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hundreds of watts per 2879 ???????????????????????

Now wait a minute! If -he- can get a 'banned' rating, I deserve at least an honorary banned rating. I'm a bigger 'C.O.B.' than he is! ... On second thought, I'd settle for 'C.O.B.' for a rating, "Crotchety Old ...uh 'Beauty' ". (Not sure I can say that other word.)
- 'Doc

(in pink letters, naturally)
 
Well lookie there, Doc got what he asked for. :LOL: Glad to be of assistance to you Doc. :love: Sorry about the colour,maybe HH to Mole can change hat for you. I couldn't find the option to change the color for you. :laugh:
 
a point to think about regarding distortion. A clean home amp used to have a spec of .1% distortion from 20-20k hz. Newer models are drastically lower. I have read before that the human ear can "tolerate" 10% distortion in some frequencies before it becomes readily apparent. The speakers we use for ham or cb have nowhere near the kind of fidelity that a good home stereo does obviously. My point here is simply that I think it's possible to have a signal with a huge amount of distortion in it before it would sound "bad" to the other guy especially with all the noise in the airwaves and the junk radios everyone hears out there. I don't like someone splattering three channels up and down either, and strive to have a clean signal, monitor it, etc. but how many people listen to what they sound like? I was on 38lsb the other day and a guy was running a 2950 with some power and sounded like total crap. Getting great audio reports from everyone he talked to...really? that radio should sound choice and it was terrible lol. He was happy cause it was loud...

It's gonna be a personal choice for each operator how far they want to push it. Are they willing to be a pain to their neighbors and other operators on the air by pushing those transistors to their limit? This has been an awesome thread for the knowledge and now people know the pros and cons of such a setup and what it will "cost" to get those extra watts.

The debate will surely continue as to how much "should" be a maximum as far as what is acceptably clean. Added filtering, signal from the source, installation, all become issues and it starts to get gray lol.

As to the other forums that can't handle a conversation, they're just weenies :)
 
Why are you guys still even messing with transistors.Dont forget a transistors rating is rated for 100 percent duty cycle cb talk isnt even 50 percent.As long as you can keep it cool then your good to go.Look at the guys who run 10kw out of a 3cx3000 in a mobile.As long as they keep the tube cool the tube wont overheat so the same goes with a transistor get a big enough heatsink and slap some fans on it or get an even bigger one and forget the fans and you will be good to go.If i still ran transistors which i havent on cb in over 15 years minus the ones in my yaesu I would want the box to have protection like shockwave spoke of each bank fused.But since most of the so called garage techs out there make most of their money in repairs would never dream of installing such a circuit.Would be nice to have a over voltage interlock to break the primary feed if the voltage got to high and also high swr protection then you would have a bullet proof box that would probably last forever.But why run a transistor box with so many pills when you could have just 1 tube which would do way more.
 

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Shockwave said:
These are good questions. 18 volts with 250 watts is 13.89 amps but you've calculated for a total input power of 250 watts. The maximum theoretical output would be when the collector dissipation itself has reached 250 watts. The input power required to do that will depend on the amplifiers efficiency. You also make a valid point I overlooked. At 18 volts it will not be possible to reach the maximum collector dissipation before you exceed the maximum current. However, with class C you don't need to come close to the maximum dissipation to make 250 watts output.

I have to admit I have learned some things from this tread also :) I thought the 2SC2879 was rated at 20 amps maximum collector current for the last 20 years. It's actually rated at 25 amps. Which explains why the 20 amps fuses in my protection circuit worked best at preventing transistor failure. It takes a few extra amps before that fuse is going to heat up and melt. Back then I didn't even have an amp meter that could read higher then 10 amps DC.
Click to expand...

So at 50% efficiency, the transistor would produce 125 watts output and the other 125 watts in heat correct? This is at 18v
 
Honestly transistors are not my thing.I build tube boxes.And sell them under the name jeannem2987 on ebay.I mess with big tetrodes also which are grid driven.Like i said before i havent run a transistor box in years.I learned most of what i know from shockwave a licenced rf tech when he used to have a shop.Now i talk alot of ham and have started on a multiband 3cx3000 for myself.With a roller inductor for the tank.
 
But i have to say with the risk of sounding biased what shockwave has said does make sense to me.Sometimes opinions cloud the truth.May want to go over everything again.
 
dxhound said:
So at 50% efficiency, the transistor would produce 125 watts output and the other 125 watts in heat correct? This is at 18v
Click to expand...

You are correct. About the only way I can see reaching the maximum collector dissipation on 18 volts without exceeding the maximum current of 25 amps would be if the amp were running under 50% efficiency. Where most of the power would then be in heat. At 18 volts and 25 amps, the input power would be 450 watts. With 50% efficiency you have 225 watts in transistor heat and the same into the load.

To be realistic, you can not run the collector current at max, the collector dissipation at max, overvolt it, and then overdrive it too! You have to cut the transistor some slack in you want it to make 250 watts reliably. That extra headroom comes from dropping down into class C and gaining the efficiency to keep the dissipation down. It's never a good idea to exceed the maximum current either because that can cause instant failure.
 
Thanks for the "plug" there Scott. You've come a long way in the last decade from not knowing how to fix you're own amp to building your own quality all band HF amp from scratch. Tubes are great and I wouldn't run anything else in the base unless it was FCC type accepted. That insures some level of signal purity. At the same time, transistors are fairly hard to resist in the mobile when you consider the operating voltages. If you must run an 11 meter solid state amp and you want it to run clean by all means add a decent low pass filter right after the amp. It makes a huge impact in the area of harmonics. You should also avoid class C for improved IMD and resist "overmodulation temptation".
 
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you can also buy a cb amp that has filtering built in , but theyre arnt many to choose from .
 
Booty,
Nitrogen cooled? Already been done, by the military at least. Knew of one of those 'surplus' amplifiers that used a single transistor and would put out unbelievable amounts of power. IF you could keep the nitrogen flowing. Friend had one, it definitely would 'pin' an 'S' meter. He didn't have the complete nitrogen system, no scavenging of the gas, and would eat up a large bottle of nitrogen in a short time. No idea what transistor, or whatever happened to the thing (something like 10 years ago now).
- 'Doc / COB
 
so 250 watts at an avg class C efficiency of 70% would be this

18v 13.9 amps 250watts 250/70 = 35.7

So 36 watts would be heat and 214 output.
Up the drive, and at this efficiency level you could theoretically hit 386 watts output before maxing the 25 amp current limit. But wouldnt that be to much for the junction to handle and internal losses would cap the output? I know Im being persistant on this, But my area is Traveling wave tubes for radar use, and apparently my understanding of RF device transistors was wrong.

Im asking as I am about to combine two Texas star dx1600's to make a 16 transistor mobile amplifier for amateur use. It will be filtered and alot of upgrades to the bias network. But this will help in my further testing of this amplifier. I want the amp to be as efficient as possible, but with no or little trade off in IN band harmonics. as it sits each transistor will have to produce 94 watts roughly to reach legal limit. Im considering more transistors to get this down to the 2879's cleanest 60 watt per device. but for that I would need 25 total 2879's or three dx1600's hmmmmmm
 
dxhound said:
so 250 watts at an avg class C efficiency of 70% would be this

18v 13.9 amps 250watts 250/70 = 35.7

So 36 watts would be heat and 214 output.
Up the drive, and at this efficiency level you could theoretically hit 386 watts output before maxing the 25 amp current limit. But wouldnt that be to much for the junction to handle and internal losses would cap the output? I know Im being persistant on this, But my area is Traveling wave tubes for radar use, and apparently my understanding of RF device transistors was wrong.

Im asking as I am about to combine two Texas star dx1600's to make a 16 transistor mobile amplifier for amateur use. It will be filtered and alot of upgrades to the bias network. But this will help in my further testing of this amplifier. I want the amp to be as efficient as possible, but with no or little trade off in IN band harmonics. as it sits each transistor will have to produce 94 watts roughly to reach legal limit. Im considering more transistors to get this down to the 2879's cleanest 60 watt per device. but for that I would need 25 total 2879's or three dx1600's hmmmmmm
Click to expand...

If you take your example of 250 watts input power at 18 volts and 13.9 amps with 70% efficiency, I come up with 175 watts into the load and 75 watts of collector dissipation. Up the drive until the collector draws 25 amps and you have 450 watts input, 135 dissipation and 315 watts output. That is only 24 watts more output then I've ever seen from the device. The transistor was designed with enough collector dissipation so that it would not be the weak link in the chain. Going past 20 volts or 25 amps is what kills this part. This is what prevents you from ever reaching the output power that would be obtained at the 250 watt collector dissipation.

I read your posts in the other tread about building combining the two amps and using it on the other HF bands. Before you go through the work of doing that, I would test a single amplifier on each of the lower bands you intend to operate on to make sure it runs stable first. Even passing this test is not a 100% guarantee it will still be stable after you combine two but it's a great start. Keep the coax going to your combiners as short as possible and the same length.

At 94 watts each transistor they will be running comfortably and with the correct mic gain setting the IMD shouldn't be too bad. It is biased but as you probably know it's not the best bias circuit or amplifier for HF. If one amp works good I'd almost be tempted to leave it at that rather then come up with twice the DC amps to feed a pair and gain a half an "S" unit. There is also the chance of a self oscillation on some frequency that could wipe out transistors instantly too.
 
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