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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

100KW Power Amplifier

ChrisA

Member
2008-01-08 12:22 am
Just for curiosity,does someone know how is made a 100Kw power amp?Things like types of tubes used,voltages involved,rectification,capacitors ,and OPTs used?I know this amps were used in transmitors modulators and still used.Would be nice to see one of this monsters.
Thanks

I don't think anyone has ever build a audio amp that large. Typically if you do need that kind of power they "rack and stack" quite a few 1000W class D amps and hide that in rows of racks under the stage (like for a rock band playing a open air sports stadium.

But people have built some large 100KW class radio transmitter amps with tubes. The tubes are huge, like a washing machine and have water jackets and are so expensive they are designed so they can be opened up and rebuilt after they have seen some use.
What makes it posable is that at radio frequencies the transformers can be MUCH smaller than at audio frequencies. Mostly RF amps don't need output transformers and will directly drive the antenna through some kind of resonant tank circuit
 
I don't think anyone has ever build a audio amp that large.

You forget AM radio, the modulator needs to be around the same power as the transmitter and it is an audio amplifier. In Australia 50Kw AM transmitters used to be commonplace so I am sure 100Kw and larger units have been built, not all AM transmitters use modulators linear mode is quite popular also.

Making the power is not difficult, high power valves are very compact compared to equivalent transistors of equivalent frequency response, I have a 4CX10000D as a mantelpiece ornament Two of them in PP will give about 30Kw with a modest 7.5Kv on the plate.
Here is one guys 12Kw RF amplifier using that valve Linear There is a more compact one on the net somewhere using 3 phase power which will fit in the boot of a car.

A 30Kw modulator would be a similar size weight is going to quite large due to the output transformer and I cannot think of any practical reason to use an amplifier this size to drive speakers multiple small amplifiers improves reliability, control and safety.

As for the techniques used, the RCA transmitting tube manual covered this a bit. PSU 3 phase full bridge choke input filter, transformer driven PP output stage, AB2 or class B. Driver stage is also PP Really it is just an upscale PP home audio amp except for the transformer coupled PP driver stage. Safety controls and instrumentation reflect the higher value of the equipment.
 
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You forget AM radio, the modulator needs to be around the same power as the transmitter and it is an audio amplifier. In Australia 50Kw AM transmitters used to be commonplace so I am sure 100Kw and larger units have been built, not all AM transmitters use modulators linear mode is quite popular also.

Making the power is not difficult, high power valves are very compact compared to equivalent transistors of equivalent frequency response, I have a 4CX10000D as a mantelpiece ornament Two of them in PP will give about 30Kw with a modest 7.5Kv on the plate.
Here is one guys 12Kw RF amplifier using that valve Linear There is a more compact one on the net somewhere using 3 phase power which will fit in the boot of a car.

A 30Kw modulator would be a similar size weight is going to quite large due to the output transformer and I cannot think of any practical reason to use an amplifier this size to drive speakers multiple small amplifiers improves reliability, control and safety.

As for the techniques used, the RCA transmitting tube manual covered this a bit. PSU 3 phase full bridge choke input filter, transformer driven PP output stage, AB2 or class B. Driver stage is also PP Really it is just an upscale PP home audio amp except for the transformer coupled PP driver stage. Safety controls and instrumentation reflect the higher value of the equipment.
Thank you for the pictures! Very nice work!
I never made RF transmiters,but have only curiosity for the HUGE things involved ,like voltages,amps, and parts.
EL156
 

ChrisA

Member
2008-01-08 12:22 am
You forget AM radio, the modulator needs to be around the same power as the transmitter and it is an audio amplifier. In Australia 50Kw AM transmitters used to be commonplace so I am sure 100Kw and larger units have been built, not all AM transmitters use modulators linear mode is quite popular also.

The modulator would be running at audio frequency and so technically be an "audio amplifier" but the impedance of the load it drives is very high compared to a loudspeaker . and so you don't have the most expensive part of an audio amp, the output transformer. We keep thinking of power tubes as being the main component of an audio amp, no the transformers are. By any way you care to measure, volume, weight or cost, the transformers are maybe 8X larger.

OK enough for technical nit-picking.

Tubes really are usful still inside radio transmitters. I just found a salvaged 2KV 1.5KVA transformer. from a very old microwave oven built back when they used to cost over $1,000. The parts used in those dys were "standard" off the self components. It's a really nice transformer, a big 6" cube of iron. Now to find some 813 tubes to build a 500W amp.

After radio the other big users of tubes are guitar amps. In a way guitar amps are more fun to build than HiFi because with HiFi there is only one goal, good clean sound. With a guitar any sound that might be in your head or that you can imagine is "good" if you like it. I'm currently experimenting with a spring reverb and with clipping a signal with diodes, kind of anti-hifi in the extreme.
 
You forget AM radio, the modulator needs to be around the same power as the transmitter and it is an audio amplifier.

For 100% amplitude modulation, the modulator need only furnish 50% of the transmitter's carrier power.
The output transformer would be a big problem. And the power consumption.
Anyone have 480 3 phase power in their homes?
 
Guys, you're missing a point here. In many/most AM/FM/SSB transmitters, the modulation is applied to the lower-power "driver" amp. The modulated signal is that used to drive the final power amp stages. Depending upon the gain of the final power amp (and any intermediate amp stages), the audio modulator can be relatively conservative. For example, in amateur HF transmitters, you can have a well-designed AM/SSB transmitter that's sized to sit on a desk--while it in turn can drive a large (or very large....rack sized) power amp. I used to have a 4CX10,000A final amp in a large rack, that was driven by a desktop-sized 200 watt HF transmitter.

Speaking of large tubes, I'm a defense contractor, and we used to maintain and modify the VHF Ballistic Missile Early Warning System (BMEWS) radars in Thule, Alaska, and the United Kingdom--the transmitters were effectively copper-lined/shielded rooms, with about seven-foot tall travelling-wave tube amps--and they used to get pulled and rebuilt whenever they reached their service life, or started to drop in output power.
 
You forget AM radio, the modulator needs to be around the same power as the transmitter and it is an audio amplifier. In Australia 50Kw AM transmitters used to be commonplace so I am sure 100Kw and larger units have been built, not all AM transmitters use modulators linear mode is quite popular also.

Yeah, 50 years ago and even then manufacturers were trying to reduce power consumption. Look up the RCA 'Ampliphase' from the '60s. I don't think there's been a plate modulated AM transmitter in a LONG time. It's done with PWM to modulate the RF output - more accurate and way less power.

 
The modulator would be running at audio frequency and so technically be an "audio amplifier" but the impedance of the load it drives is very high compared to a loudspeaker . and so you don't have the most expensive part of an audio amp, the output transformer.

AFAIR modulators have output transformers in series with B+ so they can swing between roughly 10 and 190% of B+ they are not series pass regulators.

The modulator is a linear amplifier and it definitely does not run class C though it did not surprise me to hear of PWM being used now to modulate the carrier.

A linear RF amateur amplifier is an entirely different animal to a plate modulated commercial transmitter.

As RF power tubes go 100Kw output is small Klystrons and gyratrons can produce insane amounts of power in the microwave region.