Are all CB Amps built the same?

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ghost123uk
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Re: Are all CB Amps built the same?

Postby ghost123uk » 11 Jan 2017, 18:19

Otter wrote:I've had two Moonraker L100s and they have an input SWR of over 3. RM amplifiers seem to have a proper match.

I was reading about this recently (over on here = http://www.4x4cb.com/public/item.cfm?itemID=759 ) where, with regard to the RM Model KL200, they say,
IMPORTANT : You will need a 90cm connecting lead for the CB-to-amplifier connection - it is important that a 90cm one is used rather than any other length to ensure the SWR that the CB experiences while driving the amps' input is low.

Now I've been "doing" CB since the A.M. days, and for ~16 years ran an electronics shop where I did a fair bit with CBs. I do know about a coax feeders length being tied in, on some types of installation, with multiples of wavelengths, but what's this 90cm (3 foot!!) patch lead thing? Is it summat I don't know about? (genuine question by the way).
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
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Re: Are all CB Amps built the same?

Postby kilimax » 11 Jan 2017, 18:58

It's most likely to ensure a good match from the amplifiers output network. Length of this lead (the stub) will vary the impedance seen at the output. Amps that have some form of suppression may need a specific length to ensure optimum performance.

The output or matching network can be different. Some amps have a pre-tuned or matched output, while others do not. I once read that some older Collins valve amps requited almost 20 feet ( that's 20) of coax cable as the coax stub!!
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Re: Are all CB Amps built the same?

Postby Phoenix » 11 Jan 2017, 19:12

Cheers well i would like an little bit of added assistance now the cycle is low but would like to do it correctly...hence the questions
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Re: Are all CB Amps built the same?

Postby ghost123uk » 11 Jan 2017, 19:15

kilimax wrote:It's most likely to ensure a good match from the amplifiers output network. Length of this lead (the stub) will vary the impedance seen at the output.

Hi killmax. If you have time, can you clue me ("us") up on this relationship of the patch lead length between radio to the input of the amp and the effect this has on output matching of the amp to the feeder and antenna? I thought the output network of an amp would be independent of the input, but I certainly stand by to be corrected ;) That blurb on the 4x4 website indicates the specific length of patch lead, at 90cms, is so the radio "sees" a correct impedance (and hence swr) at the amps input. It srikes me that a well designed amp would have a nice 50Ohm input without resorting to "matching" cables. I'm confused.



Oh, and it's not really thread creep, because it's about how well some amps are designed, or not ;)
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Re: Are all CB Amps built the same?

Postby kilimax » 11 Jan 2017, 19:52

ghost123uk wrote:It srikes me that a well designed amp would have a nice 50Ohm input without resorting to "matching" cables. I'm confused.


You've nailed it. A "well designed amp" :thumbup:

However, I believe you are referring to the input circuit of the amplifier, not the output matching network I was referring to. Your amp is designed to see a 50 ohm input impedance (actually it's not 50 but good enough for practical purposes) & your radio requires to see similar. Any varyiance from this will alter the match. By varying the length of coax, you alter this value. You could I guess alter this by adjustable LC network just like the ATU principal but most radios and amps have an acceptable range. Some are a bit fussy and altering the coax length will change the input impedance seen by the amp.
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Re: Are all CB Amps built the same?

Postby ghost123uk » 11 Jan 2017, 19:56

When mine arrives I will have a play, looking at the SWR between the radio and the L200, using different length patch leads. Also to see if it does affect the power out of the amp. Summat to do :geek:
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Re: Are all CB Amps built the same?

Postby kilimax » 11 Jan 2017, 20:00

Phoenix wrote:Cheers well i would like an little bit of added assistance now the cycle is low but would like to do it correctly...hence the questions


An RM KL203 will privide about 80-90 watts out. I think these are really popular but they are cheap and cheerful and while I've read that some folk can drive them with more than 12 watts, I wouldn't and I wouldn't want to use these for long periods.

Perhaps a KL405 or its replacement, keep the drive low and use the adjustable attenuator. Not sure if they have a LP filter but I'd fit an external one anyway.
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Re: Are all CB Amps built the same?

Postby kilimax » 11 Jan 2017, 20:02

ghost123uk wrote:When mine arrives I will have a play, looking at the SWR between the radio and the L200, using different length patch leads. Also to see if it does affect the power out of the amp. Summat to do :geek:


Would be interested in your findings!
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Re: Are all CB Amps built the same?

Postby ghost123uk » 11 Jan 2017, 20:06

^^^ = Watch this space Image



.
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Are all CB Amps built the same?

Postby ghost123uk » 11 Jan 2017, 20:10

kilimax wrote:You could I guess alter this by adjustable LC network just like the ATU principal

I've got an ancient little Bremi 35W (if your very lucky) jobby here that has an internal trimmer cap on it's input side. I used to just adjust that for max output, but I'm guessing that is an input tuning device.
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Are all CB Amps built the same?

Postby kilimax » 11 Jan 2017, 20:17

ghost123uk wrote:
kilimax wrote:You could I guess alter this by adjustable LC network just like the ATU principal

I've got an ancient little Bremi 35W (if your lucky) jobby here that has an internal trimmer cap on it's input side. I used to just adjust that for max output, but I'm guessing that is an input tuning device.


I remember those. All the rage back in the early 80's with the UK FM stations competing to hear each other over the S9 hetrodyne noise from everyone else talking. Blimely and to think that back then, you often couldn't find a free channel! :lol:
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Re: Are all CB Amps built the same?

Postby ghost123uk » 11 Jan 2017, 20:30

That ^^^ is one reason why I'm looking to replace my trusty Cybernet Beta 1000 with a modern set that has a "scan" facility. That way I won't be wearing out the channel change by constantly flicking around trying to find a channel that actually has someone on it !!
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
Don't just monitor, key up and talk, otherwise everyone thinks no one else is out there !!!

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Re: Are all CB Amps built the same?

Postby Marky-boy » 11 Jan 2017, 22:36

some great suggestions above, i've little experience with CB style amps, but one with a hard-switched PTT would be nice, i've heard some users on air where the amplifiers RF sensing is audibly clicking in and out, i heard some uber-cheap CB amps doing this and the resulting (transmitted) RF spikes clicking hundreds of Kc through the band.. some turned up their mic gain in an attempt to stop this, and their SSB 'dead key' was nearly as strong as when they were talking...

but the thing that worried me (apart from the obvious) about many CB amps is their heatsinks are way too small.. it may be different nowadays, i've seen some with fans fitted as standard... i was fixing an old Zetagi transistor amp years ago, orange switches, pretty gold heatsink, but the heatsink was tiny, grossly undersized for the amount of heat the 'pill' transistor inside made after even a very short length of time.

if i remember correctly, i think some model numbers were made to look like Watts output - where the model number had a W suffix, so something named like BL100W (just an example) looked like a 100watt amp, despite the device inside being only capable of maybe 70w at a push. i would expect this kind of thing is still happening too... plenty of fake RF power transitors about too.

Just my 2€ ..

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Re: Are all CB Amps built the same?

Postby ghost123uk » 12 Jan 2017, 00:10

Marky-boy wrote:one with a hard-switched PTT would be nice,

+ one on that !!! Image

I would not personally trust SS switching if using SSB.
John, on the S. Cheshire / N. Shropshire border. Muppet handle "Grey ghost" and on UK C25 most working days.
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Re: Are all CB Amps built the same?

Postby dt307 » 12 Jan 2017, 01:06

found the kl203 dont like magmount at all it will feed back and hold the tx open
i did try the right length patch lead and longer and shorter still the same
the b300p will tx into anything
but with one cap the kl203 will be happy on 80m and up (but its output will prob be so dirty) ok for a little qrp rig when you need a little boost.
the kl203 nice little amp sound clean if you keep it at around 100w,run it flat out and gets gritty
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