Its nice to have a scope and a line sampler to monitor trace. with a Two-tone audio generator, adjust the bias current to the best RF envelope trace on the scope without exceeding the Current rating of the transistor. a current limited power supply is nice to have. an Am trapezoid envelope test is the best test for linear amplifier commissioning. Congratulations on your Homebrew linear amplifier. All the best from DU DV3SB

Great video as always mate, I love to see the thought process that goes into your home-brew projects. 73

Nick, try a broadband input matching transformer instead of LC inputs. They are too narrow-band even for this application. One little ferrite binocular will do away with the relay and most of the rest of those other components. You also need a stiffer bias supply. BJT's aren't as simple as FET's when it comes to biasing in class A/B, (or close to it.)

Yeah, where to start? You're not going to get anywhere expecting narrowband matching circuits to work across broad frequency ranges. The data sheet test circuit is 30MHz narrowband input and output matching. 21 MHz was never going to happen. You need to look into broadband matching. You're on the right track with the output binocular ferrite core, but look at the data sheet, the input and output impedances are like 2 or 3 ohms(plus reactive components) So that gives you the ball park of where you need to be with transformer ratios, 20-25 to 1. You are trying to bias the transistor as if its a FET or MOSFET, voltage bias, you need to adopt a current source type base bias. Essentially you just need a series variable resistor and not a potentiometer. Often there would be a power diode to ground and the forward conduction voltage fed to bias the transistor, the diode would be mounted on the output device to give thermally tracking bias. 73 de G0AFV

Nice to see that you use the same CNC machine as me for PCB's, very good video.

I remember my teenage self cloning a CB "afterburner" with this exact transistor. No clue about the schematic just 1:1 Copying the layout, coils, component values. 😆

I need friends that gift me power transistors 😂

If you monitor the 1000uF on the base bias network, with an oscilloscope, you will probably see the voltage collapsing under drive conditions (or even going negative). As others have mentioned a low impedance source is optimum, as the transistor sinks considerable DC current under RF drive. The 2 diodes and an emitter follower solution is probably do-able. Couple the lower diode to the vicinity of the transistor. A 2 tone test would help in seeing any issues. Keep up the great work.

Learned something new with you. Tank you for taking the time to do these videos, liked and subscribed

Interesting as always and good see a video where the result is not perfect first time and is more real world. You have generous friends, I would need to find a cheaper transistor. Thanks for taking the time to make this video.

Carefully constructed transmission line baluns are your friend on the input (and output of course). You can get 50 ohm coax that is less than a mm overall diameter, this is the stuff that I use to get the required wide band match to the low input impedance. Any resonant circuits in the input path will basically make it a two band amp. Also, and I am sure you are aware of this, a large transistor like that will achieve much greater linearity if biased using a constant current method (a bias drive transistor) but the method you employ here is much safer for the beginner and I guess that`s why you are showing it. You should be able to get a reasonable return loss if you are only interested in 14 to 30 mhz using the good old brass tubes in ferrite sleeves method uncompensated. One turn to the transistor (the brass tube) and maybe seven or more turns of teflon insulated wire. The same method used when driving a push pull amp, but a few more turns on the primary. In my experience that transistor will do at least 50w until you run into some gain compression at around 55 to 60w.

Time to redesign your bias system. It would be good to give it a little of temperature compensation, even on 4x 1N4148 diodes. It should be sufficient and it will be more safe for MRF :)

That bias circuit needs to be *much* stiffer, but definitely look into the MRF433 from club sales, nice and cheap.

Been doing some similar experiments for 17m with CB Power Amps, and MRF433s; same form factor but only rated for 12.5W up to 30MHz. Some terrible feedback on some set ups, one really good one with a very chunky 5W (!) feedback resistor, from SPRAT 70. More work required, but 10W is looking good.

I real like all your vídeos

As someone else as mentioned the MRF101 is a cheap(ish) interesting device 50 odd volts opperating probably a bit awkward but you can get cheap PC supplies for that and you can always run it at a somewhat lower voltage

Good job!!!!

Nice! An interesting transistor to look into for future designs is the MRF101, which is a 100 Watt LDMOS for up to 250 MHz. Just something I thought of when I was the price of the MRF455.. While the MRF101 is a device designed for 50V, you can get quite a bit of power at 12V as well. Anyway, great video! I like the "raw" style of your videos. Unfortunately something YT (or viewers?) don't seem to favor anymore.

To me, it seemed like you were slightly mistuned on the SDR for 10m; even seemed not centred in the SDR passband. So, I think its ok on 10m. Anyway, thanks for all your hard work! 73...

I have about a dozen of those MRF455 devices. Not a bad transistor. Notice how the datasheet says 30mhz characteristics... thats not suggesting that is its maximum frequency, more like a typical frequency of use and what to expect. Motorola themselves used them in their maxar 80 low band vhf transceivers at around 70mhz to give 50W output I think. They easily are capable of 70W at that frequency in a tuned class C configuration and give around 10dB gain there. There's a number of suggestions I could give you to make your power amplifier here much better... (if you're interested) a few watts should be more than enough to get that transistor hooting at 50W at 30mhz. My suggestions are surrounding the bias and input/output matching

Great video Nick! 73, Mike, EI6AU

Hi Nick. I decided to take a look at the MRF455 info on the web, and it quickly became apparent (to me at least,) that the device was designed specifically for American CB external amplifiers, and only them. They have max input specified at about 4 watts, coincidentally the FCC's AM CB output spec. Of course, they are also spec'd only for car battery voltages, and there is almost no info regarding them at anything other than 30 MHz, just above the US Citizen Band. I don't recall any other RF power transistors that are specified at only a single frequency, and show a test circuit at only that frequency, and at 1 voltage, and only 1 voltage. Other RF transistors will show Smith Chart impedances for a range of frequencies, as well as graphs of output vs frequency. These do not. Very suspicious. These also have an unusually good gain spec for a BJT at those conditions, and at those conditions a pair of them in push-pull will put out just a bit over 100 watts, a nice round number for amplifier advertisements. What that all boils down to is that they are a single band transistor designed and optimized for 11 meters, (but conveniently for hams, also on 10 meters,) with their input impedances apparently varying widely at other freqs, probably a tradeoff for the gain at 12 volts. That would explain the difficulty of getting an input match that would work at both 21 and 28 MHZ. The transistor was never intended to cover multiple bands, just 27 Mhz. I note too, that nowhere in any of the manufacturer's specs is the 27 MHz band ever mentioned, nor is a bias circuit shown in the test circuit schematic, even though the device is clearly perfect for a CB amp. They wanted to be able to deny everything to the FCC when the transistors started showing up by the thousands in illegal CB amplifiers, which they did. (I have 3 different push-pull amps with blown MRF455's that I hope to repair and tune up on 10 and 20 meters.) But what other market could the devices have been designed for? They certainly didn't design a 27/30 MHz-only transistor for ham 10 meter transceivers. Nope, it's a CB specific transistor developed in the hay-day of the US CB craze. I'd say you did well to even get a workable match with a single transistor configuration at 21 MHz. Your input circuit design is not an indication of poor engineering, but a testament to your excellent impedance matching skills in getting the device to work in a single device configuration on a band the designers never intended it to work on.

Excellent result Welldone Did you consider an input transformer as impedance matching arrangement?

Nice! 73 de VK2AOE

Hello Nick, I sent you a message using a form on your Nick the Vic page. I was looking for an email address so I can send you some diagrams. Hope this was your page, otherwise I have bored some poor man to death with talk about RF matching. Chris.