 de KK5DR

Thanks to Adam Farson VA7OJ/AB4OJ for his proof-reading/editing of this article.