I am posting this due to the fact that I have made horrible costly mistakes. I know now that a 50 amp supply will not run a low gain 4 transistor(1446) assistant(why it is capable of pulling 66 amps). Do not base your supply on fuse. I also know that a 100 amp will not run a 1 driving 8 transistor assistant(low gain transistors 14* driver + the 8). Feel as as smart as a box of rocks right now, only reason for the post.
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How much do your assistants draw?
- Thread starter 2315 Robert
- Start date
A VERY common mistake people make is not figuring the efficiency into the equation when calculating amp draw. If a power supply is capable of providing 100 amps at 13.8 volts that means it is running max out balls to the wall to provide 1380 watts DC power. If an amp is running at about 700 watts OUTPUT it will need pretty much everything that power supply can give it to operate because the efficiency is only about 50% which is why most people suggest getting a power supply that can provide twice the power that the amp puts out. Most people make the mistake of calculating that 700 watts divided by 13.8 volts and come up with a current draw of only 50.75 amps and then think a 70 amp Astron or the likes will run it when in fact that 700 watts OUTPUT wants to see 13-1400 watts DC INPUT for a current draw of about 94- 101 amps at 13.8 volts.
How hard are you driving that 4x1446 amp (and I guess what mode?)
I can use a MegaWatt S-400-12 (36 amps / 41 amp peak) to drive a 4x1446 AB biased amp to an output of 320 watts SSB before it starts to run out of juice.
I can use a MegaWatt S-400-12 (36 amps / 41 amp peak) to drive a 4x1446 AB biased amp to an output of 320 watts SSB before it starts to run out of juice.
Interesting you mention that. I was running it on the same previously on my S-400. 3w dead key swinging 12-15ish if the meters I have are telling me the truth.. I stopped using it because I felt I was to close to its max and got the 50A. If I am following what CK is telling me that 4 transistor could pull up to 66 amps. I had 2 50 amps hooked together and all was well(equal length wire and so forth). Then I switched some connections around and mis-takedly had it to one supply only and lights out.
Bear in mind that class C comp style amps draw more current on stock volts for a given output than a class AB amp designed for stock volts.
comp style amps are very thirsty on stock volts.
comp style amps are very thirsty on stock volts.
First the ideal voltage to run so-called 12V devices off is about 14.5 Volts not 13.8V and not 12V that assumes you have a device that can handle that which is not the norm. If you routinely sample voltage at a 12V car battery while you are driving on the HWY/FWY/Inter-State you will see most car's after things warm up and settle down are between 14.5V and 15.5V which is why so many 12V devices list 16V as their limit in owners manuals.
For some reason though power supplies designed to run 12V devices off mains power never come close to replicating a real car's 12V system.
AS long as you do not exceed the official OEM white paper rating for devices the higher the voltage you can achieve without exceeding the limit the fewer amps you need.
The only reason I say 14.5 is because that leaves plenty of wiggle room on a regulated supply even if the clamping is a little slow. Also MFJ on their mobile amp the 500 blah blah back when it used Toshiba transistors said in the owner's manual " every volt below 14.5V resulted in an 85W pep reduction in power"....
Also current draw and voltage drop are both related to how much input drive you give it. If you want to reduce the current draw just turn the rf output power down! Only in the electronic arena do people toss all their common sense out the window! You would not drive your car with wide open throttle ever place you went and every time you take off so why would you run your electronics like that? When you are shopping for groceries do you sprint up and down each aisle and sprint the entire time you are at the store? No and if you say someone doing that you would think they were an idiot! Turn rf output power down to half and retune modulation.
If you can adjust the voltage up a bit without triggering the safety fault then do it! Obviously, use sound judgment!
Yes most amp's that use BJT's are at best 40%. Once in a blue moon, you will come across a nicely designed amp that might be into the low to high 60's for efficiency.
For some reason though power supplies designed to run 12V devices off mains power never come close to replicating a real car's 12V system.
AS long as you do not exceed the official OEM white paper rating for devices the higher the voltage you can achieve without exceeding the limit the fewer amps you need.
The only reason I say 14.5 is because that leaves plenty of wiggle room on a regulated supply even if the clamping is a little slow. Also MFJ on their mobile amp the 500 blah blah back when it used Toshiba transistors said in the owner's manual " every volt below 14.5V resulted in an 85W pep reduction in power"....
Also current draw and voltage drop are both related to how much input drive you give it. If you want to reduce the current draw just turn the rf output power down! Only in the electronic arena do people toss all their common sense out the window! You would not drive your car with wide open throttle ever place you went and every time you take off so why would you run your electronics like that? When you are shopping for groceries do you sprint up and down each aisle and sprint the entire time you are at the store? No and if you say someone doing that you would think they were an idiot! Turn rf output power down to half and retune modulation.
If you can adjust the voltage up a bit without triggering the safety fault then do it! Obviously, use sound judgment!
Yes most amp's that use BJT's are at best 40%. Once in a blue moon, you will come across a nicely designed amp that might be into the low to high 60's for efficiency.
Bob you got it a bit backwards I think. The entire point of converting a class B or AB amp to class C is that for any given voltage and power output it will draw less amps to make the same power. The problem with that is it limits the modes of operation you can use without damaging the amp, sounding terrible and tearing up a band. See flywheel effect as it relates to push-pull BJT circuit. Likewise if yu go from class C to class D you again can make more pout. at any given voltage/current combo and same amount of input drive. This is why you see people using class D for subwoofer amps because you can get a crazy amount of power out of a very small package as compared to other classes like A,AB,B,C parts also end up running cooler.
It's not the class of operation he is getting at but the whole design. The class C comp amps everyone is building were originally designed to run on 20 volts for competition purposes. When people copy this design and run the amplifier on 14 volts and loaf it along the efficiency not so good.
Just as the plate impedance of a tube changes with voltage and current draw. Tune up a tube amp at max drive and the efficiency may be 60% or better. In AM service with the carrier at 1/4 of the peak power the efficiency may 30% or less. When you measure efficiency at a reduced voltage or drive level than the circuit was optimized for you will see terrible efficiency.
No i don't have it backwards in this case,
for a given output nothing pulls current like a comp style cb amp on stock volts,
normally class C amps designed with correct load lines for stock voltage would pull less current than a class AB amp, you would expect something like 40% vs 55-60-% efficiency for properly designed amps,
neither will run on 18v or more making significantly more output without exceeding maximum device current or dissipation,
comp style amps are not designed with normal load lines, you don't want the same amp for stock volts as you would use in a keydown on 18-21v but that is what you get,
they are drag race amps designed for higher than stock voltage to allow more output without exceeding maximum device current, efficiency on stock voltage SHIT,
w8ji made an interesting post on Eham about what transistors can actually output when pushed into deep class C on close to breakdown voltage with the correct load impedance if imd was of no concern,
some Hambones with a little but not enough knowledge were rubbishing cbers for claiming higher than datasheet output for 2879's, w8ji set them straight
for a given output nothing pulls current like a comp style cb amp on stock volts,
normally class C amps designed with correct load lines for stock voltage would pull less current than a class AB amp, you would expect something like 40% vs 55-60-% efficiency for properly designed amps,
neither will run on 18v or more making significantly more output without exceeding maximum device current or dissipation,
comp style amps are not designed with normal load lines, you don't want the same amp for stock volts as you would use in a keydown on 18-21v but that is what you get,
they are drag race amps designed for higher than stock voltage to allow more output without exceeding maximum device current, efficiency on stock voltage SHIT,
w8ji made an interesting post on Eham about what transistors can actually output when pushed into deep class C on close to breakdown voltage with the correct load impedance if imd was of no concern,
some Hambones with a little but not enough knowledge were rubbishing cbers for claiming higher than datasheet output for 2879's, w8ji set them straight
Someone always has to preach from the datasheet. It says they are 100 watt devices so anything over that is fake. It also says the device is good for 2 to 30 mhz. It must be impossible for it to amplify harmonics too.....
Someone always has to preach from the datasheet. It says they are 100 watt devices so anything over that is fake. It also says the device is good for 2 to 30 mhz. It must be impossible for it to amplify harmonics too.....
Those of us....yes I am one of them, that preach from the data sheets are almost ALWAYS talking about decent clean watts where harmonics and IMD ARE a concern. Everything has it's limits and if you exceed those limits there is a payoff. You can have a nice decent signal running within data sheet specs or you can have a crappy wideband fuzzy signal while far exceeding the data sheets specs. Nobody I think says you CANNOT exceed data sheet specs it just comes at a price.