AMERITRON AL-572
Near Legal Limit CW. But Certainly Illegal Capable On SSB.  
by Bob Hutchinson, N5CNN

I just can't get away from the warm glowing tubes and Winter ain't over til it's over. So, let's open this "near legal limit" sweetie up, photograph it, put it together and see if it lights-up. If it does, we'll run it at the WIA bench.

                            Click Any Picture Larger. >>

Unless I'm missing one, Ameritron makes four amplifiers utilizing the same compact cabinet: - the AL-800, AL-800H, AL80B and AL-572. I believe this size is based on the original AL-80 and SB-1000 Heathkit. I believe these are all designs developed at Ameritron after its purchase by MFJ. Here, as we review the AL-572, we will see some components that apparently are interchangeable and shared by these four amplifiers.

This writer has not opened up the AL-80B, but I suspect it shares the majority of components. This AL-572 is almost identical inside to the AL-800H, which I suspect is identical to the AL-800, sans one 3CX-800A7 Eimac tube. These amplifiers appear to use the same hardware in the RF, rectifier and filter section, T/R, metering, and tuned input. Different, of course in transformers, B+ voltage, tubes, controls and some dedicated components. All utilize the full wave voltage doubler rectifier/capacitor/filter circuit made famous by Heathkit in its classic amplifiers.

The AL-80B utilizes the 3-500Z type tube that requires considerably higher B+ voltage than the 572B and 3CX800A7 tubes. Because of the high performance of the 3-500z tube in the AL-80B, I suspect the B+ is 3,800 to 3,900 or more VDC. This commands a plate transformer with about 1,350 VAC available. The AL-572, AL-800 and 800H utilize the same 2,900 VDC B+ and consequently about 1,050 VAC from the plate transformer. The AL-800H, with about 1,000 watts more power output that the others obviously utilizes a healthier plate transformer of 32 lbs. versus about 23 lbs. for the others.

The line up below is in street price/output power order. Prices are from The Ham Station in Evansville, IN and subject to change. Ham friend W5HBO always says, "it's just  function of money".

AL-80B Single 3-500Z Tube ......  $ 1,164.95  QSK-$325.00
AL-572B Four 572B Tubes ........  $ 1,279.95  QSK-$325.00
AL-800 Single 3CX800 Tube ...... $ 1,579.95  QSK-$325.00
AL-800H Two 3CX800 Tubes ...... $ 2,299.95  QSK-$325.00

In tube amplifiers, it appears that Ameritron has focused on three box designs. This box being the middle size, the smaller being the AL-811 AND AL-811H and the larger being the AL-82, AL-1200 and AL-1500. Quite a line-up.

OK, it's tent duty
It's kinda like shooting the AL-800H again. I put the amplifier on the caster equipped plant stand and worked the Olympus digital and speed lights. At one point I applied power and made a few shots with the tubes lit up.

Some assembly required?
Oh no. This amplifier arrived with everything in place including the tubes. Ameritron takes great care in cushioning and wrapping the tubes in their confined space so they can't be damaged in shipment. Like the AL-800H, this one arrives with transformer bolted and soldered in. The fuse holders required the only assembly with the fuses. A complete set of extra cover screws is included with the amplifier. Nice touch. Nope, I don't believe any other maker does this. They even send extra screws back with repaired amplifiers.

RF section
The same basic parts as the AL-800H. Fiberglass circuit board with RF coils and taps to the band switch mounted on it. More compatibility of parts among this middle size amplifier group.

Cooling of the box
And the four warm, glowing 572B tubes are handled like the AL-80B, with a muffin fan of about 30 CFM capacity. Typical of amplifier design today, the fan pulls air through the power supply section and blows it through the tube and plate choke group and exhaust it, and the heat, outside the box. See Information in another article about how glass tubes are cooled.

Although not noiseless, this arrangement is considerably less noise than the centrifugal fan in the AL-800h even though the capacity is the same. The 3CX-800 indirectly heated cathode type tubes require a pressurized cooling system to force air through the axial/radial heat exchanger configuration. Plain old fans don't do that well. Centrifugal blowers will handle it OK but create more noise and, to me, an irritating frequency along with the noise. More about this below. Yea, a fix.

Good choking
With the plate choke mounted in the middle of four tubes it serves an ancillary duty - it acts as a stabilizer to the tubes during shipment. Ameritron inserts foam packing material around the choke and around the tube and choke group of components and on top.

Pin diode QSK
I ordered this Ameritron with the factory installed Pin Diode QSK board installed, it is not really necessary for normal SSB or semi-break in CW. It is a $325.00, 14% add on option but makes SSB VOX operation silky smooth and VOX delay can be set to minimum.

The manual
Good and available. You never have to chase down a manual for Ameritron amplifiers. Just access it at their web site.

The power supply?
It has a multi-purpose transformer with a little over 1,300 VAC available for the performance proven voltage doubler HV rectifier/filter circuit introduced by Heathkit 40 or so years ago. It also provides the ancillary voltages for bias, metering, QSK, relays, etc. Available, no load, plate B+ is 2,900 VAC. Without this voltage doubler HV power supply design this amplifier would be considerably heavier and cost more to build and more to buy. Click Here for more about doubler power supply benefits.

Bench
Our test bench is equipped with a 2000 watt fan cooled dummy load, 3,500 watt low pass filter, Bird 43 and Coaxial Dynamics watt meters cabled in series, each with 2,500 watt slug. The bench has a superb 7,500 w. antenna switch utilizing Jennings vacuum relays, designed and built by this writer, for access to 75M & 40M dipoles, 20M, 17M and 15M verticals and the dummy load. Various putt-putt wattmeters are used between exciter and amplifier to measure drive power and SWR.

Accuracy of measurement
We use the average of the two wattmeters mentioned above. Both manufacturer's spec. an available inaccuracy of + or - 5% of slug rating at a mid-scale reading. This works out to be an error factor of + or - 125 watts at mid-scale. The maker's don't even mention accuracy at close to full meter right deflection. Remember, these wattmeters are the bottom of their line, most inexpensive of the manufacturer's offerings.

Will this little box perform? Does it run?
Well, of course it runs. Dollar a watt please.

I went through my systematic load test for the following bands on the dummy load. Drive signal is CW.

Data below is grouped:
Grid Current MA / Plate Current MA / CW Power Out Watts

Band	Loafing Column 75W. Drive Grid/Plate/Power	Near Legal Limit 100W. Drive Grid/Plate/Power
1.90 Mhz	75/1000/1,000W	100/1,000/1250W
3.90  Mhz	50/600/1,000W	100/1,000/1350W
7.200 Mhz	75/1,000/1,100W	90/1,000/1,350W
14.225 Mhz	75/950/1,200W	90/1.000/1,300W
18.140 Mhz	45/450/850W	90/1,000/1,250W
21.350Mhz	70/900/1,100W	90/1,000/1,300W
28.400	85/1,000/1,000W	95/1,000/1,250W

In this amplifier ten meters is enabled by clipping the green wire in the tuned input circuit area. It looks like a little green worm here.

Conclusion
In CW mode this nice amplifier is near legal limit. Although we have no way of measuring PEP, I'm sure it will exceed legal limit in SSB with 100 watts drive. Since Hams hardly ever exceed the legal limit in this manner, be careful, you don't want to be one of those, do you? If you want a big legal limit voice on side band this is a great value, probably the most legal limit amplifier for your money in the industry today at a - less than $1,300.00 street price.

Noise fix?
This amplifier is quiet. For really quiet operation for SSB use a 12 watt, 200 ohm wire wound variable resistor (rheostat) in series with the fan wiring. Notice in the picture of the back end of the amplifier a small knob under the 12 DC RCA connector. Without young perfect ears, amplifier noise with this arrangement is really hard to detect.

If you would like to publish an article here contact Bob Hutchinson, N5CNN.