Plate Voltage
One often has to consider that things may not always be as they seem, particularly when Local QRPers are asking questions. Absolutely. We had been to an annual get together a few weeks back and it was a mixture of traffic types, those who like to tinker and build homebrew gear, and of course, DXers! DXers are drawn to other DXers and no gathering of the amateur fraternity has ever proven this otherwise . . . it is one of the Inevitable Truths of DXing. Believe that, for it always has been.
We were sitting at a table with a few of the true blue DX types, talking about the Great Days of DXing and the imminent arrival of cycle 23 when one of the Local QRPers came along with his tray. He spied the one empty seat, and being a Seeker of Knowledge, this QRPer sat down and began right off with a question: "If you guys were going to build a linear, what kind of tube would you use?" he asked, looking quickly from one to the other. No one answered for a moment, then one of the big guns shrugged and replied, "Well, I guess I'd probably go with a pair of 3-500Zs . . . they're pretty rugged and not that expensive." The QRPer nodded and waited. "8877s are good tubes too." another of the true-blue types offered. The QRPer had pulled out his notebook and was writing all this down. QRPers keep methodical notes, especially when they are driven by a single purpose. Remember that. A QRPer on a mission is dangerous. Never let your guard down.
"And what about you?" the QRPer said, looking straight at us, "What would you use?" We thought for a moment, then replied: "Either of the two mentioned are good choices" we said carefully, "and the we are hearing good reports about the 4CX1600B too . . . 50 watts from the exciter will get you 1500 out. But none of us are really considering building an amp right now." We got nods of agreement from around the table and we decided to pursue this a bit further. This QRPer had an agenda and we had to know more.
"Why are you so interested in tubes?" we asked. The QRPer took a bite of his lunch and looked at us carefully, then did the double take and answered our question with a question. "Let's say someone wanted to double up a pair of those 4CX1600Bs. How much power would you expect from 100 watts drive? Am I correct in assuming that if 50 watts will generate 1500 watts, then a pair of them will put out 3 kW?" We weren't sure where this was leading and we were getting a bit out of our league. "We've never built an amp" we replied, "and a lot of this depends on the design and efficiency of the components involved. In theory, you're probably right. But what's the point? You can't run 3 kW anyhow. You'd be cited for excessive power and if you kept it up, you'd be off the air." The QRPer glared at us with his beady little eyes and ploughed right on. "Yeah, yeah, I know all that" he said, wiping a bit of sweat from his upper lip, "but let's say you could do it, what would be the best way? Get three, maybe four of those killer 1500-watt tubes and try to drive them with the exciter, or build an intermediate with a single 3-500Z and use that to drive the final? And if someone was to do that, use an intermediate amp, would 5 kW output be possible?"
Son of a Gun! How does one answer a question like that? It was obvious we had a QRPer who had done a lot of thinking before asking. So we did the obvious. We excused ourselves, and hauled this Seeker of QRO Knowledge over to the table in the corner where the Old Timer was sitting with Red-Eyed Louie. The Old Timer had built his share of amplifers and the QRPer repeated his question, referring to his notebook and getting ready to write. The Old Timer leaned back in his chair, looked up at the ceiling for a moment and then looked back at the QRPer. "What we have to answer here" he said, "is not how, but why." The QRPer looked puzzled as the Old Timer continued. "Now, you were by the other day and you were complaining that you had missed Heard Island. Remember how you said you called and called for hours and they never came back to you? And I told you that the reason they didn't come back to you was because they didn't hear you? Do you remember that?"
"Exactly!" the QRPer replied in an excited tone, "and I listened to your advice too. You guys are always telling me to remember what Albert said: That all things are relative, some more so. So I did my homework, and I researched as much of Albert's work as I could find. And I believe I have found the answer. Everyone knows of the famous equation, E=MC squared, right?" The Old Timer simply looked at the QRPer without saying a word. "Well" the QRPer continued, "I found the pages of complicated equations that Albert used to derive that formula . . . and in there, buried among the integrals and complex formulas is the answer! I now know the reason I missed Heard and how I can prevent it in the future! It has been there all along and no one noticed it:"
E (squared) = DX
"It's obvious that E is the plate voltage! And if you square the plate voltage, you will certainly increase your output power. Albert was telling us that the more power you run, the more DX you will work. Why didn't anyone notice this before?"
The Old Timer looked at the QRPer, then at us, and then turned back to his meal and his conversation about DX spots with Red-Eyed Louie. It was clear he wasn't going to comment. The QRPer looked at us for a moment, then asked, "What's wrong with him? Is he jealous because I've discovered one of the Mysteries of the Ages? I bet that's it! Maybe even with that cloud of aluminum he has over his shack, he missed Heard too!" We walked over to the door with the QRPer and patted him on the shoulder as he was walking out: "You're probably right", we said quietly, "and when you get that amplifier built, let us know how it all works out." The QRPer nodded in agreement and trotted off down the street, determined to get his order in for as many tubes as he could sneak by the XYL's budget.
When we returned to our table, we were deep in thought. While we were convinced most true-blue DXers ran maximum power, and some maybe even a bit more, we couldn't shake this picture of blasted coax, fused elements and TVI complaints! Somehow, we didn't think that Albert was referring to plate voltage. And while an extra kW will get you an S-unit or two, we had the feeling that knowing the DXpedition was working split might help too.
This story is in the public domain and may be reproduced in any format. - VE1DX
Last updated 14 December 2020