30 years ago an array of amplifier tubes were available for reasonable cost. 3-500Z, 4-400. 3-1000Z, 572B, 3CX1500, 4CX1500, 3CX1200, 8877, 8874, etc. Some are still available, but the cost is about $1.5K for the mighty 8877 tube now. Unless you have deep pockets, you best be very careful of the RF amplifier you purchase. The tube itself can cost more than your entire radio shack. Some used RF amps do not have new replacement tubes available. I would never own such an amplifier. Three types of RF power tubes are still available at reasonable cost- 811A, 572B and the 3-500Z. The 811A has a small plate with no reserve capacity during tuning an amplifier. I do not recommend the 811A tube. The better tubes left for sale are 572B and 3-500Z. The 572B can be purchased for about $60-70 each and the 3-500Z for $200. Both are from China and are of reasonable quality. Early Chinese 572B were terrible, but current manufacture has seemed to resolve quality issues. So, if you are budget minded, I would only buy 3-500Z or 572B RF tube amplifiers. The 811A amplifier is ok with drop-in replacement 572B tubes. These amps only produce up to 800 watts PEP out. Two 3-500Z can approach legal limit power output and four 572B are good for about 1200 watts output with the 2200-2400 high voltage amplifiers . Four 572B in an Ameritron AL-811H will only produce 800 watts PEP out and the three tube AL-811 will make the same power with three 572B tubes due to the lower 1500 volt power supply. If you purchase older 572B amplifiers such as the Heathkit SB-200 or Yaesu FL-2100B, these amplifiers are long over due for HV capacitor replacement that is an extra cost. However, if you are handy, a new filter capacitor bank is about $30 and you will need a $30 interface keying module to handle the SB-200 160 volt keying voltage. IMO you are better off with the newer AL-811 with three 572B tubes and this amplifier has 160 meters.