A lot of HF amplifiers are available for sale. Some are lower cost and may only produce 500 watts out. Many are not worth considering due to design issues. And, some need rebuilding. So, with the older amps you are better off with someone else that spent the money and labor on the rebuild. Fortunately many Hams enjoy rebuilding thus a rebuilt amp is not much more expensive than a stock amplifier. The power tube is a major consideration due to some are not being manufactured now and command very high prices with many NOS prices over $1K. Other tubes such as the Chinese new 572Bs only last a few years. Old Cetron 572B lasted 30 years. The only long lasting tube is the Chinese 3-500Z and some hams report the Chinese 3-500Z is still full power output after 10 years use. It is worth a few extra dollars for a 3-500Z based HF amplifier vs the 572B, 811A and sweep tube HF amplifiers in the long run. All amplifiers have a tuned input that is important for solid-state transceivers without an antenna tuner. Some can operate on 120 volts AC if you do not have a 240 volt outlet in your ham shack. I also mention transformer size as it is the 'engine' behind your amplifier. The contenders I am listing are well known not to have issues and are reliable. Some are old enough to need a rebuild so as I mentioned buy rebuilt. The best HF amplifier in mu opinion are: Heathkit SB-220 Drake L4B Amp Supply LK-450 Amp Supply LK-500 Ameritron AL-80A (Heathkit SB-1000) Ameritron AL-80B The amplifiers listed are about $800 used. The Heathkit SB-220 uses a pair of 3-500Z tubes on 80 to 10 meters and produces about 1100-1200 watts output at 240 volts AC and slightly less on 120 volts AC. This amp can be found rebuilt for reliability. Some rebuilds includes a low voltage keying interface. If not you will need to purchase a keying interface to reduce the 160 volts to 12 volts. The SB-220 has a rather light weight 23lb transformer that is not powerful for 1100-1200 wats output, but failures are uncommon. I would only buy a rebuilt SB-220 due to the age. Some operate this amplifier on 120 volts AC. The Drake L4B uses a pair of 3-500z tubes on 80 to 10 meters and produces about 1200 watts output using 240 volts. I would not operate this amp on lower voltages. Cooling of the tubes are thru a glass chimney thus the sockets and pins are kept cool. The L4B uses a 29lb transformer that is powerful enough to use for AM. Rebuilt L4B are harder to find. If you have the skills it is worth rebuilding. I believe it needs a keying interface. Many ask higher than $800 and are not rebuilt. 120 volt AC not recommended. The Amp Supply LK-450 uses a single 3-500z tube on 160 thru 15 meters (easy mod for 10 meters) and produces 800 watts output. Some report 1000 watts out on 75 meters. This amp is over shadowed by the Ameritron AL-80B thus is rather rare, however it is very well built. I do not know the transformer specifications. If one was available vs the Ameritron AL80A or AL-80B I would not hesitate to buy it. It does not need rebuilding and it has great reviews on the eham site. 12 volts keying voltage. 120 volts AC is ok. The Amp Supply LK-500 uses a pair of 3-500Z tubes on 160 to 10 meters and produces 1200 watts output. The transformer is a premium Peter Dahl hypersil at 23lbs weight. Hypersil core makes more power per weight vs standard IE lamination transformers. I believe the transformer would be easily equal to a standard 30lb transformer. It does not need a rebuild or keying interface. I believe the LK-500 is best buy of all the amplifiers and I seen many at $800, some at $600! Not recommended at 120 volts AC. The Ameritron AL-80A is very different vs the original Ameritron AL-80 that had many issues and not recommend at all. The AL-80A uses a single 3-500Z tube on 160 to 15 meters and produces 800 watts output. The transformer is 23lbs. It does not need a rebuild or keying interface. I would not hesitate buying an AL-80A vs the AL-80B. 120 volts AC is ok. The Ameritron AL-80B uses a single 3-500Z on 160 to 15 meters. Many used AL-80Bs have the 10 meter modification. The amp produces 800 watts output and some have reported higher. This amp is talked about a lot on ham sites and conservative hams recommend the 800 watt output. It has a 26lb transformer. That is a lot of transformer for a single 3-500Z and indicates the conservative design. It is still being built this newer models will cost over $1000. I find older AL-80Bs for $800. It does not need rebuilding or a keying interface. Some consider this amplifier a best buy. 120 volts AC is ok. When purchasing these 3-500Z amps verify the amplifier has a healthy tube(s) if you can. New 3-500Z are about $200 each. I mentioned 572B or 811A amplifiers are not recommended die to early failing new tunes. Sweep tube definite not recommended and only produce 350-400 watts output. However, if you cannot raise more that $550, I have seen used Ameritron AL-811H amplifiers for $550. Operate the four tube AL-811H at a maximum of 600 watts PEP out with the 811A tubes and tune with 25 watts at first to get in the 'ballpark', then tune for 600 watts out. You have less than 10 seconds to get this amp tuned or you will burn the 811A tubes weak plates. Good Luck with your hunt for an HF amplifier.