Here I go again..New prototype 2 meter amp..

[VK3ZL]

I decided to take a slightly different approach to the RF deck design of my 2 meter EME amplifiers...The originals work well but there is a lot of work setting up the RF deck, hand making and assembling the parts...Ya can't buy this stuff off the shelf..

I have cheated a bit this time...Friends got a good look at a commercial amp and took a few pics and ran the ruler over the plate line etc for me...I started this amplifier on Monday and I have nearly completed the whole job now..I will post more pics next week..

The plate line is a simple sheet of brass sheet bent to the dimensions given to me along with the tune flapper cap and pickup line...I have only used the brass plate line and set up my own flapper capacitor design using a 2 inch Teflon disc I machined up with a 3/8 offset to make it work as a cam....I used an old 3-1 shaft drive on the chassis front sub panel and the flapper combination tunes this little bugger from 143-147mhz smoothly...I put in a GS-31B tube in this amplifier but I can change it to a GS-35B tube in 5 minutes if I want...

The Pickup wire runs across the cold end of the plate line with a small series tuning capacitor to load the amplifier into the antenna coax relay..My Analyser tells me that the combination Tunes and Loads at 144.250 into 50 ohms using a plate resistance of 2200 ohms so we are in the ballpark...Will probably need some adjustments under fire no doubt..

I will present this amplifier at the SERG convention HB competition for comment next month..The amplifier will be primarily for EME work, moderate power probably 800 watts JT65 and data modes and 1200 watts SSB/CW...I am waiting on a shipment of new power transformers so may have to use a compromise trannie if they don't arrive..

I have completed the front panel ready to wire in and I am keen to see the end result...

Pics show the pre assembled case..17x17.5x9.5 high...The amplifier will use a 12-28volt 6 inch Papst fan 285cfm in my usual setup air pressurising the RF deck exiting through the tube cooler then a Teflon chimney on the top cover..The cathode compartment uses a small fan blowing air over the filament/cathode assembly..I have used this combination of cooling on a number of my amplifiers and it is very efficient..

Enjoy..Bob..VK3ZL..

[K7SGJ]

Very nice, Bob. Do post some completed pix.

[NQ6U]

Have some mercy and stop making all the rest of us look bad, Bob.;)

[N2CHX]

Awesome as always!

[K7SGJ]

Have some mercy and stop making all the rest of us look bad, Bob.;)


You and I don't need Bob to make us look bad, we do pretty well by ourselves.

[NQ6U]

You and I don't need Bob to make us look bad, we do pretty well by ourselves.

Good point. Let me rephrase that, then:


Have some mercy and stop making all the rest of us look even worse, Bob.

How's that?

[K7SGJ]

Good point. Let me rephrase that, then:

Have some mercy and stop making all the rest of us look even worse, Bob.

How's that?

Works for me. I wouldn't change a word.

[KC2UGV]

Good point. Let me rephrase that, then:

Have some mercy and stop making all the rest of us look even worse, Bob.

How's that?

I concur.

[NQ6U]

By the way, Bob, this came in this morning's mail:

9661

My first 10m SSB QSO to Oz.

[VK3ZL]

I dunno....These UHF guys always trying to outshine the Topband where real men take a stand..

Dunno the bloke but lot's of 6 meter hams hang around 28mhz...Congratulations...

Bob..VK3ZL..

[VK3ZL]

Ok, Saturday and bloody cold all day....I worked on the cathode side getting it all together and resonant on 144.250....I use a simple tunable T network here adjustable with the variable tuning cap...The GS-31B load resistance is somewhere between 40 and 50 ohms...Using the Impedance Bridge and a 50 ohm resistor from cathode to ground it is a small but tricky job adjusting the tuning cap and squeezing the little coils in and out..End result, drive vswr 1-1....This will probably still need adjustment under fire...


Bottom cover and legs go on in the morning and I will finally get around to wiring in the front panel...Pic shows the front sub panel with a few wires to be sorted out....No problem though as all circuit wiring is color coded...

First two pics are the cathode compartment showing the input relay and tunable input network and small simple 12.6 volt relay power supply..

Front sub panel with HT step start bias relay and cutoff resistor and on the right the Bias board which uses a TIP147 transistor and a TL431 precision adjustable regulator..There is another 22k resistor across the input and output of the bias board with a series fuse from the B minus line...If the board cacks up the fuse will blow and the resistor will bias the tube to cutoff..

Enjoy...Bob..VK3ZL..

[KC2UGV]

I forgot, it's winter for you down under...

[VK3ZL]

Sunday..Well, I got behind the front panel and connected up the conglomeration of wires and things today...I am pleased that I have that job behind me now....All that is left is to install the HT transformer and Rectifier stack..

Pics are the before and after wiring on the sub front panel..

This is what the amplifier looks like fron the front....The meters were a bit oversize but is all I had for this job..At least they are easy to read...

Enjoy...Bob..VK3ZL..

[NQ6U]

Looks like crap, Bob. You need to get rid of that piece of junk. The address on my QRZ listing would be a good place to do that...

[K7SGJ]

Look like crap, Bob. You need to get rid of that piece of junk. The address on my QRZ listing would be a good place to do that...

I tried that with the last one. My mailbox is still.........well.......crickets.

[VK3ZL]

Ok Carl,

I will dismantle it down to the last rivet and put it into a metal compressor, box it up and send it to you where you can re assemble it...How's that....A Bobkit....

Bob..VK3ZL..

[VK3ZL]

Didn't you receive it?? Musta mixed up the address...You didn't by any chance receive a package of old musty socks and underpants by any chance....I bet UNESCO are wondering what to do with a big amplifier....

Bob..VK3ZL..

[VK3ZL]

Monday and I am not full of beans today...Couldn't sleep last night...Musta been that last jar of pickled onions...Wonder if you can buy them in the US...100 farts per bottle..

Anyhow down to serious amplifier building business....I made all the air input holes today....Look at the pics carefully and see how carefully I have labored all day with a file carefully reaming out 27 air inlet holes...

Would I tell porky's? Sometimes I guess....I have my trusty old chassis punch which has seen more holes in 70 plus years than me I think...Three lots of 9 holes 1 1/8 diameter...Still a fairly painstaking job..

I use 6mm birdcage wire over my air inlet holes...It doesn't restrict air flow like many finer meshes...It keeps larger spiders and mice etc out of the amplifier box....Little spiders are ok...

Picture of the 6 inch Papst fan....I managed to pick up a box of 20 for $150 from an electronics clearance place in Melbourne....Their list price in the US is around $90 each....Lucky me....They are 12-28 volts dc...Rated at 285cfm....That's a lot of air....However running them into a sealed enclosure and through a tube cooler, the back pressure reduces the air flow to about 125cfm through a GS-31B tube...That's more than enough to keep this tube cool even at 1200 watts output....When using a GS-35B these fans produce about 5/8 inch back pressure using my manometer so actual air flow through a GS-35B cooler is probably closer to 80cfm, still enough to keep the tube cool at 1500 watts output....I use a rheostat and an LM317 and pass transistor on these fans so that I can regulate the fan speed...I rarely find they need more than 20 volts for my purposes..

If you don't have enough air flowing through a power tube it is like having a shag in a sauna..The tube will just get hotter and hotter until it collapses from exhaustion and probably dies...Another thing I often hear is how whisper quiet brand x desktop amplifier runs...I bet the tube feels like that guy in the sauna....If you cant hear some good air noise then you aint doing it right...My amplifiers operate with quite a bit of air inflow noise only...The DC fan does not create any motor noise and once the covers are on it is nearly silent...

Ok..Pics are those holes I labored over all day...2 hours actually...The el cheapo birdcage wire I always use and the Papst fan....Have to go to town tomorrow so no more work until Wednesday....

Enjoy....Bob..

[K7SGJ]

Didn't you receive it?? Musta mixed up the address...You didn't by any chance receive a package of old musty socks and underpants by any chance....I bet UNESCO are wondering what to do with a big amplifier....

Bob..VK3ZL..

Yes, Bob, I got those. I couldn't get the sock to load, and the underpants you sent were already loaded.

[X-Rated]

I saw some W5 lander spot one VK3ZL on 160M this morning my time. That is pretty impressive.

[VK3ZL]

I saw some W5 lander spot one VK3ZL on 160M this morning my time. That is pretty impressive.


Google VK3ZL....You will find that I am fairly active on Topband....

[N1LAF]

Schematic Software on hot standby...

[VK3ZL]

Well, my HT transformers arrived at last....Must be made with gold....Shipping worked out to $336...OUCH !!! All up no change from $1450..Anyhow I have four new custom made Toroidal HT trannies to play with.I installed one into the latest 2 meter amplifier and switched it on...Everything works a treat..I just have to connect it up to an exciter and dummy load and final testing begins...I will get back on to my new 160 meter shack amplifier when this clears the bench and complete that with it's new HT transformers..Will keep you posted..

Pics are some of the transformers and a couple more of the amplifier idling over purring like a contented pussy..

Enjoy....Bob..VK3ZL..

[K7SGJ]

Outstanding, as usual.

[mw0uzo]

Delicious! The custom wound HT transformers sound very expensive, where did you order them from?

[VK3ZL]

Delicious! The custom wound HT transformers sound very expensive, where did you order them from?
The transformers are made to my specs in Hong Kong...Excellent quality...It is the shipping that makes them expensive...Any company in Australia who makes transformers knows the international shipping fees so they can safely charge a high price to build anything knowing they have the shipping edge...In fact shipping within Oz is bloody expensive so there is no advantage ordering anything here..

Bob..VK3ZL..

[WX7P]

The transformers are made to my specs in Hong Kong...Excellent quality...It is the shipping that makes them expensive...Any company in Australia who makes transformers knows the international shipping fees so they can safely charge a high price to build anything knowing they have the shipping edge...In fact shipping within Oz is bloody expensive so there is no advantage ordering anything here..

Bob..VK3ZL..

That's interesting.

I guess the Post really sticks it to the locals.

Whenever I've gotten something from Australia, the postage to the US (at the time, California) was a lot less expensive than Europe or anywhere else outside the US.