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5kW Mobile tube or solid-state?

Unit 148

Unit 148 Mobile CYPRUS
Sep 29, 2010
London, UK
I currently run an old 8 x Toshiba 2879 mobile amplifier which will be shortly going to a new home.

I would like to step up to the next level in mobile power. Mobile power supply is not a problem as can add alternators and or batteries as required.

I operate mainly on sideband so will use an AB amp.

Would you look at a solid-state or tube amplifier? Either way I would like a high gain unit so I do not have to have boxes driving boxes.

I have up to 100W pep drive and would like a clean 5kW+ pep output

Unit 148

Oh, fun. That’s gonna get expensive!

I’m curious to see the recommendations and what you ultimately decide on.

I’d imagine that finding a quality AB solid state amp could be difficult, and that most producing 5kw plus are designed for higher drive levels.

Never did mobile toobs.
For me i would go solid state, easier to set up the dc than ac for tube boxes. Gotta get a custom made box obviously. Lower drive levels will be more achievable with a solid state amp as well
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Thanks for your thoughts. Regarding drive. Yes, that is an issue. I do wonder how much drive one would need in a non-competition amp for a tube like the 3CX3000 which I believe is fairly popular at this power level? My only concern with a solid-state amplifier that can run this power reliably from a stiff 14V rail is just how many devices one would need. Of course 50 or higher voltage LDMOS would be the way but nobody as far as I know is producing high voltage solid-state mobile amplifiers using these devices. Sorry Blackcat, I just read your post. I think you;re right.
4 x MRFX1K80H @ 65V :)
Bare boards available. Passives are less than $50 including the transformer cores per board.
I'm sure you can source a 4 into one combiner.


Stack up the $25 MRF300 devices until you get where you need to be. Standard construction. Every pair gets you 600w .

going the 12v transistor route would be crazy, they don't make any worth a wank nowadays,
big almost indestructible fets look VERY nice for mobile & not difficult to feed with 48volt + alts,
a big tetrode would be nice but who builds a decent 5kw tetrode mobile tube amp ?.

if i had a shed full of pre red dot 2879's id sell them & buy something with much more gain that i can't break as easy.
The difference between an 8 pill and a 32 pill is only one S-unit. It's really not worth the effort for daily talking unless you're into side by side competition. If you plan on going that route, you'll need to hang onto that 8 pill as a driver anyhow.

Anything like a 3CX3000A7 is going to require just as much drive since the gain is similar and then you have to worry about where your RPM's are when you key and unkey. Forget seeing any power at an idle too. The number of large tetrode mobile amplifiers I've seen successfully built are one, by Wolf many years ago. That's about as rare as a hens tooth.

There is the LDMOS option but at that power level, it's going to take at least 4 transistors but has the advantage of low drive. Realistically, these boards are hand grenades waiting to go off. The transistors are far from indestructible and pop like firecrackers when the drive is just a little too high.

The only way to do LDMOS reliably is to install full electronic protection. Something I have not seen a builder for this band, ever do or sell successfully. On the other hand, there are control and protection boards for these 50 volt devices being sold. It's just a matter of time before someone gets one working correctly in this high level RF environment.
OK on those 65V LDMOS. You say there are 4 device bare boards available - with a schematic or and parts list? I have only seen single and dual 65V LDMOS builds out there from RF Man: https://www.youtube.com/channel/UC7r5QeuynhXudskQDl7J45w/videos and https://www.youtube.com/channel/UCJ943xhv66Sy29PMrrAjCLA but no 4 x 65V LDMOS unless you guys know otherwise? Yes, several sellers with LDMOS protection boards. I have found a rather nice AC unit and would love to think the internal PSU would make room for a DC to AC equivalent. Remember this is primarily for sideband so figures in/out are pep: 90W in for 5.4kW using a single Svetland GU78b.All bands, soft-start , well regulated grid and screen supplies, plus current limiting to protect the tube, two-speed blower etc.
You can double-click on the images for super hi-res.


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That's a conservative 1500w @ 65v
The combiner is child's play.
A 6KW low pass filter? Not so much...


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