Making (Big) Waves

Posted on Thursday, March 15th, 2018 at 11:41 am in
Yaesu FL-2100Z

Yaesu FL-2100Z

The Yaesu FL-2100Z is a classic HF amplifier. It sits near the end of a long life line of multiple variations and models of the basic 811A or 572B triode valve, all using exactly the same grounded-grid core design, and all of them making nice big waves in the ether when fired up.

I was lucky enough to lay my hands on three of them over time. The first one came to me from an estate in Durban, KZN, in perfect working condition I might add, and I was stupid enough to sell it to a fellow ham that pleaded poverty. He still owes me money after selling it onwards immediately at a profit. The last two came my way in more fortuitous ways and have been a pleasure to own and I’ve made big waves with them while they were visiting my shack.

The one was nearly in original (i.e. unmodified) condition, the other lacked the row of input filters in the back. The one had 572B lamps whereas the other was retro-fitted with 811A lamps. Both were sort-of working but not really, and over many evenings I painstakingly studied the details of what was working and what was failing. It was great to have two examples with two different faults, because I could compare faults and figure out their individual problems.

Just remember, I have never worked with valves before. I don’t know them, I don’t like the seriously high voltages around them, nor the heat, and just to add injury to insult I once forgot to disable the HV interlock switch sending me running for cover as the whole thing exploded with loud noises and blown fuses. These things are like hot women: Pretty to look at but you will get burnt if you touch them.

But we made peace and progress more or less at the same time and eventually both are working. My feeling is that the circuits are relatively simple and the resulting box robust. Once you have twiddled all the knobs and read all the meters, they actually produce some nice power and can make nice big HF waves. For example, with an input of 25 watts the 572B valves (after tweaking for optimum output) spat out a good 500 watts key-down on 40m where I like to operate. The 811A valves were also willing but maybe a tad less capable at 450 watts. I must add that the 572B’s were eager to give a little more with a little more drive, whereas the 811A’s needed a slightly bigger push to give out more. Both worked very well though.

I uploaded a short video of the testing of the first one to go live at YouTube Video Link showing how easily it puts out 500W key-down even in a somewhat mis-matched feed with high SWR. It is shaking that wire mercilessly by the tail!

Sadly I have to let them both go. I started another project to build a solid-state amplifier in the same output range, the so-called DN-600, and there isn’t enough rack space in the shack for all of them. The Yaesu FC-902 tuner stays, though, as it has proven to be both accurate and useful. I compared the power it measures on its built-in meter with an expensive scope-voltmeter into a dummy load and I’m satisfied that it tells the truth. Plus my only antenna that can handle that sort of power is my (unmodified) G5RV which, as you know, needs a little impedance matching. Having said that, when I use the G5RV for WSPR (QRP 200mW) I consistently get very good spots, so please don’t come tell me it is rubbish and you lose power when you use a tuner – my experience is exactly the opposite.

I will be remiss if I don’t mention the three gentlemen who assisted me in this project. Firstly Jacques Scholtz ZS6JPS who introduced me to the AWA (Antique Wireless Association) and was always willing to give advice. Secondly Renato Bordin ZS6REN who kindly donated a secondhand 811A which came in very handy, thank you. Finally Adi Loupo ZS6CNC for setting up the valve tester and helping me test all my motley collection of good and bad lamps, and helping me to get two pairs or lamps that worked together. Thank you guys, you were demonstrating the true amateur radio spirit.

I’ll publish some more details and photos of the work I did in the next few days, I just wanted the word to get out so long. Here is one last photo of the pair of 811A’s to close off with:

811A's Working Full Steam

811A’s Working Full Steam