I began this project a while ago and now have the amplifier in a working state, although not entirely complete.
I wanted to try out the Russian external anode power tubes that are now avialable at extremely reasonable prices. At the same time, I decided to do the construction entirely with parts and metal that happened to be laying around - for three reasons. First is to use up parts that are taking up space, secondly because I'm cheap, and lastly to engage in the practice of sustainabilty through reuse.
Here's a picture of the tube and output components.

and here's a picture of the underside with the input circuit, relays, and low voltage supply for the relays.


I'm currently using an external 2000v power supply but I left enough room in the RF compartment to put in a high voltage transformer, caps, and rectifier. I may decide to do that at a later time. I was very surprised at how smooth the amp tunes. The biggest problem is that it's so easy to over drive it since it only requires about 25 watts to get a full 420Watts out. I ordered some 5Watt metal film resistors to create a 6db pad for the input, which should help with the spikes I'm getting on voice peeks. I'm going to leave the input circuits for the harmonic suppression.

I've made up a two tube socket and will begin working on the two tube version in a bit. One question that I need to answer for that amp is if I can run separate filament chokes for each tube. I have some nice 24 V center taped transformers at 2 Amps. The tubes take 12V at 2 amps, which makes the transformer ideal for two tubes. I was thinking about putting the filaments in series and running them across the 24v, but I'm not sure how that would affect rf input. Anyway, I can experiment and see. As you can see from the photo of the bottem of the one tube amp, I made the filament choke from a ferrite core, which is one of about 25 that I have. Since the filaments are only 2 amps, you don't need heavy wire and can put a fair amount of windings on a relatively small core.

At the very least, it was a fun project.

arnie - W1GCI

Hi Arnie,
Nice looking amp. Do you have any pix of the front panel? It appears you used 2 band switches - one for input, and the other one for the pi network output. Also, the plate choke looks like a single layer of wire without any gaps. Do you have any resonances? I am replacing the plate choke on my Dentron Clipperton L because the original plate choke has a resonance at 18MHz which is not good. I just purchase one of MFJ Ameritron chokes figuring they have already worked the problems out. Let us know how the 2 tube model goes.
Jerry W5JH

Hello Jerry. Nice to see you in here again. I hope the move went well.

Yes, the RF/Plate choke is a single layer. It's one that I had in the junk box. I haven't encountered any resonances with it - I've only run the amp on 80, 40, 20, 15, and 10 and had smooth tuning - no bumps. I'm sure I measured the inductance prior to putting it in, but I can't remember what it is.

I haven't prettied up the front panel yet. I'm planning to create the letting using waterslide decals that I'll print on my inkjet printer. I've never done that before but it looks simple enough. I also don't have nicely matched knobs yet - I'm saving the good ones for the two tube amp. Here's a picture of the sockets for the one and two tube models - along with the tubes. The grid plate is 1/16" brass while the two cathode/heater plates are double sided PC board. I was fortunate to have a good supply of finger stock.



I've made a few changes since I originally posted and took the pictures. Yes, there are separate switches for input and output networks. I added a pad with a collection of 5 watt metal film resistors that tamed the input, preventing the overshooting from the transceiver. I also changed the plate loading cap from the wider spaced 225pf three gang that you see in the photo to a receiver type 900pf, which makes tuning a lot easier. I also added ALC circuitry in an attempt to control the drive power. There's still plenty to do and I'll likely be making other changes.

arnie - W1GCI

Hi Arnie,
The tube sockets are nice. The only other ones I have seen are by K4POZ. He makes a nice looking socket. I thought about building one of the GI7B amps but haven't got enough parts to get it started. Maybe someday...
It has been my experience that ALC circuits do not work very well. Most of them sample the drive level and create a negative voltage. The negative voltage is usually adjustable and is fed back to the transmitter to reduce the output. The problem I have with the circuits is that in order to get any ALC voltage, you will need to "overdrive" the amp. When that happens, the amp has already flat topped before the clamping action of the ALC kicks in. It is sort of like closing the gate after you let the horses out! What I like to use best is the adjustable output on the transmitters. Almost all newer transceivers have a power out control. I just turn it down to prevent over driving the amp. If you are using an older transmitter without a power control, the swamping resistor on the input of the amp is good. I feel these techniques work a lot better than ALC.
On my SB-200, which I rebuilt about 7-8 months ago, I drive it at 50-55 watts to get 600 watts out. I once made the mistake of driving it with a full 100 watts. At 100 watts of drive, my SB-200 puts out over 900 watts! However, it obviously would not last very long at that power out. After the rebuild, I expect the SB-200 will last another 40 years - longer than me.
Jerry W5JH

Agreed!
Very good job!

Mike Y

Thanks all for the comments. I took some pictures of my very basic power supply. On the top, you'll find the home made power transformer. Everything else is below. There are two switches - the first to enable A/C to the amp; the second turns on the high-voltage.

Okay, I messed that one up - pulled the trigger too soon. Below is a picture of the inside. You'll see an 18.5 volt transformer. That was added as a bucking transformer when I thought that the power transformer was putting out too much voltage. As it turned out, it wasn't needed. It's disconnected but not yet removed. Once it's gone, you'll see there isn't much too the rest of it. No soft start circuits; it's not really needed. The 120V EIA connectors came from an old PC power supply. One is the normal detached cable type and the other is the expanion device connector, which makes a good way to get the 120V A/C to the RF deck.



I think the components from this supply will fit into the RF Deck, but I think I'll keep them separate as I may want to experiment with some other (6 meters, 432) RF Decks and not want to build a separate supply for each.

arnie - W1GCI

Arnie,
What are you planning for 6 meters and 432? Are you planning to use the GI7Bs also? I have some 4CX250Bs that can be grid driven with 2.5 watts or less for full output. A pair of them will put out 600 watts on 432MHz. I have run a pair on 2M moon bounce at 1200 watts out. The trick is to keep the grid current to less than 1mA and run the plate at 3000V or so. You also have to push a lot of air through them to keep them cool. I never lost a tube in two years of moon bouncing. The little tubes are pretty rugged.
Jerry W5JH

Hi Jerry,

Yes, I was planning to play more with the GI-7B tubes. I don't really need an amp for six- I have an 8877 on that band as well as on 2 and 222mhz. I just thouh it would be fun to see what that tube would do on that band. I don't currently have a 432 amp, but I do have 50 watts out of my FT-847 so driving the triode would be no problem. I was going to build a 432 with an 8874. I have 6 of those tubes but none of them are any good. So the GI-7B seems a likely candidate.

I'd like to try EME some day. I never have except on a rare occasion when the moon was on the horizon, but even then I was less than completely successful. I've built a number of the pair of 4CX250 type VHF/UHF amps in the past for club contest operation but I really like the triodes better. What's interesting is you can get the new GI-7Bs for about what it will cost you for used 250Bs these days and the sockets will cost a lot less if you make them yourself.

Anyway, this is just for the fun of experimentation - like having a new toy.

arnie- W1GCI