200 or 300 Watt A M Xmtr, doing the math

[W4RFM]

My latest plan is to build a 300 watt (actual output) rig using a 4-125a with 1500 Volts @ .250 A for 375 watts input or 296 watts out.  I plan to modulate this with a pair of 811a's with 850 volts @ ,250 for an input of 212 watts or 153 actual useable watts output.  I further plan to use a 100 watt modulation transformer with the modulation reactor  and cap design from:  http://www.hamelectronics.com/k1deu/pages/ham/transmitters/am/pages/using_audio_reactor.htm
The modulator plate load impedance is 3400 ohms, so using the equation .0045 X 3400 ohms, I get a reactor of 15 Henrys, and a capacitor of 2.9 or 3 Mfd.
AM I correct?
Thanks

[KE6DF]

I'm not a guru, but here are my thoughts FWIW.

1. With 375w input, you probably won't get 300w output. You would need more like 400 - 425W input to get that output in the real world with circuit losses. Maybe two 4-125a's.

2. A 100w modulation transformer seems too light weight to me. You would need more like 150w or 200w.

3. A 15Hy mod reactor is marginal. It kind of depends on what the secondary inductance of your mod tranny is since the heizing inductor is in parallel with the secondary of the mod transformer as far as audio is concerned. Find two 15hy chokes and put them in series.

Dave

[ke7trp]

Yeah.. 300 watts out, THats more like a 500 watt Transmitter of the day, Globe king, Johnson 500. 

Also. if you must use 811's then run them in class B.  Its a simple hookup and you get some decent power.  More like 1250 to 1500 volt range.

To me, I would build the modulator output to a minimum of 75% of the output of the rig. But I like alot of modulator.  Nothing worse then a rig that cant fully modulated the carrier and you sound PW on the air.

C

[W4RFM]

I picked these vaules based on the Typical Operation table son the NJ7P tube data base.  I am using a 100 watt mod transformer because I have one, if I had a 150 watt, I would not use the reactor and cap hook up. I am taking that step to stretch the xfmr capabilities 50 watts.  Also, I am trying to use voltage and current combinations that will allow for reasonable plate load impedances, to match what iron and tank components I have.
I am open to any and all suggestions.  Heck if I had the mod iron, I would do a 500 with a 250 watt Mod. and line up the 4-125's and 811'a in windows.
Please comment one and all.

[W2PFY]

I am using a 100 watt mod transformer

I think John may have wrote about using the smaller mod transformer in the article. when used with a reactor, since the mod transformer has no DC on it, you should be able to get double the audio through it. I think you'll get all the audio you need on voice but it would be another matter if your were to put a sine wave through it. John says it's closer to 2.7 times the rating of the transformer. I uses a similar scheme using a backwards hooked up audio transformer with a 30 Henry choke. The audio transformer is rated at 60 watts and I was able to get a great sounding Apache running over 110 watts out with 150 percent modulation. I was told it sounded very good by our class E guru, Steve WA1QIX. I have never gotten that kind of report out of any other transmitter from Steve so I guess it says something about getting more of of a transformer than it was designed for. When I was running just ten herry's people still said it sounded good but I was getting talk back form the reactor. Adding more inductance cured the talk back.

Another thing to consider while it may not be scientific, the more inductance you have in the reactors, the lower in frequency the audio range will be. If you don't have a very deep voice or you don't care about bass using less inductance would be acceptable.

My Westinghouse MW-2 rated at 2.5 KW output with a pair of 833A modulators only used a six Henry choke and the DC blocking cap was 1/2 uf 6000 volt. It was very difficult to get any hi fi audio through it but equalized strictly for speech, it was fine.

[ke7trp]

With the cap and choke, I think the 100 watt iron will be ok. It depends on the match and how hard its pushed. Also mount on wood.

I think that one stiff supply at 1500 volts for the 811s and the 4-125s could do it.  Running those 811s down at 850 volts is not suggested.  Sounds like a realy neat rig.

C

[WQ9E]

Bob,

The tube data sheets don't include power lost in the output network, sort of like rated engine versus actual rear wheel horsepower in a car. 

That being said, the difference between 375 versus 325 output watts won't be noticeable at the receiving station.  So running the legal limit versus "almost" the legal limit is more a case of bragging rights.

[KM1H]

A pair of 4D32's will do an easy 200-250W out at only 700V and a couple more loafing in AB1. Easy on the PS budget too.

Carl

[W4RFM]

See below, new plan.

[W4RFM]

New thread started.