Amp Wattage Inputs

[Pamm r]

I have a few boxes CB only, Does anyone know where to look up how much input a box can take.
I hear low key box and High key but don't know anyway to tell.
I have auto units, tnt 350, skeleton 100, Eagle 350, Palomar 250.
And house units RM KLRM 550 transistor box, and a old Contex box

 

[LeapFrog]

Well, you can always start low and work your way up (regarding input power), starting with too much input can cause immediate damage.

With any unknown box that is RF sense keyed, I'll use a radio that has convenient carrier adjust and start at about half a watt and slowly increase the dead key.

A lot of people will disagree with i'm about to explain next..

I've heard people say as a loose rule of thumb "use just enough carrier to key the amplifier without observing any relay chatter".

The problem you can run into with too little carrier, is that if the radio is "peaked" or modified for "swing", the amplifier may produce a lot of distortion (over-modulation).

I turn up the dead carrier and use an occasional "Audiooo" to see where the amplifier starts to back swing with audio (on AVG reading meter), that tells me I am over driving the device(s). I will use this method to find a reasonable "maximum carrier output" number, and use that as my goal to reach (P.E.P.) while using a carrier (giver-or-take) 1/4 of the maximum dead key output, this leaves head room and hopefully will stop you from having a problem over-driving, flat-topping or sounding bunched-up/pinched.

If It keys 25 watts and "swings" 50 AVG, (so roughly 100 P.E.P.,) that is good!
You don't want to see any back-swing with an average reading meter, and you would ideally see the needle kick-forward (about 1.5X times the carrier).

You wouldn't want to have the amplifier produce an overly large carrier, this would sacrifice the amount of headroom or "swing potential" of the box,and your audio will sound "off" with too much carrier.

You can run the #'s on the power output devices and find a datasheet for each part (MRF455 or 2SC2879, or w/e is used) to see the manufacturers rated, maximum input specification.

It is hard to really see what kind of input to use without observing the output on a 'scope... That's one way to get a better idea of what is really going on, an oscilloscope will tell you more than a watt meter.

 

[Tallman]

Check your model numbers on this site. This is the most complete site when looking for older/discontinued amplifiers.
http://cbradio.nl

I managed to find all of my technical data for the Silver Eagle 777. It is fairly rare.

 

[kopcicle]

More rule of thumb but for different reasons.
Find the data sheet for the device(s)
Find the gain in db.
Now do the math...
for a 10db device.
watt in 10 out
2 in 20 out
4 in 40 out...
If 5 in doesn't get you 50 out you are heading intro what is called compression. Back off to 4.5 and look for 45 out .
This is just an example. It would follow 1 in 15 out, 2 in 30 out for a 15db device.
At some point I'll do what I can to explain bias, efficiency and total dissipation but for now I'll just do the basics.

For the above 10db example if the 1 watt in draws 1.5 amp at 13.8 then approximately 20.7 watts total which fits well with the just over 50% efficiency expected. if it results in 10 watts out. Then, if 2 watts in draws 3 amp at 13.8 then approximately 40.14 total and 20 watts out.
The whole point here is if you draw more current without gaining the expected output you have gone to far.

If 4 watts in draws 6 amps but you don't quite reach 40 out with 82.8 total dissipation then you are headed into compression.

Okay that is the simplistic version without including the bias into the total dissipation or estimating the losses in the broadband input and output transformers. Many amplifiers also have input padding in the form of a resistor network that is variable for variable drive/output or just to make the driving radio VSWR happy.

Finally design limitations.
Heat, some just don't have enough heat spreader/heatsink or fan. Heat is the enemy.
input network, The wattage rating of the input resistor network isn't up to dissipating the excess from the radio. Again, heat is the enemy.
broad band input and output transformers, As a cost saving measure the transformers are never large enough (well, almost never). Even when they are large enough they still have to dissipate heat. The heatsink does nothing to to cool the transformers. Once again, heat is the enemy.
VSWR , transistors will not tolerate large VSWR. Although there numerous complications from a high standing wave the first and most important is device junction temperature. Here it's temperature rather than heat because thermal breakdown of the device junction is caused by excessive temperature because the device package can not transfer heat away from the junction, through the heat spreader, to the heatsink, into the air stream and away from the amplifier. Junction temperature is the enemy.
I won't touch on Oscillation, Inter Modulation Distortion, harmonic energy, or output low-pass/band-pass filter losses . I'll just leave this here and go pet the dog ...

 

[Tallman]

You are correct about junction temperature! One engineer I worked with was too lazy to do his calculations on this design. Since I was co-engineer and junior in this company I ended up doing all of the calculations for my part and his part. He was a little sloppy in his design work and most of his power transistors would fail quickly. To cover my butt I sent him memos and cc'd the manager of engineering. I even went so far as to recommend changes to his compensation networks. He whined to the manager and had me called in on the carpet. After he left the manger thanked me for my diligence and put me in for a pay raise. I don't know what happened in the meeting he had with the manager after I left. He was kind of sheepish after that.

Never beat the donkey who is carrying your load for you.

Impedance mismatch in a circuit (SWR) will take a boarder line case and destroy it by exceeding the junction temperature no matter how good the heatsink is.

 

[LeapFrog]

After reading Kopcicle's & Tallman's post, I realize I am just a CB'er who once took apart his mom's TV remote control as a kid. Really, my "help" is not exact or precise, barely accurate.. I think i'm going to stop posting and trying to help people, I don't know enough at this point and I don't wish propagate misinformation or CB folklore.

Sorry Pamm r, and wwdx crew. This frog is 10-7.

 

[Tallman]

After reading Kopcicle's & Tallman's post, I realize I am just a CB'er who once took apart his mom's TV remote control as a kid. Really, my "help" is not exact or precise, barely accurate.. I think i'm going to stop posting and trying to help people, I don't know enough at this point and I don't wish propagate misinformation or CB folklore.

Sorry Pamm r, and wwdx crew. This frog is 10-7.

Don't do that! I enjoy reading your post and not just being facetious either. Myself and others I won't mention by name are well studied in the art of electronics.
Your views are quite accurate and are from a practical learning experience. Keep 'em flying frogman!

 

[LeapFrog]

Thank You, Tallman I appreciate that! Some times I feel like the descriptions I share are not "good enough" or detailed enough in contrast to others, but looking at things from a practical experience angle helps me feel not-so-bad for lacking the specific and technical knowledge I hope to obtain.

73

 

[nomadradio]

Not sure about the TNT350, but the rest of them are all built to accommodate only a barefoot CB type radio. None of them are meant to take the higher power you get from a "10-meter" Galaxy, Connex, RCI or Magnum radio.

A fairly-dependable rule is that the older the linear, the less drive power it can stand.

73

 

[kopcicle]

Thank You, Tallman I appreciate that! Some times I feel like the descriptions I share are not "good enough" or detailed enough in contrast to others, but looking at things from a practical experience angle helps me feel not-so-bad for lacking the specific and technical knowledge I hope to obtain.

73

Detail comes from practice and experience. Different people learn in different ways. Who's to say what turn of a phrase will make sense to someone ?

keep at it

 

[Tallman]

Detail comes from practice and experience. Different people learn in different ways. Who's to say what turn of a phrase will make sense to someone ?

keep at it

"All it takes to learn is an open mind."
"Anybody who claims to know everything DOES NOT.."
"Never beat the donkey that is carrying your load for you."
"It is better to ask questions than try to bluff your way through and get caught with your ignorance hanging out."

Definition of EXPERT: A has been under pressure.

 

[LeapFrog]

Thanks I appreciate that gentlemen, today is a new day and I'm feeling better about the situation already; my apologies for hijacking the thread.

I have had a few 1 transistor driver boxes and a few 2 transistor boxes, they always seem to run cooler and happier with a radio that keys under 3 watts.

 

[Pamm r]

After reading Kopcicle's & Tallman's post, I realize I am just a CB'er who once took apart his mom's TV remote control as a kid. Really, my "help" is not exact or precise, barely accurate.. I think i'm going to stop posting and trying to help people, I don't know enough at this point and I don't wish propagate misinformation or CB folklore.

Sorry Pamm r, and wwdx crew. This frog is 10-7.

Hey, thanks, I read all post replies and thank everyone for taking time to answer

 

[Pamm r]

Not sure about the TNT350, but the rest of them are all built to accommodate only a barefoot CB type radio. None of them are meant to take the higher power you get from a "10-meter" Galaxy, Connex, RCI or Magnum radio.

A fairly-dependable rule is that the older the linear, the less drive power it can stand.

73

Thanks, my radio I have keying 2 watts but swings to 40. The box doesn't seem to want to take that much swing so does that mean that the swing matters also?

 

[nomadradio]

Yes, it matters a lot. An amplifier from 1979 or 1983 that was meant for a radio with a max modulated power of around 20 Watts will be overwhelmed by peak power twice that much.

All amplifiers will have a "ceiling", a maximum peak input that it will amplifiy. Pushing it harder than that just sounds bad and causes overheating. The 1970s solid-state two-transistor amplifiers would have a "ceiling" of around 9 or 10 Watts PEP. Even back then, this was way too low to sound right with a typical mobile radio. The routine fix was to add resistors in the input circuit that brought the radio's peak power down to a level the amplifier could tolerate and actually amplify. Slicing off the voice peaks in your audio waveform just doesn't sound all that good.

Only tool I ever found for establishing this is a 'scope. Don't know any other way to find the amplifier's upper drive limit.

Where the resistors go, and the value to use is different for every amplifier.

And for the size radio in use.

If the amplifier has a "Low" selector that may do the trick all by itself. But if you spot a resistor attached to the High/Low switch that looks overheated or burned, you need a bigger resistor.

Or two.

73