I have gone crazy again..70cm

[VK3ZL]

Gidday all, I was asked by a friend to build up a 70cm amplifier for EME work..Requirements are that it will run 400 watts of data and CW..I had a think about it and looked at a couple of projects that had been around the internet for a while..I finally decided that a GS-31B Russian triode would fill the bill..

The Plate line is made from a 12cm length of 4 inch copper tubing...I had to machine 1/2 mm off the tube cooler to make it a neat fit into the copper tube secured with four brass bolts..The original projects have the tube sitting vertically..I decided that laying the tube horizontal would be a far better way to engineer my project..

In the anode compartment I made up two brackets to support two cams, one for Load and one for Tune..The cams are 1 inch diameter 1/4 inch Teflon with a 5/8 hole offset from center.
I machined two brass connectors threaded to fis the cams rigidly and a 1/4 inch hole to accept the tuning shafts..I have used fiberglass shafts in the Anode cavity...The shafts terminate into two old 3-1 slow motion drives salvaged from old 1940's radio gear...These drives hold the cams in position so that the flapper capacitors won't move the cams, detuning the circuit..

The Tuned Input on the cathode side of the amplifier uses a 1/4 wavelength line made from a sheet of brass and fingerstock to fit neatly on the tube Cathode ring...Tuning is achieved with a 7pf air trimmer and Drive is applied to the Cathode line with a series 7pf air trimmer capacitor..This gives us four tuning controls on the amplifier front panel..

Cooling...As the RF box shows I have mounted a 6 inch Papst 12-28 volt axial fan directly below the anode compartment..This pressurises the Anode compartment the air blowing directly against the tube and exiting through the tube cooler and Teflon chimney and the back of the amplifier..All electronics in the amplifier are my standard design, Step Start HT, W4ZT Bias system etc...All wiring is color coded of course..

This is a job nearing completion..I expect I will have it finished in another day or so ready for testing..

My experience with 70cm is fairly limited..I did build a 4cx250B 70cm amplifier around 1980..I am expecting no more than about 40% efficiency with this amplifier but we will see how it turns out..It has been a real mind bender figuring out how best to put it together so that I can access the tube for removal easily and also make any mechanical adjustments on the tuning systems if needed..

Enjoy these few pics...I will post another batch when the amplifier is complete.

Bob..VK3ZL..

[VK3ZL]

70cm amplifier progress..These pics show the Teflon chimney..This fits on the back of the amplifier and is easily removed using four bolts...Once removed I can remove the Tube copper Plate line and access the tuning flapper capacitors and also remove the tube if needed....This was an important part of this project..

Picture 2 is the Cathode component showing the 1/4 wavelength line and tuning capacitors, filament and Bias attachments..On the Left side are two small 12 volt muffin fans blowing air directly across the Cathode Filament and exhausting on the right hand side of the compartment....Testing these two fans I have found that they give a really good blast of air across the cathode so there shouldn't be any heat problems there..

Pic 3 & 4 are just preliminary setting up the front sub panel and some wiring, step start and Bias circuit..
As you can see, a bit of work getting everything aligned and working properly in the mechanical side but the hardest is over..

I will post some more pics as soon as I have the project completed ready for RF..

Cheers..Bob..

[NY4Q]

Very nice. Please post your test results when you seal it up for delivery.

[K7SGJ]

Another job well done, Bob. Very well planed out, as well. I imagine that part can be just as demanding as the build. I've been meaning to ask, have you ever built one for anything higher than 70 CM? Also, what's on the drawing board for the next project?

[KG4CGC]

Is there a museum for your stuff?




































There needs to be.

[mw0uzo]

one man production line.... awesome

[VK3ZL]

Another job well done, Bob. Very well planed out, as well. I imagine that part can be just as demanding as the build. I've been meaning to ask, have you ever built one for anything higher than 70 CM? Also, what's on the drawing board for the next project?

I have always wanted to get on 23cm but to date havn't built any equipment..There is a big GHZ following in VK and many of them modify commercial solid state amplifiers and other stuff up to 24ghz...I saw quite a lot of it at the SERG convention in June and there was some impressive work..

In years gone by there were a few 23cm amplifiers built using the 2C39A tube water cooled...These are 100 watt amplifiers...I don't know what other tubes, apart from the Russian GS-15 Tetrode, are available but I have heard around the traps that some very nice ex commercial amplifiers that can be modded to 23cm to produce KW plus are around....Just think using 1KW at 23cm and a large stacked array or 24 foot dish would be like..One of my ham friends in VK has just such a setup for EME..

Nah, I doubt I will ever produce anything higher than 70cm...I am getting too old, 70 plus, and my hands are not steady enough for really fine work...

Bob..VK3ZL..

[VK3ZL]

Well, I have finally got around to tuning this beast and beast it is....I will need advice from those of you who have had UHF amplifier experience..

Running 10 watts into this amplifier the following results >>>

Input drive and load controls peak and resonate beautifully..

10 watts produces 400ma of plate current indicating that I am getting good drive on to the tube..
However output Tune and Load although responding vaguely show only 15-20 watts output..
I played around with the Tune and Load flapper capacitors with no better results..

The amplifier is built to a design originally from PA3CSG..Two local VK hams have built this amplifier with success..

My thoughts lean towards the Teflon Chimney which extends to the half way point 6cm of the plate line possibly introducing losses into the circuit...Before I start chopping bits away from the chimney and other mods I would be interested in any thoughts that may help me to sort this all out..

The tube is new, run in and sound..Tested it in a 2 meter amplifier..The fact that 10 watts drives the tube into conduction reaffirms this...

Pic shows the current setup...Further design information can be found if you Google VK3HZ 70cm amplifier....

73..Bob..VK3ZL..

[NQ6U]

I don't know what to say, Bob. If I had a problem like this, you'd be the one I'd go to for help.

[koØm]

Hey Bob,

I was looking at Tony King's (W4ZT - SK) site and, I saw reference to an amp that you built back around 2006.

vk3zl.jpg

Mel

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