6m qro

It's time to add some more power on 6m.  After looking at a lot of drawings, specs, and schematics I finally settled upon the Russian GI-7B.   It seems to be very popular with the 6m and up crowd, and there is plenty of HF conversions using them as well.  I was put in contact with a few hams that have not only built amps using these tubes but they built them from scratch.  Very impressive! 

I looked at my options for the HV power supply, the GI-7B runs at a plate voltage of 1500-2500V, and a current of 400-600mA.  I started considering donor amps for conversion.  The HeathKit SB-200 was my first choice having experience with them, but they are hard to find reasonably priced these days, and if you do they are heavy to ship! At about this time I got an offer of a Yaesu FL-2100B, which is very similar in design as it runs the same tubes, a pair of 572B's.  I see many conversions for the SB-200 and even a couple for the FL-2100B to put them on 6m using the 10m position on the band switch.  The problem in doing this is the 572B's are only rated to 30Mhz, and even though they will make power at 50Mhz they seem to have a rather dirty output and don't run that efficiently.  This is where the GI-7B comes in!

Introducing the GI-7B for those who are unfamiliar with it.

The GI-7B is a good choice for this application, as it works quite well with the power supply that used to power a pair of 572B's.  Here is the data on this tube.

The first thing you notice is the filament is 12v, where as the 572B's are 6v.  This means a new filament supply is needed.  However, this can be accomplished very easily with a small secondary transformer.  I plan on using a 12v 4a transformer that I have on hand for this purpose, more on that later.  Here is a schematic that I drew up, with help from Sid KG4NVZ.  Click on the image for a full size version.

When the amp arrived the first thing I did was test it and then strip it down.  The goal was to remove everything that wouldn't be needed and salvage the parts that would be re-used.  Here is a look of the bottom side while I was busy gutting it.

The RF compartment was stripped, tube sockets, plate choke, band switch, tank coil.  Then the new tank coil was made, it's 10awg copper wire that I salvaged from my junk box.  Refer back to the schematic above.  My plate and load capacitors are not the perfect value yet, more work will be done on them later when I hook up the MFJ Analyzer and adjust everything, for now they are just being used as-is to organize everything and determine the layout.

Now the tube and socket arrived!  You will notice that mine is slightly different compared to the stock picture above, this is due to mine being the GI7BT variant, the ceramic portion is larger in diameter then the metal grid ring.  The "T" is more shock and vibration proof, and has a larger ceramic structure.  It is designed to be used in Tanks.  I ordered both of these from Ron K4POZ.  If your looking to use one of these tubes I suggest you get one from him.  Each tube is tested, and they hand build every socket.  Definitely worth the money, the fit and finish on these is superb, a nice snug fit on the contacts to the tube, and very well thought out!  Head over to his website for more info on them, Click Here, where you can also see more pictures of the socket as well as plenty of pictures of completed amps and those being built with this tube.  Tell Ron I sent ya.. HI HI

I fabricated an aluminum plate to cover the back of the chassis and close up all the extra holes from the previously mounted parts, tube sockets and plate choke.  Then the new tube socket was attached by drilling through both layers.  For now that's all that is holding the extra plate in place, that will change later as more components are added to the rear of the chassis. 

Here is another shot with the tube installed to check clearances.  You can also see that the filament wiring is done as well as the RF input to the cathode connection on the socket.  It sits nicely above the two fans.  I decided to mount it this way so I could also change the fans to something of a higher flow later if needed.  I thought about mounting the tube side to side but it wouldn't have been as easy with the transformer and caps tight up against one side, and the cabinet tight against the other side.  I could have gotten two tubes in here side by side but the power supply isn't capable of enough current on the plate to run a pair of tubes.  I think this way should work fine and still gives me a little room if needed.  I test fitted some fans to the open holes when I removed the original fans for cleaning and oiling.  I could easily mount some 120v 80mm fans in their place for more flow.  I am also considering a Plexiglas divider between the caps and the top of the tube to concentrate the air movement to around the tube.  This would also give me an insulated divider to mount one end of the plate choke, more on that later!

Now for the rear compartment of the amp.  Originally this housed the input coils, the wiring to the tube sockets, and a few other things.  Now it will house the filament choke, still in its stock location but rewound per the specs on the schematic above.  I shortened the stock choke by about five turns and after testing it I'm pretty convinced it should be fine.  I also fabricated a piece to fill in the area where the input coil plate was mounted originally.  This compartment will also house the GM3SEK Triode Board, that will be coming up soon!  This will allow easy access to that board for any future adjustments after the amp is setup and running without having to be exposed like the chassis is now, and there was no room anywhere else for it!  The white/blue wire will be a subject for later, for now its just hanging out..

Today I got a couple things done on the amp.  First thing I did was install a plexiglass divider in front of the tube, this will keep most of the airflow going past the tube.  I also removed plates from the plate cap and spread out the plate coil to get everything adjusted.  I used an MFJ analyzer connected to the output of the amp with the relay triggered using a set of jumper leads.  There is also a 2.2k resistor from the anode of the tube to the chassis ground to simulate the in use conditions as described in the MFJ Analyzer manual and many other sources online.  With some time I was able to get it to a perfect match at 50.0Mhz, and it only requires a slight adjustment up to 51.0Mhz.  It will go higher with some adjustments and careful tuning but I really wanted it to cover this range without a lot of adjustment and I seem to have met that goal using the analyzer, I don't plan on using it above 51.1Mhz anyway.


A picture of the analyzer with the amp's output matching network under test.  It's sitting at 1:1 for SWR and 50 Ohms for resistance.  I don't think it can get any better than that!


The filament transformer and a few other small parts arrived today from RF Parts.  Here is the transformer mounted and wired, you can also see the blue plate choke from RF Parts, part number Z-50 as shown on the schematic above.


Here is a shot of the input network.  Pretty basic, a hand made coil and a variable cap.  There is also a variable cap to ground from the normally connected contact relays to adjust the SWR when bypassed or in receive.  It's pretty common to have impedance problems with a relay, that's why you need to be able to provide some adjustment.


And here is what I plan on working on this weekend.  The Triode Board arrived today from its journey, all the way from GM3SEK in Scotland!  Ian makes a great product and the manual covers the board and installation well.  Not to mention if you have a question he is very quick to answer!  If your looking at building an amp using a Triode or Tetrode make sure you go have a look at his website and view the manuals, I think it was a worthwhile purchase for adding protection to the amp and tube, not to mention the metering, bias, and having indicators on the front panel for everything!  Click Here to go to his site for a look!


Another batch of work got done today.  The first thing I did was create the indicator and switch panel for the front of the amp.  This gives me the information on the amp status from the Triode Board.  You have a TX, Ready, Grid/HV, and Anode indicator.  When you power up the amp the Grid/HV and Anode led's come on while the warm-up timer counts down and the tube filament is on but not the high voltage to the plate, then it trips a relay which powers the primary on the main transformer switching on the HV, once that's done and if no faults are sensed then it turns on the Ready led.  Of course TX comes on during transmit, and the Grid/HV and Anode led's are then off unless there is a fault.  For more info on this I suggest you download and read Ian's manual for the Triode Board.  I know, its just labeled with my label maker, I might do something better down the road to improve its looks but this will work for now.


I also got the HV Control relay, it's inline with the line power to the main transformer, mounted along with the small 16v 1A transformer for power to the Triode Board.  To do this I removed the plate with the terminal strip that sits over the capacitor bank.  Then I removed the metal uprights and drilled them for hardware and fastened the transformer and relay to them and then reinstalled them and finally wired them up.  Then of course I had to reinstall the terminal strip and reconnect everything but in the end it was worth it because these two components are now hidden and out of the way, there wasn't much room anywhere else for them.


One last thing for today, I test fitted the Triode Board to the rear compartment and drilled holes for the board standoffs.  The board is almost done, needs the trimpots yet, I have to wait until I get the meters I bought for the amp so I can test them and see what their internal resistance is before I select the trimpot value and install those.  It also needs to be tested offline and have its IC's installed before I permanently mount it in here.  I had just enough room to sneak it in there!  You can also see by all the wires hanging out I spent a couple hours today wiring everything back to the board compartment.


The Triode Board was wired up today!  It's snug in its new home, just enough room to get away with but it turned out to be a good location to mount the board in the rear compartment.


The meters arrived today, all the way from the UK.  I couldn't find anything size wise that would work here in the US, probably due to the amp being a foreign built model to begin with.  These meters are slightly taller than the stock ones, so two new holes had to be drilled for the upper mounting studs, the bottom hole from the stock meters was reused and an additional hole drilled for the other stud.  The original meters used two studs diagonally from each other, these have one on each corner.  I used the Meter program from Tonne Software to create the new scales for the meters, a very handy program that only has a slight learning curve to use, mainly due to it wanting everything in millimeters instead of inches.  Have a look at his software HERE and give it a shot!


I made a short video showing me doing the initial testing of the control board and power supplies in the amp.  You will notice I edited the video for time so we don't have to wait for the entire three minute warm-up cycle.  The quality could have been better but I didn't want to stress the internet connections of some that are less fortunate.

The amp was finished today!  Here is one final shot inside after finishing reinstalling the tube and the parasitic before its first test.


A look at the completed front panel.


A look at the completed rear.  You will notice the terminal strip was replaced with a new plate and an RCA jack for PTT keying.  Also the rear compartment cover has been drilled for four screws from the back, this way the cover over this compartment can be removed for adjustments without having to removed the entire cabinet first to gain access to the screws.


After extensive testing and adjustments here is the final numbers:

Input 20W
Output 500W
Plate Current 400mA
Grid Current 70mA

Some will ask, "Ok But How Much".  I have a total investment of $300 into this amp.  A half a kW for $300, that's not bad in my book!

copyright © 2013 Scott Lichtsinn | design by dcarter

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