Screen modulated 4-400A (4) in BC rig

[John K5PRO]

While bowsing Harold Hallikainen's broadcast history website of old documents and manuals for BC equipment I found a Continental Electronics 314D 1 kW AM transmitter manual, dated 1962. My gosh, that thing used a screen modulated quad-pack of 4-400As with (4) 807s in it. It had ~ 15 dB of inverse RF to audio feedback to reduce the distortion of screen modulation. Had a knob to control the level of the feedback to make specs. You adjust it during your proof!  Overall efficiency was quoted at 22% for 85% modulation. They ran 4100 volts at 650 mA in those four final tubes, giving a PA efficiency of 37%. I can see why this thing wasn't a popular model, compared to the BC1T, BTA1R1 and 20V2 of the era that used high level modulation with class C finals.

Still, it was a neatly laid out box, with built-in dummy load (called phantom) and at 1100 lbs, 32 x 32 " footprint, it was fairly well sized for the early 1960s when big was in. I haven't heard anything of these, and I doubt we will find any around anymore. With the Gates Vanguard I and II, and the RCA BTA 1N1 (also 22% efficient), this will certainly be a great electric shack heater, while burning those kilowatts.

The site:
http://sujan.hallikainen.org/BroadcastHistory/

[The Slab Bacon]

Necessity can be the mutha of invention! Some years back I set out to build a high power AM rig, but at that time I couldnt find a big piece of mod iron. (at least not at an affordable price) After a few discussions with other hams, I decided to build a screen modulated 4-1000A. A lot of the "AM elders" laughed at me when I was discussing building it over the air. None of them laughed when I put it on the air. It loafs along at "legal limit" power levels and puts out a whopping signal. It has pretty much ran like the proverbial "swiss watch" for many years untill I recently crapped out a plate transformer. (what do you expect fron a 60+ year old transformer)

Many hams used to put down anything other than plate modulation because of the old 1KW power input power limit. Why build a lower efficiency rig and get less output when you can build a plate mod rig and get 60-70% efficiency for 600-700w output as opposed to 300w with screen or cathode mod rig.

Now, all that matters is what comes out of the coax connector. This invites much more room for alternative forms of modulation: grid modulation, AM linear, etc!!
Its all fun, and it is fun to sometimes try new (old) ideas. It dont matter how you get there, as long as you are there!!

                                                      The Slab Bacon

[AB2EZ]

John

An illustrative example of the efficiency comparison of screen v plate modulation*

Case 1 Plate modulation

Carrier output power: 375 watts
RF output stage efficiency (at carrier, and also for all modulation levels):70%
Electrical power input at carrier: 375/0.7 watts = 535 watts
Modulator output power for 100% modulation: 535 watts x 0.5 = 268 watts
Assumed average modulator electrical power consumption: 268 watts / 0.5 = 535 watts
Total electrical power consumption: 1070 watts

Case 2 Screen modulation (done properly)

Carrier output power: 375 watts
RF output stage efficiency (at carrier): 30%
Electrical input power at carrier (and also at all modulation levels): 375 / 0.30 watts =  1250 watts

*Note that with screen modulation, the rf output tubes must have a higher plate dissipation rating than for plate modulation... but, then again, there are no high power modulator tubes

Stu

[N3DRB The Derb]

one of the best linear driver transmitters is the Viking Challenger. Screen modulated with a pair of cheep tubes, some weak points, but better han a dx-60 IMO. 100W in cw tune for tuning yer maul, and switch to AM and power drops to 15 watts carrier at about 150% positive mod. I ran mine into a SB 200 and I was loud and proud.

[steve_qix]

Hi Stu,

I won't disagree with your numbers, but I don't think the application of these figures is neccessarily valid, particularly for ham radio 

The *average* power consumed will be CONSIDERABLY less for a high level modulated transmitter than for a linear.  For instance, using realistic figures that I have personally obtained using tube equipment:

2000V * 200mA = 400 watts input.  Efficiency of this RF amplifier: 80%.  Power output (ignoring tank losses, which were small): 320 watts.  Modulator resting current: 40mA, resting power: 80 watts.  Total power in: 480 watts; 320 out, efficiency: 66.7 % (at carrier).  The *average* modulator current is not high with speech     I suppose I could have dropped the modulator resting current, to get even more efficiency!

For the same output, using your figures of 30% for the linear, the power input would be 1066.7 watts input (at carrier), for the 320 watts output, as opposed to 480 watts input for the same power output using class C with a modulator.

Of course, the whole discussion is really somewhat moot.  The efficiency of a ham radio transmitter (using tubes), operating at a 10% transmitting duty cycle (90% of the time listening or not on the air), the consumed power of the tube heaters alone will drop the efficiency to something very, very low (probably a few percent!!!).   

So, it probably really doesn't matter much in the overall scheme of things.  If you really care about efficiency (as defined by use of energy for transmitted output), you have to go solid state.

Regards,

Steve

[AB2EZ]

Steve

Yes, I agree ... your calculations represent what would be possible with a very efficient (80%) Class C rf amplifier, and an efficient modulator... biased for very low resting current.

You are an acknowledged grand master at designing and building very efficient transmitters 

In my Ranger, the RF output (at carrier) is about 40 watts with the B+ set at 550 volts and about 125 mA of plate current (ignoring the screen power to keep things simple). So the efficiency of the rf amplifier is around 40 Watts / [550 volts x 0.125 amps] = 40 watts / 68 watts ~ 58%  .  I'm sure that I could tweak the Ranger to do somewhat better... but nowhere near 80%.

My KW-1 produces 420 watts output at carrier with 2100 volts B+ and 320 mA of plate current. So the efficiency of the KW-1 is 420 watts / [2100 volts x .320 amps] = 420 watts/672 watts ~ 62.5%  [When Bob Sullivan tested it with the normal plate voltage of 2500 volts and the normal plate current of 400 mA, he measured 1000 watts in and 600 watts out => 60% efficiency]

With respect to the modulator, a stock Ranger modulator runs at 50 mA of resting current with 550 volts on the plates of the modulator tubes. That's 27.5 watts of resting modulator power (as you point out, not including the filaments).

So the overall efficiency of a stock Ranger, at carrier, is: 40 watts /[68 watts + 27.5 watts] = 40 watts / 95.5 watts ~ 41%

If you upgrade the Ranger to 6550's, then the resting current of the modulator is around 100 mA, and the efficiency drops to 40 watts / [68 watts + 55 watts] ~ 32.5%

The modulator of the KW-1 draws 50 mA of resting current (only considering the 810 modulator output tubes) at 2100 volts. So the resting power of the modulator is 105 watts.  The overall efficiency of the KW-1 at carrier is 420 watts / [672 watts + 105 watts] ~ 54%

As you point out, when you add in everything else... the electrical power consumption of a stock KW-1, in AM mode, with no modulation is specified in the manual as 2280 watts. The corresponding stock KW-1 carrier output power is 600 watts. So the wall plug efficiency at carrier is 26%.

For comparison, the wall-plug efficiency of my Class E transmitter + Class H modulator, at carrier = 50% [i.e., 400 watts of electrical power in, and 200 watts of rf power out]

Best regards
Stu

[steve_qix]

For comparison, the wall-plug efficiency of my Class E transmitter + Class H modulator, at carrier = 50% [i.e., 400 watts of electrical power in, and 200 watts of rf power out]

Now all you need to do is to build a pulse width modulator for your class E rig, which would boost your power line efficiency to better than 75% (the pulse width modulator itself is around 95% efficient, and the RF amp should be around 90% - take away some loss in the power transformer and probably 10 watts here and there...).  The class H modulator is only around 70% efficient at carrier.

Regards,

Steve

[Jeff W9GY]

I've had my homebrew pair of screen modulated 4-400's on air for several years now.  It's a great running rig with about 275 - 300 W carrier.  I could get more output, but I'm limited on headroom due to the plate HV available. Excellent audio with WIDE frequency responce capability.  Yup, the efficiency issue of this type of modulation is no big deal in amateur use...