He dips his plate meter. 22 Amps at the dip!, 11kv. Must watch.

[KD6VXI]

http://criticaldistance.blogspot.com/2014/05/two-shortwave-transmitter-sites-visited.html?m=1

Check Wooferton.

It's a complete rundown of a quarter megawatt TX.   He explains each stage,  which class of operation,  cathode followers to provide power for the ab2 stages,  etc.

If you've ever been interested in international shortwave,  this is cool!

Then they power it up and dip and load,  going from a bit over a hundred kw to 220.  Link coupled,  the links are insane.

--Shane
KD6VXI

[W2PFY]

I watched that video a number of times and it's fantastic! On one of the driver transmitters (5KW) they show that it is driving the final stage or linear in grounded grid.  ( The schematic is printed right on the front of the driver) The driver is just straight RF but the linear is where the high level modulation is applied. I wonder how they can achieve 100 percent positive modulation?

I think I read somewhere that with these high powered transmitters, that the idea was to have a big phatt carrier with about 60-80 percent positive peak modulation with the intent to not have audio distortion during selective fading conditions?

Does anyone agree with the above or am I full of it? 

[W3RSW]

Uh oh, don't let Opcom watch these. He'll be converting to steam boiler cooling, plug in coils and doors upon doors with interlock keys befor you know it. 

11kv from the national grid no less.

A great tutorial for number of turns in a coil vs. current for a certain Q.
Amazed that relatively low frequency coils look like 2 meter strip lines or should I say "strip bars." 

"So I sent Ulrich down to the corner hardware for a couple of Jennings 40 Kv caps."

Great vids. Tnx fer posting.

[KD6VXI]

I love the 5kw.  An Exciter!   

Interestingly,  the modulators on the quarter megawatt amps are 180 kw.   So,  they where able to attain a hundred pct plus....   150 pct would be in the neighborhood of 180 kw.   So figure 120 pct pos peaks and it would still be run clean.

The final is biased class C.   No DC on the mod tranny,  it's choke (modified Heising I believe) fed.

Maybe K5PRO or one of the ex transmitter jockeys can chime in with more particulars.   I'm going to Google  at it to the big screen in the living room.

--Shane
KD6VXI

[w8khk]

That was a very interesting presentation, Shane.  Thanks for sharing it.  Long program, but the detail was excellent.  He was very knowledgable, and gave performance and design details not commonly heard on the typical youtube videos. 

The size of the "coils" at those low frequencies was quite a surprise to me, but when you consider the power level and currents, much less L is needed at those lower impedances.

[John K5PRO]

Indeed thats exciting big iron! Loved that! /The Marconi with triodes and grounded grid driving gounded grid is classic design, as V. O. Stokes would cover in his book on Radio Transmitters. The newer RIZ using tetrodes in AB1 is a lot more like what I use for RF. The tubes are similar but I think those were either TH537 or TH573 tetrodes in the film. They use hypovapotron cooling which is the mixed boiling/condensing water in the anodes.

 I designed our 'penultimate' stage (called IPA stateside) with a pair of the green Thales TH555A running class A, 40 Amps plate current at 15 kV DC. These are push pull driving grids of a pair of EEV BW1643J2 triodes, plate is floated at 15 kV but grounded for 2800 KHz RF. RF output is from the two cathodes (filaments). Output Z is only 10 ohms or so. This is used to power a large ferrite tuned cavity, each toroid is 1/2 meter diameter approx. There are maybe 36 cores stacked. Protons feed through the center vacuum pipe and a ceramic gap provides the location where the two cathode follower drive it. All class A.

I need to make some videos but so far the rig is running 24/7 for part of each year.

Recent work has been building and installing 3 x 480 kW average power amplifier systems at VHF. These don't look as exciting as those big vaults full of copper pipes and vac caps in the BBC film. The cavity amplifiers are each running half this and are combined in quadrature. Plate V is 23 kV and current per tube is about 90 amperes.  Class B. There are quite a few papers in the public on these rigs, just look up 201 MHz and my name..  I use Thames tubes there also. The IPA is a single TH781 tetrode in a cavity circuit making about 100 kW.

[Opcom]

Uh oh, don't let Opcom watch these. He'll be converting to steam boiler cooling, plug in coils and doors upon doors with interlock keys befor you know it. 

I know my limits. Anyway I gave my ML-892 to a friend.