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.

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.

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

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 if 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.  

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 wit the match as shown on the right.

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 capacitor, then through a coil, and then a small 4 to 20 pF capacitor that's to deck, and then another coil to the grid connection of the tubes socket.

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 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.

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.

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.

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!

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.