I have a dual 813 GG amp that I have been running for the last year or so. 80 watts input will give about 800 to 1000 watts out depending on what band its on. On 75 meters I can push it to 1200 watts out. I admit to abusing the tubes pretty badly, but they seem to keep taking it and delivering good service. Most of the time I don’t push it past 800 watts because on the other end it’s pretty hard to tell the difference between 800 watts and 1.5 kw. As a rule I don’t exceed 125 Ma for grid current. As long as you don’t make the plates glow you will be just fine. A 30500z is made to show some color on a graphite plate, however an 813 is not designed to show ANY red on the plate during normal operation. If you get a red plate at all during tune up then you need to get faster or don’t drive it so hard.
Please see my review on 813 tubes:http://www.eham.net/reviews/detail/8820
I have been using a set of 813’s that are over 40 years old in my new home brew GG amplifier for the last year. These tubes are tough as nails. The factory anode dissipation rating on an 813 with a graphite plate is 125 watts, however in practical service in a grounded grid amplifier they will live a long happy life at 175 to 200 watts of plate dissipation in ssb service. I run them with 2800 volts on the plate and others using these tubes in HB amps have successfully run anode voltages up to 4KV. Now obviously this kind of service is way past the OEM rating and borders on abusive treatment. After a year of running these tubes in my amp and heavy contesting, I can attest to the toughness of the 813 tube. They require a very small breeze to keep the seal temps to a safe level. A 3 inch muffin fan is plenty. This kind of service is fine for SSB and CW service ,but I would not use them on high duty cycle modes such as RTTY, SSTV, PSK31 etc…
813 tubes use to seam like they grew on trees ,but as of late the supply of NOS tube looks to be drying up at warp speed. The 813 tube is currently being produced in china and Russia as FU13, GU13 and 813 part numbers in both metal and graphite anode varieties. The graphite plate version is the only one I can recommend for transmitting service. These tubes that are imported into the USA currently are made at one location in China and one location in Russia in a production run of several thousand at a time. Then they are private labeled by many other brands such as Green Stone USA, Sino, Shuguang and others. As of march 2012 the street price of an imported tube is 50 USD and a NOS tube can range from 30 to 100 USD depending on condition and dealer.
At this time availability of import tubes is unclear as there have been a lot of quality control issues in recent production runs and several well know domestic dealers and online retailers no longer carry NEW imported 813 tubes. The 813 has developed a huge following in the high end audio amplifier market and that demand seams to be driving production of tubes and sockets for the time being.
From the perspective of using an 813 in new equipment, my personal feeling is this is a good choice because they are durable, cost effective and will in most cases out last and perform most designs using 811 or 572b tubes. The 813 tube has a very high amount of output capacitance and thus you will have to make a very careful design is you want good performance on 10 and 12 meters. However on 15-160 meter you could expect 700 to 900 watts of output from a pair in GG service depending on design and anode voltage.