Modified CB amplifiers for 4m

[Brian Howie]

I'm looking at how feasible it is to modify a CB amplifier for 4m .

The valve ones can be done quite easily.

The one I'm looking at is the RM KL203-P which uses Mosfets . ~ £45
200W PEP .

http://www.rmitaly.com/index.php/en/products/amplifier/item/download/46_7
5821a2090dee0be64b2631102dcc8ab

The design looks broad band, so with a few tweaks it might work. I
don't have info on the Mosfets

Has anyone modified these or similar for 4m or 6m ?

Brian GM4DIJ

--
Brian Howie

[Brian Reay]

In the absence of data on the MOSFETs, the obvious concern is their
frequency performance. Generally, you could perhaps take the view that,
even if they weren't intended / designed for use at 70MHz, if you can
'get' 200W pep from them at 27MHz, you can probably get something worth
the hassle at 70MHz. The fly in the oinment is that these 200W pep may
be 'CB' 200W, ie it may well not perform nearly as well as claimed-
especially when it comes to not only power and linearity.

Have you tried it into a dummy load on 28MHz and checked not only the
power but for linearity?

In the past I've a converted CB set to 70MHz (an LCL which had a PLL and
mixer ripe for such conversions) but, while it worked, it was more of an
experiment than a serious project. Obviously the RF stages needed a fair
bit of work and then there were the crystals. I had a selection of
crystals and found some 'near enough' to prove the idea but it would be
cheaper to buy a PMR radio than buy the 3 crystals needed.

[lordgnome]

On 18/11/2016 14:46, Brian Howie wrote:

Don't know this kit, but the only CB amps I have had the misfortune to
deal with have produced such vile harmonics as to be unusable without a
following filter.

Les.

[Brian Howie]

In message <o0ukvg$5ae$1...@dont-email.me>, lordgnome <l...@nospam.null>
writes

Wise words.

There's no filtering as such. The circuit is sort of tuned. I make the
2nd harmonic about 18dB down ( simulation).

The amp I use at the moment is an A200 brick , which has a a 3 section
LPF.

Brian
--
Brian Howie

[Brian Howie]

In message <o0tf5o$4bk$1...@dont-email.me>, Brian Reay <no...@m.com> writes

Since posting, I discovered the devices are thought to be HUF75345 from
what I can find.

Http://www.mouser.com/ds/2/149/HUF75345S3S-1010615.pdf


The input gate C is an eye-watering 3nF and the rise time doesn't look
great even for 27MHz, so I'm suspicious.

Some people have put in IRF540 as a replacement.

I've not got one yet. At £45 it might be worth the gamble, but I was
trying to find out if anyone had successfully modified them. I've drawn
a blank, but will try the 4m forum. The Bremi valve amplifier can
easily be put on 4m.

I've a Spice simulation that sort of works on 29MHz. The FFT plot looks
a bit whiskery, and it doesn't look very linear. I've had to make a
guess at the inductors. If I can get it to work on Spice at 70MHz I'll
buy one to try.

I'm only looking for 160W . The higher power versions of the CB amps
seem to use more of the devices in parallel.

Brian
--
Brian Howie

[Brian Reay]

If the existing device is 'flakey' at 27MHz, as your investigations
suggest, I would think 160W at 70MHz is wishful thinking. If you have
to replace the PA devices as well as the inductors etc, you aren't
really saving much (as you will almost certainly need to change other
components to suit the new PA devices to get the bias etc right).
Chances are, you will reuse the box/heatsink and the PCB, which may not
be up to the level of rework required. Does the relay look good enough
for 70MHz? (assuming it uses a relay, 1/4 wave lines aren't really
practical at 27MHz).

It looks more like a project for a box you have lying around or have
picked up at a junk sale, rather than buying especially for £45. The CB
set I converted was one I had lying around, I think it came in a box of
PMR stuff bought at a junk sale and the PA coils had been 'got at' and
were knackered away.

[mm0fmf]

On 18/11/2016 14:46, Brian Howie wrote:

I acquired one brand new for silly money (£5 ISTR). Yes, there are no
filters and the gain drops rapidly with frequency. With 0.5W of carrier
on 10MHz it gives 20W out. With same drive on 14MHz that is about 17W.
By 18MHz that was down to 12W. I tried 5W of drive and it would give 45W
on 24MHz and 40W on 28MHz. I never tried any higher bands. I'm not using
it on air without a filter of some kind.

[RustyHinge]

Much of the later (FM) CB add-ons are pretty poor all-round. Some of the
earlier AM 27 MHz stuff is a lot better.

--
Rusty Hinge
To err is human. To really foul things up requires a computer and the BOFH.

[Brian Reay]

For AM (at least in theory) they were designed to be linear in
operation. For FM, they could get away with being class C. The latter,
as you may know, has the advantage of being more efficient but are, of
course, not linear.

[Brian Howie]

In message <o30vdd$ifq$1...@dont-email.me>, RustyHinge
<rusty...@foobar.girolle.co.uk> writes

I put the same request out on the 4m group. Opinion was it was a
no-hoper. Basically the voltage is too low for good linearity, and the
output power was a bit optimistic even for 27MHz

As a sanity check I modelled the G4IZH amplifier and it did 150W OK with
about 1 W in. The output is clean on a two-tone test.

QRO 4m Amplifier http://www.g4izh.co.uk/qro-4m-amplifier.html

I may build one of those.

Brian


--
Brian Howie