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Doughboyx
Junior Member
Username: Doughboyx

Post Number: 38
Registered: 9-2005


Posted on Thursday, March 29, 2007 - 10:54 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

This is the problem, I have a rebuilt Palomar TX600 with 4 2sc2879 Transistors in it. Well you can say it's custom built, but anyway. I had this amp in my vehicle with a big coil antenna that I got from a truck stop locally. The swr's was good with the amp off about a 1.5 if I can remember. And with the amp on it was a 1.8 and I was happy with that. But I did realize that custom amps swr's should be at least 1.3 or less. So I have been running this amp set up this way for a long time, several months or more maybe. Had a watt meter from radio shack in line and the wattage I was seeing was a little high I thought hell a loose meter. But it was ghost watts do to high the swr's was so high and now the amp will only put out 300-400 watts depending on the input drive 20-40 watts.

And I want to know did the high swr's hurt the output on these 2879 transistors and if so. If I replace the transistors should this fix the amp or is there a problem some where else in the amp. And if you blow 1 transistors in your amp say a 4 transistors will you still have output from the other 3 transistors.

Thanks any help would be appreciated
Doughboy
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Bruce
Senior Member
Username: Bruce

Post Number: 4404
Registered: 9-2003


Posted on Thursday, March 29, 2007 - 1:03 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

WELL looking at the devise spec sheet 400 is about right .....

See how much current your pulling ask someone who is using a simular amp how much current his pulls..... and how much he gets out.
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Romstar
Member
Username: Romstar

Post Number: 50
Registered: 3-2007


Posted on Thursday, March 29, 2007 - 1:36 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Those transistors should only give you about 400 watts out with a 40 watt drive. Even then, you are going past the specified input drive for them, and chances are they have been driven past saturation since you put the unit into operation.

If it is still working, you should run 20-30 watts in, and expect 350-400 watts out.

This will extend whatever life they have left, keep them running cooler, reduce your intermodulation distortion, and lower the chance of saturating the transistors.

If you are buying a 600 watt amp, you should expect less than that in real watts. If you really want 600 watts out, buy something that says it will do 800 or 1000 amps, and drive it properly.

It will last longer, it won't bleed like crazy, and you'll most likely have a much better sounding signal.

Check the spec sheets on those transistors, and you will see a lot of what I am talking about.

Romstar
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Doughboyx
Junior Member
Username: Doughboyx

Post Number: 39
Registered: 9-2005


Posted on Thursday, March 29, 2007 - 5:00 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

This amp with the output as it is 300-400 watts, is on par with a 2 pill amp 2879 transistors. This amp output should be 500-600 watts with 20 watts input at least. And my question is with the output this low if or should I replace the 4 2sc2879's will I see 500-600 watts. And this amp has two fans on the heat-sink.

With 20 watts input the amp will put out only 300 watts. In order to see the 400 watts output, I have to put 30-40 watts into the amp. I shouldn't have to drive this amp that hard to see these numbers. Do you understand ????
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Sonny
Intermediate Member
Username: Sonny

Post Number: 156
Registered: 5-2004
Posted on Thursday, March 29, 2007 - 5:46 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Dough, the real world is like what Romstar said when people buy a 600 watt amp. it will not do 600 watts and last. Most will do only 2/3 rds the stated wattage and last like a 1,000 watt might do 800 etc.
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Romstar
Member
Username: Romstar

Post Number: 53
Registered: 3-2007


Posted on Thursday, March 29, 2007 - 8:16 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Doughboy,

That amp, with those trnsistors, with those ratings simply won't push out 600 watts. If you could get a real 600 watts out of those four transistors, you would smoke them in about 1 minute and the last whimper from the amp would most likely be heard across all 40 channels.

I understand your predicament perfectly.

You are trying to get something out of those transistors they were never designed to do in a way that is damaging to them, and pouring out all kinds of harmonics and noise.

As you saturate those transistors you have caused damage, their linierity is shot and as you try to over drive it to compensate you are getting worse performance.

Hook any of these things to a real watt meter. Drive them at their proper input ratings, and see what happens.

Wattage out doesn't mean squat if you sound like Donald Duck talking on helium at the bottom of a grain silo.

The 2879's just plain won't do 600 watts with only four of them. Heat sink and fans be buggered. Replace them, reduce your drive and be happy with a 300-400 watt output that comes at least close to being clean.

Otherwise, find different transistors or buy a new amp.

Romstar
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Bruce
Senior Member
Username: Bruce

Post Number: 4407
Registered: 9-2003


Posted on Friday, March 30, 2007 - 11:06 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

WELL .......

Back in my WIPPER-SNAPPER days on 2 meter AM ( 1960's ) the 829B tube was KING !

This early 1930's tube was the best linear amp tube you could buy off the WW2 surplus market for under 10 bucks each NEW .....


LONG before transistors would work well at 144 mhz these tubes made many a young HAMS night providing close to 100 watts output for 5 watts drive which many of the AM rigs of that day ran.

What has this got to do with a 2879 CB amp?
EVERYTHING ......

SEE even if thoes days people just had to get a bit more out than that darn thing was rated at ...CCS being about 45 watts and ICAS about 60 watts ...... Now there was one rated level that didn't show up in tube manuals of that day .... that was called OIBD ( ONE INCH BELOW DISTRUCT )

Yep we used to drive them so that you could see the white hot grid glowing through the red hot plate ...... 100 watts output ......

The good old days just like thoes 400 watt transistors getting 600 watts out .......
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Doughboyx
Junior Member
Username: Doughboyx

Post Number: 40
Registered: 9-2005


Posted on Friday, March 30, 2007 - 11:54 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Ok Romstar

That is what I was asking. What is over driving a 4 2879's as I check the internet 20 watts input should produce 500 watts easy. And I have seen my amp do just that too. So all I am asking is with the high standing wave I had in the past was not enough to blow the transistors. But just enough to have damaged the transistors to where they wouldn't perform at full spec.,(would the first course of action be to replace them) thats all. And why would I buy a new amp when I have one ??

And I only drive my amps with 2-3 watts that swing 20-40 watts into them so I don't over drive them. That is why like the straight 4 pills High Drive amps.

Romstar, are you saying a Texas Star DX500 only produces at 2/3 the watts that's only 335 watts and the rest is harsh and trash right.
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Romstar
Member
Username: Romstar

Post Number: 56
Registered: 3-2007


Posted on Friday, March 30, 2007 - 12:53 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Doughboy,

With very few exceptions, when I see an amp I also see a lot of signal noise, bleed over and interference.

I can't speak to any of the new DX500's but everything I have seen tells me that they aren't really any better than the old products.

When amps are marketed to the CB crowd, the manufacture's know they are producing a "grey market" device. The majority of them squeek by the rules by being fairly wide banded, or just barely locking out the 11 meter band. In almost all cases it is a simple matter to make these units talk on the CB bands.

That's where problems start. When amps are marketed to the amateur crowd, the companies know they have a lot more demanding users who are within their licensed rights to run amps on their allocated bands.

This results in amps that are much better filtered, and the specs are way closer to what the parts can handle reasonably well without causing TVI, RFI and other problems like premature product failure.

Bruce's comment on tube amps is correct, in many cases you can drive those tubes to within an inch of their lives, and still get reasonable service out of them.
It doesn't translate to semi-conductor devices. The inherant signal to noise ratio in any semi-conductor is exponetially higher. Not to mention the power handling characteristics of the materials and the size of the device.

Ever seen a transistor the size of a coffee cup? I have. Specifically designed and built for high power applications.

The best piece of advice I can give you is to look at the way your amplifier is built inside, and then to examine the spec sheets for the devices they used to build it. This will tell you what a reasonable wattage would be for the drive, and how much to expect out for that level of input.

As to your question about SWR, make sure you have the absolute lowest SWR you can manage with the radio on it's own. Then hook up the amp. A CB band amp will most likely present harmonics to the antenna system, and those harmonics if they are not filtered will increase your SWR. This situation will worsen if you use too much input power.

Replace the transistors as your first course of action. As for buying a new amp, it depends on the cost of the transistors and your time versus the cost of a new amp brand new and ready to go.

As for hash and trash, it goes right back to the design of the amplifier. If there is no harmonic supression built in, and no filtering, you are going to get a lot of bleed over as you approach the output limits of the unit. The increased distortion will raise your SWR. Combined with a higher than spec'd drive rate for the transistors themselves, these two things will combine to degrade the performance of the amp and also contribute to your being heard on five or more channels on either side of where you are transmitting.

Check what it will cost, and make a decision from there.

Romstar
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Kid_vicious
Senior Member
Username: Kid_vicious

Post Number: 2465
Registered: 9-2004


Posted on Sunday, April 01, 2007 - 10:44 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

doughboy,

you might be able to see 600 watts out of that amp.
if you supply it with 18 volts, and drive the p!ss out of it.

thats what the custom amp guys do to get their specs.
some just flat out lie.

the thing to remember here is that the number you can make a wattmeter read really doesnt mean that much.
its just a relative indication of output.

if you over drive the input to that amp (40 watts is too much) you will drive the transistors into saturation.
at this point they might show 600 watts on the meter.
but that is not on the channel you are operating on.
those are false numbers generated by all the splatter and harmonics being produced by the over driven amp.
the wattmeter cant tell what freq. the power is on, it just shows them all jumbled together.

also, you have to take efficiency into account.
that amp is class C or AB1. both have an efficiency of about 60-65%.

so if you look at the data sheet for a 2SC2879, you will see that it is rated at about 120 watts. max.
4 of them would give you 480 watts.
IF! they were operating at 100% efficiency which they will never be.
you can expect between 80-100 watts from each transistor.

an SWR of 1.8 is not going to hurt anything.
if the antenna is tunable, i would try to lower it, but if thats it, then thats fine.

make sure you are NOT using RG58 coax, or the coax that came with the antenna.
it is junk.
get some RG8x. the good stuff.

what is the SWR going INTO the amp?
measured with the SWR meter built into the radio, or with a meter in line between the radio and amp.
this should be less than 1.5.
as long as its less than 2, you should be fine.

make sure you have a good name brand antenna.
there are MANY junk big coil antennas on the market today.


transistors dont usually weaken. they blow.
if your numbers are less than before, i would look at the other components in the system.
ever considered that your antenna connection may be corroding?
your coax has gotten kinked, or nicked?

also, you will notice bigger numbers if the engine is running with the RPM's up to about 3000.

there are many reasons that could be the cause of your lowered output.
best of luck,
matt
anyone wanting a "clean signal", just look to the left and build one of these!!!
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Patzerozero
Senior Member
Username: Patzerozero

Post Number: 3582
Registered: 7-2004


Posted on Monday, April 02, 2007 - 6:28 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

WHAT was rebuilt inside that amp? was it just a transistor change & whatever capacitance/inductance change to make it 'work'? did they re-use those little tiny coils? that could be holding things back as well......
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Doughboyx
Junior Member
Username: Doughboyx

Post Number: 41
Registered: 9-2005


Posted on Thursday, April 05, 2007 - 11:19 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Kid and Pat

Sorry it has taken so long to reply. Had a bad stomach virus over the weekend. But doing allot better now.


Pat, yes whole inside has been redone with larger caps and inductance, it was beefed up.


Kid, I did see between 500-600 easily with 2-3 watt dead key swing to 20-25 into this amp and now only seeing with the same input maybe 300-325 and this was after talk on it with swr's 2.5 for two month or more. I didn't know it was that high. So that is why I was asking about replacing the pills in the first place. This tells me bad pills somewhere, I just have to find them.

Thanks everyone who responded. I will replace the pills first and see what happens from there. Money is tight right now so I will talk on the amp as long as I can or until it dies whichever come first.
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Patzerozero
Senior Member
Username: Patzerozero

Post Number: 3602
Registered: 7-2004


Posted on Tuesday, April 10, 2007 - 4:09 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

doughboyx, there is probably not a thing wrong with your amp. the higher-then-recommended-to-use SWR probably DID allow the wattmeter to show 500+ watts. the 'better' SWR you see now is showing a truer output. i would do what is needed to get the SWR even lower, make sure i have GOOD coax, & check things again with a couple different wattmeters. while 300 is a bit low, i doubt there is anything wrong.
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Doughboyx
Junior Member
Username: Doughboyx

Post Number: 43
Registered: 9-2005


Posted on Monday, April 16, 2007 - 11:09 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Pat,

Did what you said I changed the coax from antenna to wattmeter have 3 ft. jumpers inline swr's with no power is 1.3, with amp on it's 1.3 now the amp only puts out on high about 200-250 watts, mid 150 watts, low 80 watts. This is what I am talking about this amp should be putting out more than that. I wanted to hear from people that have been through this type of behavior with their amps. To get an idea if I am heading in the right direction as to start with replacing the pills.

Now that the wattage has started dropping more it look as if this amp does in fact needs the pills replaced after all. Money is slack at the moment so I will replace them later. So thank to everyone who responded.

Doughboy "73's"
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Onelasttime
Junior Member
Username: Onelasttime

Post Number: 37
Registered: 8-2011
Posted on Thursday, August 25, 2011 - 10:38 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Are you kidding me???? Have you looked at the SWR rating for the 2SC2879???? Their is a reason that I will not by a 2290 pill amp or 1446 etc......the 2879 is a beast for being able to take high swr's especially if you avoid higher then normal beta rating on match quads and such. For mobil linear's this is about as good as you can hope for in a 12 volt transistor from a durability and performance perspective. I think the Motorola MRF 455 sound better but they do not put out as much power so their is a trad-off for nice low inter-mod and fantastic sound on SSB.

As for power output like other's have said most linear amp manufactures that market to CB'er's over rate their amps. THey usualy use higher then 12 volts like 14, 15,16 volts and they normally rate them with a twin tone which modulates the amp to full peak output only 1/2 as much as a single tone would so it is kind of like rating it on SSB in terms of the output power difference as compared to how a CB'er is going to use it and check power output. Plus many amp companies have no problem over driving their amp to get the number's they want.

On top of that you have the matching of a pair, quad or more of transistors not all matched pairs are going to be 100% they might be in the 70%-90% range more often then not because high beta matched pairs and such cost more money and are less durable to high swr's so not what an Amplifier company building for the CB market would really want to spend extra on.

I once got a set of Motorola matched quad that had all 4 transistor's putting out far more then the listed 100% rating. I think they where ratted at 80 watt's of dissipation and they where putting out what worked out to be 120 watt's each in the unit. I paid dearly for that level of output in a matched quad set of transistors. What happens when a guy get's a unit that is doing all it can do putting out 65-70 watt's per transistor on transistors that are rated at 80 watt's each in the white paper's???? On top of that few units can come close to the full power output rating of it's individual components.

Voltage x .707 is what most use for RMS but I like to use .6 because few amp's ever come close to the RMS output of their transistors because they are the sum of their parts and often a lot of compromises are made. ON top of that what class is your amp is A,B,C AB1 AB2?????? The Generic power rating for the 2SC2789 Transistor is 100watts so 4 of them is 400 watts peak but often at 14.5 volts you will get 120-140 watt's out per transistor PEP.....So that still put you short of the 600PEP rating they make you think you are getting. If you do not have a scope and signal generator to set this up right and you took your PEP say it was 540 and multiplied that by .6 that would give you 324watt's you would be able to run that on AM for a long long long long time assuming you let the key up from time to time so other's can talk too and never have an issue.On the other hand running her hot at 450 watt's with long winded sessions and poor cooling is a recipe for short life cycle and early repair.

Your SWR though is not a problem at all.

Oh and if you do want to run her at higher power levels do it on SSB they do not get as hot because even though the power level output wise will read higher the duty cycle is gentler then AM.
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Moonraker
Intermediate Member
Username: Moonraker

Post Number: 298
Registered: 3-2005


Posted on Monday, November 21, 2011 - 10:25 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

An old thread but I do agree the swr is more than good. The 1.3 or lower swr comes from the competition amp builders. When you are running them at 15-20 volts and over driving them the swr becomes more critical.
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Silverbullet516
Junior Member
Username: Silverbullet516

Post Number: 22
Registered: 7-2012
Posted on Thursday, November 22, 2012 - 3:47 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Thats a 1x4 box i would look into the driver. I have its brother the boomer 600hd. Did the transistor swap and left the mrf455 as a driver. I had the same power drop off it turned out that i was over driving the driver side. Replaced the 455 with a 1446 and all was good again.. hope this helps
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Kc2kdv
New member
Username: Kc2kdv

Post Number: 3
Registered: 11-2012
Posted on Thursday, November 22, 2012 - 9:24 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

This site is awesome**** everyone has great input.Hey doughboyx,im running a ms500 (4)2879 inline with a "1999" Magnum Delta Force that has powerful swing. 6 gauge wire throughout the car,with a Wilson 1000. SWR 1.1/1.3 dx 5 watts 40pep out of the radio 100 watt dead key from amp and it swings to 500/550 all day. GROUND GROUND GROUND GROUND IS THE KEY!!!! I PERSONALLY WOULD PULL EVERYTHING OUT AND START FRESH. Good luck and be patient. What kind of radio,and wattmeter you have inline with the box? I have to be honest and say it does splatter if i crank the audio,i usally have the d104 setting almost off and the mic at like 9 o'clock. anyway, your set-up should work***you just have to get everything lined up perfect.. "Dont over drive it..."

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