The AM Forum

So there's the question. The 3CX2500F3 has filament and grid leads, and it has no grid flange. How are/were these mounted? By the anode? With those leads, a socket can not be used. I'd appreciate any pictures, links, or manuals that show how the trick is done! Could be educational.

The tube simply sits on the chimney. The grid and filament leads connect with lugs. The anode is external of course, so you just use a clamp to attach a lead to it. Quite simple! It is of course necessary to mount the chimney on a pressurized plenum so the tube can get adequate cooling across the base and up through the fins in the anode. I'll send some pics when I get a chance!

I have in the past made a plate with air holes that fits snug around the grid ring. You can clamp it down with that small flange that is on the grid ring. I then mounted the whole thing the chassis using door knob caps as spacers so as to bypass the grid to ground. You can then apply bias to the grid. I did the same thing with a 3CX10000A3. I then adjusted the bias till I had the idling current I was looking for and drove the cathode. There was not as much gain as an A7 but it still puts out good power.


John N8QPC

not the exact same tube, but it uses the same mounting arrangement.

http://w2dtc.com/3cx3000f7.htm (http://w2dtc.com/3cx3000f7.htm)

Wow some folks are getting ready for Winter, building flamethrowers..........that will clear the air!!!!!

The tube I am considering is an F3, no grid flange. It's interesting, I got the pics, it just sits on the chimney in a little groove. (haha no mobile use!) That amp gives some good pointers though. If I go with this I was considering running grid-driven because it's a low-u tube and takes 150V bias.

You can get a plate made that holds the standard 3000 from the CB community.  Remove the grid ring on the tube, it will mount with three washers.

Matter of fact, you can PROBABLY remove the three collets and socket the tube.  I've found that being the case wtih many Eimac ceramics......

I have a friend with about 45 of the 3000 'socketless' sockets available, let me know if you want one, I'll forward his contact info to you.  I've mounted a 3000 A1 and A7 in them.  Chimney available at Home Cheapo.  Less than 100 dollars for the entire air system assembly for a 3CX3000.

--Shane
KD6VXI

I'd keep the wire leads...no finger stock to crap out or replace. ;)

73DG

Maybe this will help. The tube is held in there by one set screw and the plate is mounted down with the wires going up to the transformer or terminal block. Too bad the tube in the picture is a dud.

There was a company that made sockets like this called Rotron, or Rotran, or something like that. The base seals still need cooling which was normally done by ducting some of the main blowers' air up over the top and back down with a nozzle pointed down at the base of the tube. 

I think it was made by Rotron. There are very rare at least on eBay where I purchased this one. I only seen one other and I missed that bid. I think I paid something close to 50 bucks for this one.

I have a couple of 3X2500F something. They use glass between the base pins instead of ceramic. Came from an old TV transmitter.  This one uses a socket, and I have two of them.  Also have a couple of filament transformers. One tube would run a cool KW carrie output on AM in leen-yar service. Of course, no idea if the tubes are  still good.

I have a couple of 3X2500F something. They use glass between the base pins and external anode instead of ceramic. Came from an old TV transmitter.  This one uses a socket, and I have two of them.  Also have a couple of filament transformers. One tube would run a cool KW carrier output on AM in leen-yar service. Of course, no idea if the tubes are  still good.

Those were probably 3X2500A3 yubes. More likely the low band part of the TV spectrum just above 6 meters.

.

I'm having evil thoughts. Why don't you let me test those things out for you..

According to the old Radio Handbook the dissipation is supposed to be 2500 w.  The filament draws 50 amps. Question is, has it gassed up sitting on the shelf over the years.

well, maybe it has, but wouldn't it getter the gas by the usual procedure for large tubes?

You are right, I was looking at the 3X3000.

Here's the old 3CX2500 Eimac data.
http://bunkerofdoom.com/tubes/eimac1/image101.jpg
http://bunkerofdoom.com/tubes/eimac1/image102.jpg
http://bunkerofdoom.com/tubes/eimac1/image103.jpg
http://bunkerofdoom.com/tubes/eimac1/image104.jpg

I'm giving up on the multiband 3CX2500F3 idea. The flying leads are a wild card no matter how many ferrite blocks I imagine sliding over the grid wire. Neutralization is another way of saying negative feedback, and I can see how it would be done, and swamping too, but then the drive is 400W peak with a scheme like that assuming always having 2-3x the minimum drive.. GG is out, another thread estimated 1KW driver there.

I do not yet have the knowledge to say that it will work when finished. To build it on the chance I could tame it, - that is a lot of work to do first, and then see if it could actually work. I could end up with a wideband jammer.

If I had in my possession a 10KW FM transmitter, that would be prefect, holes punched, right size tube, everything in place..  hardware development time greatly shortened and getting right down to the RF hackery.

This puts me in a similar place as you Don. I have a 4CX5000A on the shelf, a good pull doing 8KW on FM, and it has not been lit in 10 years. No socket, chassis or the rest of the stuff though.

But I still have evil thoughts about a CCS duty 5-6KW amp. Not because I want the output, but because it will last my lifetime and usually the bigger amp running at 25% power is always cleaner than the smaller one running full tilt.

Those are the type of projects you need to bring to light when its "CQ Corntest, CQ Corntest" season.  :P

Would the maximum effect be to announce it was completed at that time or just bring the plans to light?

Fire that bad boy off and dont say a word, see who notices it.  ;D  ;D  ;D

Hope that does not mean I'd have to participate in the cointest!

Pat said:
 :D  :D  :D

no you don't have to use it for the contest, but you could fire it up on AM during the contest and see how many contesters notice it when they come on to shoo you off the air because it's contest time and you're taking up one of their precious frequencies.

That grid collet should come right off.  Have a plate made that is the same size as the socket for the grid on a 3000 and you can mount it to that plate.  I use 3 or 4 washers to hold the grid down.  Even in a trailer mounted mobile, no problems 2-3 years later.  You don't NEED the flying grid lead.

ALSO, if you need a chimney, Home Cheapo makes a DANDY for about 8 dollars.  They call them recessed lighting fixtures.  Get the replacement parts kit.  It's a press fit for the tube to slide in.

There.  3CX3000 based amplifiers with no socket, no expensive chimney, and you can still play with the big boys :)

--Shane
KD6VXI