Radio With Amp Questions

[thetez]

Hey,

Im not sure if i want to do it, and please dont get all "Its illegal, blah blah" on me.. i know its illegal.

So ive got my CB Radio... I am thinking about putting a Pre Amp into my cb system.. i know thats not illegal... From what i hear the guy who i can get it from says he talks on average 15+ with it... but some times i get that anyway stock....maybe his radio sucked, or mines better, or the amp didnt actually work. Anyway. Then i was thinking about putting an "Dun dun Duhhh!" Amplifier on it, a smaller amp, maybe 40w to match the "15+" that i should be able to hear from the pre amp. Now i have no use for the amplifier or the Pre amp 90% of the tim, but some times im just far away, and no ones on ... and i get lonely and maybe flick it on then... i wouldn't wanna hurt other peoples communications. Or maybe set it up as a base station instead... Yes i have an antenna that can hook it up, and im in a 4 wheeler.

What do you think about this set up.. in terms of efficiency... and well... just other thoughts on it?

[Roadkill48]

What kind of radio are you using????

[thetez]

Why... does it matter? seriously, i dont know? Ive got a cobra 19, ive got a cobra 29, and ive got an old school midland from 87... "brand new win the box" and the same version just used, its a pretty sweet radio ive got that hooked up now..

[thetez]

Originally Posted by thetez

...From what i hear the guy who i can get it from says he talks on average 15+ with it... but some times i get that anyway stock....maybe his radio sucked, or mines better, or the amp didnt actually work....

OOH! thats why.... i think.... i like the radio, but currently am using a horrible magnet mount antenna, ive got a Wilson silverload fgt 4' sitting behind me, im waiting for the coax to get to me, then im using that instead

[kd5drx]

Ok lets go at this a different way. The amp in question is probably what is referred to as a low drive amp. meaning you only need 1 to 3 watts to drive it.
Now if you have a stock radio out of the box it will probably put out right at 4 watts so the radio needs to be what they call matched to the amp this takes a bit of doing since you have to put it on a GOOD watt meter and see what the amp really likes. for it to act right it should swing double what it dead keys IE key 20 swing 40 on a good meter. This is 100 modulation and none out side the band. Now the truth about what the amp will do for you. Every time you double your out put you gain 1 s unit so if you take a 4 watt radio you need 8 to gain 1 s unit now you need 16 to get another s unit now you need 32 to get the next so if you are getting 40 watts out of this amp then you will get 3 s units of gain that can make the difference between being heard or not at a distance. That is why us Hams don't run allot of power the first 500 is the most important after that its all wasted usually. The best thing a CB operator can do is work on there antenna system a good antenna system will get you more gain than an amplifier every time and it will help with the receive as well.

[thetez]

How easy is it to tune the amp? if i can tune an antenna can i tune an amp? and a CB shop might not do it right... since its not legal and all...? A Ham should be able to do it right? ... if i can find one

[kd5drx]

you don't actually tune the amp you tune the radio to the amp and any GOOD CB shop should be able to do it. It's not magic and most hams won't touch it because it is illegal and would cost them there license if caught.

[thetez]

What... do you have to open the radio? or is it the same as tuning the antenna.... And i might know a ham who would do it... if i can get in touch with him ... but im not sure if hes got the equipment... is that stuff hams should have?

[kd5drx]

You have to open the radio and use the bias adjustment for the final transistor and yes any ham should have a good watt meeter and a power supply. It is not difficult if he has the schematic for the radio in question. Or you can get the tune up points off the web for the specific radio.

[BobC]

The whole thing is not something you should try at home.
It takes a bit of bench hardware to get the adjustments correct.

A 40 watt amplifier is useless & isn't worth the effort of hooking up.
If you're not starting out with a minimum of 150-200 you're wasting your time & effort.

Stay away from the units called "modulators".
These are a bastardized version of an amplifier.
Their main purpose is to distort the waveform to make it appear louder locally.
In affect, it does make you a bit "louder" but at the expense of everyone around you on another channel.
You will be causing a fair amount of bleedover onto surrounding channels.

The idea of "matching" your radio to the amp is simply making sure that at any given power setting, your radio does not overdrive the amplifier & get distorted.

Another benefit of "matching" ensures that the amplifier presents a good swr to the radio when switched in.
You can have a good antenna swr but when the amp kicks in you're really transmitting into another transistor & a bunch or wires to a relay & back out.
All that extra lead length can create a poor swr for the radio.

The actual "tuning work" can take a while because you need to create the proper setup for testing & the radio needs to be set first.

Generally, you set the radio for 100% max modulation.
Once set, you continue injecting a tone or two into the radio mic input at 100% mod.
You then increase of decrease the radio's carrier power until the amplifier's output is on the verge of "flat topping"/compression.
Back it off just a little from there.

This ensures that even at the highest power setting of the amp, you will not overdrive the amp & create a bunch of splatter on other channels & the amp sounds "clean" on the channel you're using.

If you intend to use the amp on ssb as well, there's a bit more tuning to be done.
SSB requires the best tuning because if there's any distortion it will show up on SSB.

Switch your test radio to SSB, ptt & inject a tone or a pair of tones as are commonly used.

Increase or decrease the radio's "alc" adjustment so that the amp just begins to distort (flat top) & back off a little from there.

Now, no matter what setting the amp is on, you cannot overdrive it.

There's a last adjustment for ssb amps & that is the bias adjustment. This is usually not adjustable & requires a few trial & error parts changes in the amp.

The same bias(predominantly for ssb but effects AM as well) adjustments are variable inside the radio & they must be set correctly before performin the amplifer tuning.

A final checek to make sure the swr is good going into the amp & thru the amp when off.

It's tuned. However it's only tuned for the combination of that radio & that amp.
You might get away with using a different radio as long as the next one shows similar output power as the original.