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Author Topic: SPOKEN 250 HF Linear Amp. (solid state)  (Read 24616 times)

Posts: 5

« on: September 16, 2009, 09:48:56 AM »

Hello out there, This de EA5HIY (chanced callsign)
I am looking for tech info for this Ampl.
is there anyone out there how can help ?
pls my email is
73,s de EA5HIY (ex PH2JM, PJ2JM)

Posts: 1012

« Reply #1 on: September 18, 2009, 01:15:59 PM »

Hmm,, I will bite, but i will bet it is a CB amp...

Posts: 5

« Reply #2 on: September 18, 2009, 02:39:58 PM »

Hello Erik, thanks fr yr reaction. The amp says HF and trying it on HF it works on 80 aswel on 10 and every freq inbetween BUT I am not shure its cleen and because I have no info on it nor any schematic its dificult to tell.
its broadband in the finals 2x mfr545 or something alike and with docs i can make filters and/or do mods on it.
it does keydown 125 watts so PEP ?
if you have info ?? pls mail me directly
Tx de EA5HIY Johan

Posts: 3

« Reply #3 on: December 25, 2012, 02:53:41 AM »

Hi Johan, hi all

would like to give some news on the forum about my Spoken 250, you remember we had some talks about it.
Finaly I decided to make tests they reveal that t's a real broadband PA, not only for CB band, so I'm quite happy.
It's used here to follow my homemade 5 W PA board on a FT-767DX where its original 100W PA module were removed by the previous owner.
Back to the Spoken, it can deliver up to 220 W CW with about 10 W input. Didn't test with a higher input power but I guess it can deliever more than this is SSB.
I don't know the passband of the RX amplifier, well actually I don't mind. Didn't test on the 160 m band either.

The ROS is always below 1.8 and most of the time around 1:1. In fact, the more you cross the attenuators, the more the adaptation is good, sounds logical yes.
Mine was giving a high ROS because the resistors of the attenuators were burned probably due to a too high amount of input power.
While repairing I have l noted on a sheet of paper some parts of the schematics, so to identify the values of components in case of repair needs.
I don't know wether I can post the schematic here or not, in any case I would be pleased to send to anybody who might need it.
It gives the values of the 5W/10 W rear attenuator, the values of the Res and Cap of the front LO/M1/M2/HI switch, the Cap of the input stage of the MRF454 pair, and the output LP filter. I have several pictures as well, showing the PCB.

As a conclusion, if you find such PA (and need it) don't hesitate !

Wishing you a merry Christmas time,
best 73, GL de F5IHN David


Posts: 3160

« Reply #4 on: December 25, 2012, 04:37:48 AM »

The MRF454 is a common RF Transistor in the 100 Watt class HF Amplifier.
2009 discussion on QRZ Q&A about HF linear amplifiers based on the MRF 455 transistor.

The HF amplifier transistor design requires switchable (for each band) output filters,
 such as 5-pole or 7-pole Chebyshev filters.
Omission of the filtering is typical with poor, cheap CB designs -- that are RFI generators.

Motorola Application Notes (circa 1970-1978) by Helge Granberg, K7ES(sk) and Tom Bishop.

Communication Concepts sells various PC boards and parts -- from Motorola Application Notes.
G3TSO DIY built an HF amplifier based on Motorola Application Notes and CCI boards

Simplified QRO Ampliier Designs - Chapter 12

ARRL Handbook article 1981-1983 detailed construction of MRF454 HF linear amplifier by helge Granberg, K7ES (sk).
FoxDelta sold a project kit, based on this schematics and design.

YO5OFH design and schematics for 100 watt HF amplifier with Output filters.
« Last Edit: December 25, 2012, 04:55:58 AM by W9GB » Logged

Posts: 6765

« Reply #5 on: December 25, 2012, 08:01:15 AM »

2JM:  It would be interesting to know about the PA output circuit(s) 

How is it bandswitched?  Does it have several output circuits that are switched in or out or does it have just ONE??

A Pessimist is Never Disappointed!

Posts: 3

« Reply #6 on: December 25, 2012, 11:56:53 AM »


as thruly mentionned by W9GB, it's a wide band PA for CB and unfortunately does not have several band filters, only one low pass output filter probably around
30 MHz. Its a Butterworth type, 5 components so would be quite efficient.
However when properly managed regarding its input power, I believe it's a good value, regarding its low price. There's a pair of MRF454's, it is well bulit inside
and you can control easily the output power thanks to several attenuator cells, making at the same time a good impedance matching at the input.
It's like other real amateur radio amplifier actually, you have to know how to use otherwise it means spurious and
poor emission quality. Even a ham PA can be awfull on the air, frequently heard on the air ...
In my case it's perfect to replace the original PA module of my FT-767DX (same as FT-707, well known).


Posts: 2358

« Reply #7 on: December 25, 2012, 12:11:16 PM »

As thoroughly mentioned by W9GB, it's a wide band PA for CB. Well so far that is about the only good part from what i have read. 1 filter @ 30 MHz yea that awesome why the need to make things complex and add more filter's when heck we don't need them  Cheesy

Posts: 168

« Reply #8 on: December 25, 2012, 12:21:58 PM »

You DO need low pass filters to meet FCC requirements on harmonic radiation. If the amp doesn't have it, you need them externally. One problem you can meet is that amp may not be stable if not loaded with a load of 50 + j0 at frequencies  out of the operating frequency band - which can happen with filters or an antenna tuner.

Posts: 3

« Reply #9 on: December 25, 2012, 12:58:33 PM »

of course the whole emission chain doesn't stop at the output of the PA.
After the PA you need a tuning system to match properly the antenna. In my case it's a center feed antenna with a nice real symetric tuner (Johnson Viking, well known). That's my ouput filter.
Probably more efficient than a nice PA from a nice brand name with its internal matching circuit, used with uncontroled input power, a "miracle" antenna matching, bad ALC, etc.
To be honnest I don't mind, I play CW with 50 W, don't like power ... and this rig is my second rig.


Posts: 5688

« Reply #10 on: December 25, 2012, 02:41:30 PM »

Let's see what we have so far on this thread: 

A pair of real Motorola four-fifty-fours.

A description that says the amplifier appears to be very well built.

Amateurs decrying use of said amp because of its association with CB. 

My take is that if the biasing is up to snuff, all that is really needed to make this thing work just fine in Amateur use would be some added lopass filtering and a bandswitching arrangement to switch the right ones in between amp and antenna.  Use of a good quality narrow tuner would likely go a long way towards snubbing harmonics as-is, but I myself would add the filtering.  There are filter kits or plain boards available from places like CCI if their still in business, dunno.  I'd likely just roll my own pi networks for such a beast. 

And I'd certainly take a long hard look at the biasing scheme, not too hard to change that if its iffy looking. 

But other than those two issues, a pair of MOT 454's or equivalent and a clean looking build would also describe many a PA board found in many an Amateur transceiver. 

And, as already mentioned by others in this thread, many an Amateur transceiver or linear amp has been heard splattering all over the place despite output filtering.  Quite a few seem to be able to do it using Hollow State...


Posts: 168

« Reply #11 on: December 26, 2012, 03:32:09 AM »

>In my case it's a center feed antenna with a nice real symetric tuner (Johnson Viking, well known). That's my ouput filter.<

Does that give you harmonics 50dB down as required by the Radio Regulations and ERC/REC74-01?
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