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70cm PA


#1

Evening All
Not SOTA particularly (unless you count using it whilst chasing!), but as we are all mates anyway here go’s.
I have been asking around for ages to try and find the classic early 80’s design for a single 4CX250b amplifier for use on 70cm. Loads of responses along the lines of “oh, I had one”, but nobody so far could supply a diagram or build notes. Has anybody in SOTA land any info about the elusive design sitting in the bottom of a drawer that they could lend me/photocopy/sketch on a fag packet??
In desperation…
Tnx
Tim
G4YTD


#2

In reply to G4YTD:

I’m sure there are several designs in the 1st UHF Compendium or 2nd UHF Compendium. I’ll have a look to confirm.

The question that needs to be asked: why a single 4CX250? (*) The effort needed is almost the same as for a K2RIW PA with 2x 4CX250s. Or you could build something using the filth cheap GS series valves from Eastern Europe/Russia.

Andy
MM0FMF

(*) of course your single 4CX250 might be used as the driver for the real PA in which case fine. :wink:


#3

In reply to G4YTD:

Hi, Tim.

I’m almost certain there was a design using copper tubing for the lines that was publised in the VHF/UHF manual from 20 or more years ago. As it happens, I have a copy (at home). I’ll have a look later on this evening.

Good to work you when you were on Wendover Hill. What a super evening for sitting in top of a hill (SOTA summit or not). Slightly better than now and not a very good forecast for the weekend either.

73, Richard


#4

In reply to G4YTD:

The 7th RSGB Radio Communications Handbook (1999) includes a design by DL7YC for a single 4CX250B amplifier for 432MHz (pages 8.56-8.60).

73

Richard
G3CWI


#5

In reply to MM0FMF:

I guess from your enthusiasm that you got your K2 working! Mine struggled to produce more that a few watts - and I know others had similar problems despite the published fixes.

73, Richard


#6

In reply to G4ERP:

Mine managed 1kW with a pair of 4CX350Fs.

73

Richard
G3CWI


#7

In reply to G4ERP:

No but a friend had one that was a doozy. I’m sure it would produce over 850W key down for a sniff of drive. I can remember using it on the RSGB 70cms FM contest. We ran it to about 175-250W FM and it was happy and docile. Then there was a bang of biblical proportions and all the lights in the shack went out. The aging EHT transformer had let go and that, as they say was that.

In reply to G4YTD:

The 1st UHF Compendium has two single 4cx250 designs. One uses a 1/2wave anode line and with 2.3kv on the anode it will give 500W (cw) with 18dB gain. The other is 1/4wave coaxial line. With 1kv on the anode that gives 160W out. Obviously that needs loads more volts on the anode!

I can copy these for you but not for 3 weeks (I’m packing bags and will be away from the office). I still think for all the effort involved you’d be better making something with 2 valves in it. And you need expensive Eimac sockets for UHF. So the Russian valves may still be a better idea.

Andy
MM0FMF


#8

In reply to G4ERP:
Hi Richard
Good to work you too, and thanks for taking the time to set up the /p station to give it a go…
The activation went well, and qualified my 100th Activator point, the steak at the Hotel was cooked to perfection, and the client obliged the next morning with a King sized order number. in all a good visit!

OK on the VHF/UHF manual, I had a copy of that too, possibly in the huge pile of books that now live in the loft since children arrived! I will dig out the nuclear war survival kit and venture into the attic this weekend. Thanks for the reminder.
Have a good weekend
Tim
G4YTD


#9

In reply to G3CWI:
Thanks Richard
That was the callsign I was trying to remember. The “YC” was a solid design, very repeatable and with reasonable performance too. Just had a look at my copies of RC Handbook, unfortunately 5th edition so pre-dating YC by a good few years.
Cheers
Tim
G4YTD


#10

In reply to MM0FMF:
Hi Andy,
I know what you mean with the unearthly bang. Our contest group had a pair on 2m, we had been loafing along at 399.5w for quite a few hours when the smoothing caps decided to let go. Confetti & sticky stuff all over and a forced fold back to the QRP section!
My mid 80’s first love was 2m (especially big amps, 250b, then 2x 250b, and finally a 4CX1000A for MS, happy days), so didn’t do much on 70cm. I don’t have the UHF compendiums unfortunately. So if you have the time when you get back, a copy of the articles would be much appreciated.

I have an SK620 (new in box!)and a new Eimac 250B that have been kicking around for ages, also a client gifted me a Plessey transformer recently - good for 1650v AC @500Ma, so I thought a single for 70cm when coupled to my 3Kw genny would make quite a noise on 70cm. The genny is an Ebay job, wildly exaggerated at 3Kw output, but happy to supply the juice for a single 250 (I use an old Dressler D200 for car/p operations) hence the choice of a single 250…

I have a mate in Canada who makes valve bases for the Russian GS tubes, he recently sent me a couple of Gi7B bases to play with. They are in a 6m Amp at present giving around 750w key down into the Bird & dummy load. Drive needs to be around 50w for this output. Very cheap tubes though at £15 max each, although you do need to spend the time and effort in re-forming them (wind up the heater voltage slowly over 12 hours whilst blowing lots through the cooling fins).

I do enjoy a good yarn about proper amplifiers as you can probably tell.

Thanks for the offer of the diagrams, and yes please.

Enjoy your travels.

73’s

Tim
G4YTD

** Just found some old design notes for a 7213 Amp for 70cm, it needed 3.6Kv at 2A and 80w of drive for 1500w output. The scary thing was I built one whilst at Uni but couldnt stop it fusing the house we lived in whilst testing at anything over about 150 watts output. young and stupid…long before interlocks and outercasing!! I think it ended up in Wales at a new home with 3 phase running EME…


#11

Hi Andy (FMF)
Having made no progress with a decent reliable design -
Could I give you a quick prod to ask if when you have 5 mins, could you ping over the details of the single 250 on 70cm please?

:slight_smile:

Thanks

Tim
G4YTD

tim AT g4ytd.co.uk