seen some big rigs on flebay all from same seller looks like a station selling up may be as thers 3 rigs and a cdj lol thort id share the moment

Link?

Here Al - https://www.ebay.co.uk/usr/benjhall91?_ ... 7675.l2559

I did speak to the guy about them - they seem OK.

Ugh! Is that what people are putting on the air these days? No wonder they get taken off by OFCOM. There's no way that these can be clean enough.

Here's a hint: Modern FETs have as much gain as the valves that I was brought up on. With valves at VHF, it's essential to screen the input networks from the output networks to prevent coupling and spurious oscillation. A FET amplifier needs every stage screened individually, and each section of the amplifier screened off. If properly screened, "neutralised" and with fast fuses, it's possible to make PAs almost unconditionally stable and indestructible. It's also essential to have the (properly constructed) output filter screened from everything else. Just take a look at a proper, professionally designed and built PA!

I remember my first experiences with modern FET PAs - the first devices I used much were the MRF150 and the SRF1616. The gain from these devices - if properly biased and matched - was as much as two high gain bipolar stages. My early amplifiers took 3 or 4 Watts in for 150 - 190 Watts out in a single stage. Later versions, using either pairs of FETs or dual FETs were capable of 300 Watts from just 800mW of drive. This really is valve territory!

My valve PAs used to have the bottles through the chassis (on the cheap ones), with the input networks beneath the chassis and the output networks above. It made tuning them up a bit tricky, but the screening meant that they were stable and clean. More expensive rigs had each bottle housed in a screening box with air blown into it, with the input networks inside the boxes and the output networks above the valves. In the latter days of vales, I experimented with broadband networks to make "no tune" PAs, and found that with the power tetrodes (like the QQE06-40 for example) it was possible to make a 20dB amplifier that needed no adjustments! The thing that killed off valves (for me) was the expense of the transformers and high voltage capacitors. I built my last pirate valve rig in the early 90s (for 6.4MHz AM) and still miss working on that bizarre technology!

They look like old designs from about a decade ago. I’d at least stick some filtering on the output.


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Albert no one really cares about military spec transmitters when it's only going to last around a week to (if your lucky) a year.

It's as clean as it can be at the low prices.

Lmao Military Spec is still good. Especially when it demonstrates that we’re making an effort and emitting a better output when compared with the RSLs and their sproggy BW gear.

An old fact, the BBC engineers used to whack their equipment with a big rubber mallet hammer to see if it could mess up. You could tell if they were having a bad day with wifey, as they’d beat the heck out of the rigs. Apparently they survived.


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I remember tuning a UHF wavemeter with a mallet!

A five quid low-pass filter might make your rig last longer though cos it'd be a little harder to find

Sadly, at a couple of hundred Watts, the output filter costs much more than a fiver - the capacitors can cost a fiver each! We actually make our own output capacitors in some cases, since the "Arco" brand ones (the only really good ones) are often unavailable in the values we want. We use mica insulation spacers as the dielectric, and sheet copper for the electrodes. They're usually dipped in Conformol to prevent corrosion, and they're marked with painted coloured stripes to identify their values. We make values from 10pF to 175pF, usually in batches of about 100 of each value.

Some of the comments on this thread are really quite amusing, and the continued arrogance of others just amazes me.

Now, having seen these type of rigs on the bench more that a few times I can tell you that the filtering and stability are absolutely fine. The harmonic content of the long black 300W rig and the smaller black 150W rig will be less than -70dBc, which, last time I checked was good enough for legal spec. The other silver rig (a Dutch/ London hybrid) will be slightly worse in this respect but nothing that would cause any issue (prob. about -65dBc).

You have to remember that these types of rigs are the mainstay of Pirate radio in London which is, until recently on of the most policed areas in the world for unlicensed radio. Do you really think that rigs like these would be allowed to stay on air if they caused problems? Would the builder still be around producing these designs? Have a look on the PCB of the long black rig, it's been around since 2009!

Why would anyone accuse BW of producing unstable equipment? What a ridiculous comment!

And Albert, I think you need to be careful with those double standards. Was it not only last week where you told us your high powered rig had been returned to you because it was causing interference, and upon inspection the switching power supply artefacts were evident at -48dBc?
How or why did it ever leave the bench in that condition? Those humble London rig efforts would perform a lot better than that, I can tell you!

....and as for hand making 100s of metal-clad mica capacitors, what a waste of time! You know you can buy them, right??

wow proper deep comments bros!i only shared the thought and moment as 1.some one might want to buy some rigs and 2.another station gone or may be revamping ...ether way its a station or was a station : )and in my opinion i'm no expert .if your gona get nabd ya gonna get nabd whether it be by a rig thief or ofcom .but suppose equipment matters as well as to how easy you get nab

Analyser wrote: ↑Mon Sep 25, 2017 4:06 pm
And Albert, I think you need to be careful with those double standards. Was it not only last week where you told us your high powered rig had been returned to you because it was causing interference, and upon inspection the switching power supply artefacts were evident at -48dBc?

It wasn't one of ours that caused interference - it was an Italian thing that was supplied by a bloke in Cyprus (you might know who I mean). They'd done a lot of filtering on the mains side to prevent conducted crud, but the SMPSU was a heap of junk!

Analyser wrote: ↑Mon Sep 25, 2017 4:06 pm ....and as for hand making 100s of metal-clad mica capacitors, what a waste of time! You know you can buy them, right??

You can buy them, but they're generally poor tolerance (even the expensive Arco ones), and they seldom work well close to their rated working Voltage. The "home-made" ones don't flash over at the slightest provocation.

I now need to find a reliable way to make high power trimmers for those older "optimised" bipolar PAs. I have two old PAs with TP9380 devices in them, that have both failed in the same way - a minor mismatch has caused the output trimmer to flash over, and has burnt a track on the board. The transistors are intact because the "FF" fuses on the boards failed quickly enough to save them. Mending the tracks won't be a problem, but finding 100pF high voltage trimmers at sensible prices is proving to be a PITA.

I've got one of our PAs on the bench tomorrow - I'll take a couple of photos. The only significant difference between ours and the ones that the Dutch lads sell is that we have more screening between sections and stages, and they're in purpose-made boxes with big heatsinks forming the lid. We use "SMA", "BNC" and "N" connectors for the RF side of things. There are (usually) 20-way logic connectors for the monitoring and control functions in the module.

I have one prototype board with stepped power output, in an effort to reduce the range of numbers of FETs we need somewhat. It's an attempt at a cheaper design, but I have no real hope of it working out.

Albert H wrote: ↑Mon Sep 25, 2017 10:58 pm
I now need to find a reliable way to make high power trimmers for those older "optimised" bipolar PAs. I have two old PAs with TP9380 devices in them, that have both failed in the same way - a minor mismatch has caused the output trimmer to flash over, and has burnt a track on the board. The transistors are intact because the "FF" fuses on the boards failed quickly enough to save them. Mending the tracks won't be a problem, but finding 100pF high voltage trimmers at sensible prices is proving to be a PITA.

My solution to this would be to parallel a number of fixed capacitors in place of your trimmer. Good trimmers are expensive and hard to find and always fail over time because of dust ingress/ moisture/ they can't really handle the current.
Either Unelco or ATC porcelain capacitors are a good choice.

Or put an inductor in series with the capacitor in the shunt element and tune the inductor instead of the capacitor.

Anyway, it shouldn't be too difficult to use 1206 (or larger) capacitors. Buy a selection of values with low voltage ratings and change them to suit your performance needs, then buy 500 V ones at 10p each from RS once you've established the values you need and fit them. That should be good for a kiloWatt.

Thought they were Dutch/London hybrid rigs. Tbf I heard a lot of good things about them in the past. Just thought it’s good practice to fit a lpf in this day and age.

The local station using a bw rig can be heard on both 107 & 105 fm. Not my job to sort it.




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Analyser wrote: ↑Mon Sep 25, 2017 11:13 pm Good trimmers are expensive and hard to find and always fail over time because of dust ingress/ moisture/ they can't really handle the current. Either Unelco or ATC porcelain capacitors are a good choice.

I've used ATC for years. I find that they're OK long term if they're in a clean environment. Our PA boxes were always "closed" on the component side, for exactly that reason. Our long term failure rate was always low.

You're right - fixed capacitors can be the way to go, but there's often little saving when it comes to good quality parts. The convenience of being able to tune a PA to a new frequency in seconds also often outweighed the advantages of fixed values. It was possible to stock a boxful of identical PAs, and tune them for the customer in a few minutes. In the days when I had people from five or six stations turning up after a batch of raids, it was good to have gear ready to go in minutes!

thewisepranker wrote: ↑Tue Sep 26, 2017 9:27 am Or put an inductor in series with the capacitor in the shunt element and tune the inductor instead of the capacitor.

Anyway, it shouldn't be too difficult to use 1206 (or larger) capacitors. Buy a selection of values with low voltage ratings and change them to suit your performance needs, then buy 500 V ones at 10p each from RS once you've established the values you need and fit them. That should be good for a kiloWatt.

That's close to the approach I use for some PAs. I've been surprised by the behaviour of some of the newer FETs - the "cookbook" values for a PA seldom work terribly well! I generally hate working on PAs and output filters. I much prefer the cleverer end of the rig - link receivers and exciters!

Filters never come out right. You take a bit off of the inductance to allow for the inductance of the tracks, then you have to add a bit because your turns calculator makes lots of ideal assumptions. So you open the coils up a bit because you've overcooked how much to add and now you've changed the Q.

Then there's the capacitance of the 1206 (or whatever) pads on the PCB and the capacitance to the bottom layer...