12KW on HF Ham bands ?!?

[Phil1977]

Is the vacuum tube the right choice if you build a 12KW 30MHz amplifier nowadays?

I just ask out of curiosity, I just wouldn't be surprised if you could reach this frequency range with Class-D or Class-E transistor amplifiers in a cheaper way today.

[radiolistener]

Linear solid state amplifier for a short wave and such power is possible, but I suspect it will cost a lot of money and will be a bit complicated in comparison with old school tube amplifier.

Guys that I know using tube amplifiers for high power.

[Phil1977]

Linear solid state amplifier for a short wave and such power is possible, but I suspect it will cost a lot of money and will be a bit complicated in comparison with old school tube amplifier.

Guys that I know using tube amplifiers for high power.

Yeah, dissipating the heat of linear amplifiers is for sure easier with tubes at this high power level.

But what about digital amplifiers? If a CPU is switching hundreds of amps with many gigahertz, then it somehow sounds possible to get 30MHz without producing much heat.

[radiolistener]

But what about digital amplifiers? If a CPU is switching hundreds of amps with many gigahertz, then it somehow sounds possible to get 30MHz without producing much heat.

As I read somewhere, modern commercial 500 kW AM transmitters really using PWM. As I remember it allowed to implement solid state AM  modulator with 50 kW output, but 500 kW amplifier still on the tubes.

[vk4ffab]

But what about digital amplifiers? If a CPU is switching hundreds of amps with many gigahertz, then it somehow sounds possible to get 30MHz without producing much heat.

As I read somewhere, modern commercial 500 kW AM transmitters really using PWM. As I remember it allowed to implement solid state AM  modulator with 50 kW output, but 500 kW amplifier still on the tubes.

None of those big mega stations are using anything remotely modern. They are on tubes because that same transmitter cabinet has been in use for over 40 years. When radio Australia shut down, they only had 1 spare tube and new tubes for it had not existed in years. The problem they all have is spare parts. Mate of mine is a tech for a large AM/FM radio chain here, all their TX sites are solid state, with hot swap able amp modules making up the transmitter rack.

[CaptDon]

Those hot-swap module units are common. The output is usually one or two loops around a ferrite core which isolates the R.F. output voltage from the module power supply voltages. The R.F. output links are in a large series chain and there is a bridging switch so that when the module is pulled out the series R.F. link remains completed. The D.C. power to the modules can all be parallel due to the isolation of the R.F. output transformer. We ran a solid state Gates/Harris to replace the old Gates/Harris MW-5 which was a PWM modulated 5KW (6100 actual) AM transmitter. The MW-5 was more finicky about mis-match due to icing/etc. than the old tube type RCA BTA-5 and the solid state Harris was more finicky than the MW-5. The solid state unit would roll back the output power if the mis-match was above around 1.3:1

[A.Z.]

Those hot-swap module units are common. The output is usually one or two loops around a ferrite core which isolates the R.F. output voltage from the module power supply voltages. The R.F. output links are in a large series chain and there is a bridging switch so that when the module is pulled out the series R.F. link remains completed. The D.C. power to the modules can all be parallel due to the isolation of the R.F. output transformer. We ran a solid state Gates/Harris to replace the old Gates/Harris MW-5 which was a PWM modulated 5KW (6100 actual) AM transmitter. The MW-5 was more finicky about mis-match due to icing/etc. than the old tube type RCA BTA-5 and the solid state Harris was more finicky than the MW-5. The solid state unit would roll back the output power if the mis-match was above around 1.3:1

All fine, but then... move to Ham HF bands, there we haven't the luxury of transmitting on a single "fixed" frequency, we may (and probably will) change frequency/band, now imagine what this means for a Ham using a linear like the one in subject when it comes to match the antenna... also since, at that power levels, we will want an almost perfect match, otherwise we may just decide to move to US (if we aren't already there) and only power on the linear on 4th July

[Bud]

With a digital amp you'd be spitting stuff all over the spectrum. You'd need good filtering on the output to meet out-of-band emission specs.

[CaptDon]

Single fixed frequency??? Those hot swap modules are as broadband as a barn door!!!! They aren't tuned, they are transformer output. I am looking at some old LW/MW Harris designs and I believe the exact same modules were used from 200KHz to at least 1.8MHz, almost a decade span. My material is from around 1998 when competition for the solid state dollar was big and everyone was pushing 'reliability' and 'ease of repair' because us tube dinosaurs never thought solid state could survive lightning strikes and plasma ball lightning rolling out of the phasor cabinets. We all kept a tube type around for a backup for many years until solid state could prove itself worthy. Modules back then ranged from around 250W to 500W each. I've been away from the line of work for many years but I bet modules are easily pushing 1KW each today. My old workplace has a 10KW solid state FM unit now. I was gone by the time they installed that one. I guess power reduction at 1.3:1 VSWR is how they bought 'reliability'. A decent antenna farm shouldn't shift that far but 6 inches of rime ice covering most of a 4 tower directional array is going to do some weird things, Including totalling cars and demolishing antenna dog houses!!! 50 to 100 pounds falling 250 feet makes a mark!!

[iMo]

2000kW midwave TX with solid state PA..

https://en.wikipedia.org/wiki/Transmitter_Solt

https://youtu.be/LWyIwCoA8l4

[Phil1977]

With a digital amp you'd be spitting stuff all over the spectrum. You'd need good filtering on the output to meet out-of-band emission specs.

I suppose blocking the PWM frequency of a class-D amp is easier and more efficient then blocking harmonics of an only slightly clipping class-A or class-B.

The times are over that SMPS and digital amplifiers are generally noisy.