A simple GI7b 6m amplifier

Click here for my 50mhz GS35b commercial amplifier and associated problems.

This GI7b amplifier is based on the YU1AW design that can be found on the internet, it's sometimes called the Lazy Builders amp. And to that extent, it is! - (Please see notes below regarding using the GM3SEK control board with a toroid transformer)


My GI7b amplifier started out following the YU1AW design as shown here on the LEFT. In the building of the anode compartment I made some changes, mainly because when looking at the YU1AW design, and information that can be found using Google, it appears that certain parameters are not that stringent. The size and diameter of the coils is one area, some versions of the YU1AW design have one coil, some have two and all pertain to be the same design. Subsequently, there is a reasonable amount of latitude to 'play around' with, there's more on this below.


The real reason I opted for this design of amplifier is because back in 1998 I acquired a couple of N.O.S. GI7b tubes. And after sitting on my shelf ever since I thought it was about time I did something with one of them.


The next item I picked up for the project was the socket. I didn't go for one of the many hand made sockets you can find available these days, not that there's anything wrong with those! It's just I saw this socket on eBay and it was cheap, so I bought it.

Sockets can be bought from Alex UR4LL here: UR4LL

Another socket available can be bought from K4POZ here: K4POZ

Or, why not build your own following this design by N8GPQ: N8GPQ


The blower I've used, some people will recognise as the unit pulled from the GS35b amplifier of mine, this little blower supplies enough air flow to meet the requirements for the GI7b tube, and once I'd cleaned it up, it worked fine.
The cabinet is the 19 inch rack mounted unit I've had sitting here for some time. This picture shows the cabinet with the transformer I've used, plus the GI7b tube and my original tune and load caps. Included here is the junk 'voltage doubler' PSU board that I'd also planned on using, but due to it's design, haven't.


I quickly realised that although I'd created a grid compartment (the blue/silver box) as my first step in this little project, this wasn't the way to go. So, pencil and paper to the ready and I designed a complete grid and anode compartment as one unit. That may sound grand, but it isn't really! All I did was I'd drawn what was required. i.e. a big box sitting on a little box, then I created drawings that would allow some one to cut my some sheet metal out of aluminium so I could simply bolt the sheets together. (see below) 

This is what I drew and then had cut in aluminium. I simply made the compartment to fit my 19" rack unit, making sure that the tube, caps and coils would fit and that was it... At the same time, I had holes cut in the front panel for the C1 and C2 capacitors, plus the hole I required for the GI7b socket, and for the hot air exhaust. I also had the holes cut for the RF in and RF out.

Once the panels arrived here, I quickly realised that you have to be perfectly sure you've got your measurements right! On some panels I had forgotten to allow for the 2mm wall thickness, not a huge problem, but it meant that  the panels although bolted together, there were some places where they didn't quite site right.

The new cut panels can be bolted together like so>>



Here's the set of panels bolted together using aluminium angle. I also had the holes cut for the C1 and C2 capacitors. I used 6mm nuts and bolts to hold it all together with a set of captive nuts for the anode lid.

This is another view of the anode compartment, this time with the lid fitted and showing the hot air exhaust.

Here's the anode compartment with the GI7b in place, C1 and C2 capacitors and L1 and L2 coils fitted. Yes, TWO coils! I noticed that some copies of the YU1AW circuit had just one coil, others had two coils. Through experimentation I found that I needed the two coils.

There's a 1,000pF door knob on the top side that is where the HV comes in via a porcelain feed through, the door knob capacitor is to decouple the DC. Then there's the RF choke, details can be found on the web on how to wind this, but I simply copied a choke I had made for yet another 50mhz amplifier. The DC connects to the bottom of the choke and the top end to the GI7b tube itself.

The green wire is a resistor 'test set up' that will simulate the presence of the HV DC, then, it's possible too use an analyser on the RF output to test the resonance of the RF circuit, as I have an MFJ-269 analyser that's what I used. This makes adjusting the coils and caps very simple to do while winding your own coils.

Then, on the left of the GI7b there's the DC blocking door knob and a copper strap to C1. C1 consists of a small vacuum variable in series with the 5pf door knob giving me circa 5 to 15pF, on the left of C1 is C2, a 30 to 110 mF vacuum capacitor. These two capacitors do not need to be vacuum caps, I've used them as that's what I had here.  (please see the notes i've added below, July 2012)

With my MFJ analyser connected to the RF output socket, and with the resistor connected to simulate DC voltage on the tube, I was able to 'play around' with the coils and C1, C2. Eventually I ended up with the match as shown on the right.

 (please see the notes i've added below, July 2012)

Here's a copy of an amplifier by 9A6C that uses TWO GI7B tubes, in the end I used the coil dimensions given in this circuit for my single tube amplifiers anode RF circuit.

The grid compartment is a T match circuit. In simple terms, the DRIVE RF comes in and connects to a 1,000 pF disc (block) capacitor, then through L1g coil, and then a small 4 to 20 pF capacitor that's to deck, and then the L2g coil to the grid connection of the tubes socket I used the same dimensions as the L1 coil in the 9A6C design above and made two coils the same. But I ended up having to adjust the coil length on each to make a better match with the air cap to deck. Once all fitted you then have to 'play around' with the circuit to improve the input match . I found that the more you adjust the coils/cap you quickly learn that there is plenty of 'leway' in achieving a good match.


The heater connections are fed their supply via two 'feed through' capacitors then through a bifilar wound choke before connecting to the socket. The bias supply is connected to this heater supply on the outside of the grid compartment. Cooling air flow comes from the left via the square hole, air flows around the grid compartment and up through the socket to cool the larger anode fins before leaving the amplifiers out of the top cover.


With my MFJ analyser connected to the input and with the tube fitted into the socket, it was possible to test for a match. With adjustment of the coils by pulling them apart, or squeezing them together. This, along with adjustment of the capacitor  got the match shown here on the right.

The heart of any good amplifier is the control it uses, and why try to re-invent the wheel!

The control board seen here on the 'top right' of the picture is a GM3SEK triode board. This board has been modified for use with the torroid transformer also seen in the picture. The modification is something that Ian, GM3SEK has come up with and is the first time it has been implemented in this way, as such, it's still in the 'beta test' stage. Click here for control board modifications.

The transformer supplies heater voltage plus two 700 AC windings, and with these connected in series to a HV board from WD7S (a full wave bridge rated at 4.5KV)  gives me a stable 2KV of DC. The board has provision for a HV 'glitch' resistor, but I have used these connections for a high rupture, high voltage fuse. The fused HV then leaves the board (red wire, top middle) and connects to a 100w 50ohm glitch resistor that is fitted on the rear of the brown main board.

240v control and step start relays are located on the bottom of the cabinet on the right of the transformer. These relays are controlled by the GM3SEK triode board. The manual for this board gives details on how to formulate the step start system.

The red/orange wires shown on the bottom right hand side of the picture as the heater supply. I found it easier to connect these to the RF deck via a 'quick release' chock block connector.

Here's a picture of the RF deck fitted within the 19" cabinet. At this point in time the amplifier is almost ready to try.

This next picture on the right shows the cabinet with the front panel waiting to be bolted in place. The front panel is already fitted with grid and anode current meters, C1 and C2 control knobs and the four LEDs required by the GM3SEK control board. There is also a 240v power switch and 'stand-by' switch. All can be seen on the next picture. I fitted this panel with a multi ribbon umbilical cable to allow for its removal with out any problems. If I need to re-test the amplifier at some point in the future like this then I have a temporary set up just four LEDs and meters ready to connect with a longer umbilical.

It's all a bit of a squeeze!


With the blower fitted to the RF deck, and the TIP147 device that comes as part of the GM3SEK triode board kit fitted to a suitable cooler (black fins on the right of the blower) there isn't much room left!

In this picture on the right, you can see that I fitted a pair of TX/RX relays (under the left hand side of the anode compartment). I've used two CX520 relays. These are also controlled by the GM3SEK control board.

Grid and Anode meters are shown here on the left. The Anode is clearly marked as 0-350mA as being normal, with anything above 350mA being abnormal. The Grid meter is marked out as 0-80mA as normal and anything above 80mA as abnormal. The control board will trip the amplifier at 90mA.

Meter design software can be found here: Tonne Software The basic software is free and easy to use, but the full version allows you to use colour on the display. I've used the full version for the scales on my meters shown on the left.

The GI7b amplifier in its finished state. And, as with all of my amplifiers, it has been given a name and this one is Piglet.

This has nothing to do with the Disney movies, i decided on Piglet as I consider the amp to be the daughter of my GS35b amplifier that acquired the name 'The Pig', the reasons for that can be found elsewhere on my web site.

So, the question most people will want to know is "how does it work?"

Well, from the outset I was looking for an amplifier that would deliver 300w.

As it is, I can get 350w out with both the grid and anode reading within normal parameters for about 40w of drive. 300w is achieved with 30w of drive. If I'm careful, with some very fine tuning it will produce 400w, but would then often trip on grid current.

I dare say that with a few more volts on the anode, over 400w would be achievable. But that was never the intention.

The amplifier was first used in the RSGB 50mhz trophy contest in June or 2010, and it performed admirably throughout, never missing a beat or complaining about anything.

Drive  Anode           Grid              Output

-         50ma            -                   -        

9w      210ma          5ma              100w

14w    250ma          14ma            150w

19w    300ma          27ma            200w

32w    370ma          62ma            300w

39w    380ma          80ma            350w

 400w is achievable, but more often than not the amp will trip on grid current


 Update: July 2012.



In July this year i switch on the amplifier and when it was warmed up i tried it, but unfortunately i couldn't get the normal power and when i investigated why, i found that the load cap was arcing.  It actually looking like a 100w lamp!

So, the picture on the right shows the capacitor removed. 


The picture on the left shows an air spaced variable capacitor from my junk box, this one is 20pF to 110pF, and fortunately for me, it just fits... :-)

Once fitted, it was a simple case of re-fitting the coils, but i then found that L2 (the coil on the left), needed to be changed as the match wasn't good any more. So i wound another one and started with 7 turns on a 15mm drill bit as a former. See below for a better picture. 




The coil, initially 7 turns, still didn't work so i removed two turns to 5 turns. This was better, but still not right. I found that simply opening up the spacing between coils altered the match to something getting close. The more i pulled it apart the better :-) The picture shows the finished result.

Here's the match this time round. This is the analyser feeding in on the anode output with a 3.1k resistor across the tube itself to simulate the 2kvDC.

Here's the formula for working out what size resistor you need to simulate the HV on the anode.

Once finished, If you want to test it at lower power, decide what plate voltage and plate current will give the same load impedance as the high power load you are designing for.

Example: 3000 volts at 1 amp (using 1.8 factor) gives 1666 ohms. If you decrease the plate voltage to 1/2 and the plate current to 1/2 you will have the same load impedance.

1500 volts at .5 amp (still using 1.8 factor) still gives 1666 ohms.

(V / amps)/1.8

(3000V / 1a)/1.8 = 1666 ohms.

For this GI7b amplifier, it would then look like this:

(2000V / 0.35)/1.8 = 3174 ohms

Or, to be more precise

(1960 / 0.35)/1.8 = 3111ohms






Questions and/or comments are always appreciated.



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