I started using an old 1970 Yaesu FL-2500 due to the Chinese 811A and 3-500ZG tube failures in my amps. One Taylor 3-500ZG is $230. I sold my AL-811 and Amp Supply LK-500zb amps. The Yaesu FL-2500 uses five 6KD6 sweep tubes. Vintage sweep tubes do not have failures when operating within specifications and the 6KD6 is $25 in NOS. The 6KD6 tubes in NOS are about same cost as four 811A Chinese tubes. Findatube sells NOS 6KD6 for $25 each. Most sweep tubes were never rated for linear amp service, but RCA did rate the 6MJ6 (6LQ6) at 200 watts peak input in RF service. It has a 30 watt plate dissipation. The 6KD6 are 33 watts plate dissipation so I assume can be operated at 220 watts peak input. Five tubes should perform and last years with 1100 watts peak input. That produces 660 watts at 60% efficiency and the 100 watts to drive the tubes is added in the output making for a 750 watt PEP out amp. Some have stated on eHam reviews getting 800 to 900 watts PEP out, but is basically no difference on the receive end in signal strength. The Dentron, Amp Supply and Ameritron RF amps use four 6LQ6 tubes thus should not be used beyond 500 watts PEP out. Note the expensive 6LQ6 can be replaced with much cheaper 6ME6 tubes. The Yaesu FL-2500 does not have a tuned input, but is less than a 2 to 1 SWR thus modern HF radios have a built-in antenna tuner that will flatten the SWR. Most older tube radios will tune a 2 to 1 input SWR. The Yaesu requires a rebuilt power supply due to the age. Three 500 volts and two bias capacitors are all the amplifier requires. The bias caps are 220uF at 63 volts. The HV caps are only 100uF each in series for 1500 volts (Do not replace with 450 volt capacitors). 100uF is only 33uF at the high current thus makes upwards of 100 volts AC ripple at peak current. I used three 560uF replacement capacitors for much lower AC ripple, The power switch is already stressed using three 100uF capacitors, so I installed a soft start relay and a 4 ohm 10 watt resistor in series with the 120 volt AC line. Double the resistor value for 240 volts. 4 ohms at 120 volts limits the inrush current to about 20 amps and having the relay coil on the resistor output side delays the relay about 1 second. Then, the relay contacts short across the resistor taking it out of circuit. 1 second prevents the small resistor from burning out. I used a superior design wire resistor with the winding on the outside vs the totally enclosed cheap ceramic resistors, The Yaesu FL-2500 is 160 meters thru 10 meters. It weights 44lbs or about 15 lbs more than the Dentron, Amp Supply and Ameritron sweep tube amps. Most of the weight is in the power transformer thus is very heavy duty for a 750 watts PEP out amp. It also uses a separate filament transformer making the design even more conservative. Producing 750 watts PEP out makes for only 1 S unit less than a legal output RF amplifier. They usually sell for about $300, but can be difficult to find for sale at times. If you are tired of Chinese tube failures as I become, a sweep tube amp is an interesting option now. Only other option for tube reliability is an Ameritron AL-811 with three NOS Cetron 572B tubes that costs at least $100 each. Some old tubes can become gassy, but I never seen a gassy sweep tube or Cetron 572B tube. $800 for a used Ameritron with three NOS Cetron 572B or about half the cost for the Yaesu FL-2500 with new 6KD6 tubes. Some will state sweep tubes fail in a year, but that in not true even remotely so. If a sweep tube amp is not enough for you, go buy a $2000+ used 8877 amplifier with a $1200 spare tube if you can find one.