Linear amplifier adventures continued!

For several months now I have been playing around with a set of loaner amplifers which have allowed me to dip my feet in the amplifier pond before actually purchasing one. Up until recently I always thought it would be fun to have an amp, but I didn't have the proper antenna configuration or available power in my shack to pull it off. I solved the power issue earlier this year by installing a 30 amp, 240VAC circuit, and I recently upgraded my random wire antenna's Unun to a 1500 watt model, so I'm now capable of running full legal limit (FLL) if needed. That said, I do not feel the need to run FLL when you can get slightly more than an S unit worth of improvement by jumping from the classic 100 watts to 500 watts (~7dB gain). Tripling the power to 1500 watts yields only another 4.7dB (total of 11.7dB) and introduces an entirely new set of challenges required in order to run that kind of power. Not the least of which is budget! If you'd like to read part 1 of this article, click here

Given that I have a limited ham radio budget, I decided to picking up one of the mid-range amplifiers made a lot of sense. By mid-range, I mean something between 500 and 800 watt output. These amplifiers will net you a full S unit increase while being somewhat budget friendly...if you buy them right. More on that later.

Fortunately for me, I have a friend who has picked up a couple of amps on auction and from a local seller who allowed me to bring them into my shack and test them out. This included an Ameritron AL-600S and a Yaesu FL-2100B and an accompanying LDG AT-600 Pro II tuner. I wrote about the Ameritron, along with my shack wiring project, in a previous blog, so I'll pick up where those left off.

Shortly after I reluctantly gave the AL-600S and the AT-600 Pro II back, I found the same tuner for sale on QRZ's "swapmeet" for $235 (they go for $410 new @ HRO), so I quickly snapped that up. Shortly thereafter, my buddy acquired a vintage Yaesu FL-2100B linear amp from a local seller and asked me to put it through its paces. Naturally, I agreed!

Yaesu FL-2100B

The Yaesu amp is a totally different animal from the Ameritron, not only because it's, oh, 4 decades older, but more importantly, it uses tubes for amplification vs. solid state components. Now being a bit of a nostalgic type, I was intrigued to see how the old Yaesu compared with the much newer Ameritron. I was surprised!

Before I dive into the Yaesu specifics, let me say that the claimed output wattage on most amps is pretty much marketing hyperbole. It's similar to fuel economy--a claimed 50 miles per gallon (21 KM/L) may be, in reality, more like 40 (17). The same holds true for amplifiers. An 800 watt amp may be theoretically capable of producing 800 watts, but in reality you probably don't want to run it much higher than 600 watts unless you like replacing finals or power tubes. Likewise, they sell high end 5 KW amps that, naturally hams won't run over 1500 watts. Right? ;-)

Back to the Yaesu. The FL-1200B is, in theory, a 1200 watt amplifier. Inside it has a pair of venerable 572B power tubes, and being an old amp, it requires -16 to -18 volts to key the amplifier at 125-200 mA, something that modern radios, like my IC-7300, are not designed nor prepared to provide. In fact, if you connect your radio's keying port to one of these old amps, you stand a fair chance of bricking your radio. So, you need to get an opto-isolated keying interface, which safely allows you to key these amps from modern transceivers. The one I used was RadioDan's RBI-1 interface, which runs around $80. Cheap insurance IMO.

In terms of interfacing the Yaesu to my IC-7300. it's super simple thanks to the RBI-1.  Simply connect the 7300's "SEND" jack to the RBI-1's yellow "keying" jack via a shielded RCA male to male cable and you're pretty much good to go. Note: it's also advisable to change the 7300's TX Delay to 25 or 30ms in order to give the amp a chance to power up before the transmission begins. 

Two items to pick up that are critical for QRO operating are a dummy load with a relatively long derating curve for the power you're planning on putting into it (this means that it can take the power for a longer time before getting too hot), and a good wattmeter. Both are critical for accurately putting amps through their paces without the variants involved with using an antenna system for the load. 

Things get a little more interesting when it comes to transmit wit the FL-1200B. Tube-style amps definitely require more tuning than the Ameritron AL-600S. With the Ameritron, so long as you choose the right band, you're pretty much good to go. You use the front panel "ALC" dial to vary the output power, and that's pretty much it. In contrast, tube amplifiers typically have three knobs: band, plate, and load(ing). Band is self explanatory. The plate knob varies the output tube's plate voltage, and the load knob varies the tube's grid current. Most amps comes with a set of recommended values for each band, and then you can tweak it from there. 

Tuning a tube amp is similar to an antenna tuner; varying the plate voltage and grid current changes the output impedance and therefore the amp's efficiency. What most people do is figure out the best values for each band/frequency range they want to work, and then create a cheat sheet with those values so it's easy to set going forward. With the Yaesu, I found tuning pretty simple; you key up the amp at a low power level, vary the plate control for the lowest voltage (you will see a "dip" at one spot), and then vary the load to achieve the desired output level. If you'd like an even deeper treatment of the tube amp tuning process, here's a great article on the ARRL website. One major tip I got from that article is to use CW for tuning, setting the WPM rate to 50, which takes it easy on the amp while providing accurate readings. 

I discovered while putting the amp through its paces that one of the tubes was weak and behaving oddly, getting really bright and humming loudly, so we wound up replacing both tubes with a "balanced" set. Fortunately, 572B tubes are readily available (highly rated Penta Labs tubes were $99 each at DX Engineering). That made a huge difference and now the amp is working great. I never got 1200 watts out of it, but it was working comfortably at 800 watts when I gave it back. All in all, a great amp IMO.

After getting the QRO bug by playing with two borrowed amps, it was time to get my own. I decided to get a used amp since they're much more affordable than new ones. For the past few months, I've been keeping an eye on ham radio auctions and Schulman Auction out of Kansas City MO, one of the top auctioneers of ham equipment, had a new auction starting up. 

There were several nice amps in the auction, most of which were out of my price range or needed work, but I did find one amp that ticked all of the boxes: an Ameritron AL-811H, which is still in production (reportedly the best selling amp of all time), sells for $1495 new, and can run on 120V. It appeared to be in very good condition, other than one oddity: the previous owner had replaced one of the stock 811A tubes with a compatible 572B (which is a better tube) for unknown reasons. Fortunately, Schulman always tests the equipment they sell (a major advantage IMO), and it was working pretty well, albeit not putting out the full 800 watts it's theoretically capable of producing (more like 450 watts). 

I decided to bid $501 on a lark, figuring I'd be outbid since there were already 28 bids. Lo and behold, when the auction closed, I won--by $1! 10 days after that, I received the amp in good condition, double boxed with plenty of bubble wrap, which was a good thing as it looked like UPS had an elephant sit on it! FYI the actual selling price, after paying the auction fees and shipping, was closer to $650--still an incredible deal for a 4 year-old amp in good condition.


My used Ameritron AL-811H Amp...and the box!

After unboxing the amp and installing the tubes, which were shipped in separate boxes), it's all set up and working great. I plan on replacing the tubes with all 572Bs at some point in the future, but since it's working, I'm happy for now. I have noticed that tuning this amp is a bit trickier than the Yaesu. While the controls are similar, the process described by the manufacturer is different, although it's probably for the best as they go to great lengths to make sure you understand how to properly use the amp, ensuring you get long life out of the tubes. I suspect the previous owner may not have been so careful and blew one of the tubes, but who knows. And even though this amp works fine on 120VAC, I will probably rewire it for 240VAC since I have it available in the shack, and it draws half the amperage that way. It's a fairly easy project; just change out the plug and a couple of fuses and make one small modification to the amp which involves cutting a lead and soldering in a new one. I did that for the other Ameritron amp and it was pretty straightforward (details in the original blog).

To wrap all of this up, I'm now thoroughly experienced in both QRP and QRO ham radio, and I have a new appreciation for both ends of the spectrum. And while I still really enjoy the challenges of QRP, sometimes it's just fun to turn on the cloud warmer and bust through a few pileups! 73 and let me know if I can answer any questions. 


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