The constant reminders not to overdrive is about effective as telling me to drive my car slow when I was 18. Waste of time.
-
You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
BBI using eBay LDMOS Boards?
- Thread starter LeapFrog
- Start date
dont worry 99.8% of cbers have been over driving transistors in rf amplifiers for 40 years they still have not learnt ....
even running a 2sc2879 @ 100w pep per device will make most real rf engineers roll their eyes and have "S.A.D" spectrum analyser disorder ......a condition only known to affect men that understand IMD
even running a 2sc2879 @ 100w pep per device will make most real rf engineers roll their eyes and have "S.A.D" spectrum analyser disorder ......a condition only known to affect men that understand IMD
I love watching some of the YouTube builders go: "two hundred bird from this 2 pill, well the watt meter tells me the datasheet lied!" (not literally)
Splatter watts.
You can build anything (in your private shop), selling things (without a great legal disclaimer) can become another story.
A real HF rig for demonstrations, and a good disclaimer (in the video, and at the time of sale or inquiry) would only serve to help things i.m.o.
Last edited:
The most I ever got out of a pair of 2879s was 900 carrier...400 with modulation and a tad over 1kw pep.
Running at 21 volts they drew enough current to make the short 8 gauge power leads get hot. Each poweread was roughly 8 inches long.
It would warm up one of those MFJ 1500 watt dry dummy loads rather quickly. Splatter watts indeed but it would run with those big sweep tube amps for a 10 second key. Those things all belong in a landfill.
I agree with you on those sweep tube boxes, and 900 watts carrier
, swinging back to 400 under modulation!?!! Wowzer so that's what "pushing it" can be for two 2sc2879's! No wonder people loved to volt these boxes!I agree with you on those sweep tube boxes, and 900 watts carrier
The amp design everyone copies was built to run that kind of voltage. Normal operation wasn't part of the plan but they work and are simple to assemble.
Don't look at the carrier too long. Start yelling into the mic so the current drops before the smoke gets out.
Volting a comp box like that is like squeezing a couple thousand horsepower out of a V8 for the drag strip, if you're a competitor you know the risks and accept the downsides as a part of competing. It doesn't become a problem until people who don't know any better start using them for daily drivers and expect all the reliability of a more conservative design.
I take it some CB'rs never gain the concept of having a box "Loaf Around" @ say 1 Kw, and instead try to superchage a v-6, add nitrous and expect the low end torque numbers of a v-8, as a daily!
On a side note the HuaGao Semi (2SC2879) looks promising, if the part had better efficiency (the original) I could see building a big box, but I'm just not into it as they sit.
If somebody's going to go through all the effort of getting a High amp service line, at their home in a residential area they better get the most efficient design they can find.
The math doesn't lie, you will get to a certain point where you can only get so much out from what you have coming in.
Imagine what the current demands (@ 220v) would be while producing1.21 JigaWatts using a 50%
efficienct box!
Last edited:
They also overlook the fact that 1 good transmitting tube would last them the rest of their lives and they wouldn't constantly be looking for overpriced tubes for old TV sets.
The money they waste keeping some if that old junk alive is ridiculous. Tube failures, blown pills and at least a trip or two to the CB shop every year.
A single 3-500z has the plate dissipation of 12 sweep tubes and they drive 6 or 8 sweep tubes to twice the pep a single 3-500 will do. Sounds like a formula for a money pit. They still get shut down by a real amplifier running the same pep wattage.
For the big boys there are better options but I'm still hoping the HuaGao pills work out even if the efficiency is a bit lower. What seems to be missing from the hobby right now is a good quality small amp that will give the little guy a couple hundred clean watts, a decent 2879 replacement would go a long way to getting us there.
The LDMOS stuff and your better tubes bring a lot to the table if you want to go big but for more modest stations they're more work than should be necessary for more power than needed.
Last edited:
It's going to need true AB "thermal tracking" biasing and a built-in filter.
Once they are in the wild, some people are going to start hacking them into Class-C boxes for every last watt, or strictly for FM or CW use.
A nice, clean, true ten/twelve meter box (2 or 4 transistor) could be made using the HG 2879, I think this would keep anyone from needing a massive power supply..
And yes the 50v operating voltage of the LDMOS is an obstacle for most of the guys who are "appliance operators", imo.
Once they put too much drive in and blow it, that $300 for the part alone will make them re-think what they think they know.
'Brave New Linear'
Sure; but does a clean 360 watts walk the dog on 400 dirty watts?
ANS: 400 dirty watts may have up to 20% dirty watts (or more), or 80 watts out of 400 watts = 320 watts. Dirty watts are read on a watt meter; but they are NOT on the same freq as the operator thought. These dirty watts are harmonics of the original signal and not on the same freq. Sooo, all the dirty watts have done is fool the owner because he doesn't know that a watt meter cannot tell the difference.
Last edited:
- No one is chatting at the moment.