811 Triode
The 811 has to be the most useful of all higher power transmitting tubes.
I got started in ham radio in 1959. That year the December issue QST had an article on the use of four 811s to build a medium power linear. The basic idea was to have one RF deck with the four 811s, one power supply, and five separate, switch selectable, pi-net output circuits covering 80 thru 10 meters.
Now, nearly 50 years later, that is still the most logical, economical, and reasonable approach to high power.
The 811 is still in production. At least in China and it is one of the tubes the Chinese know how to make.
Used to be you could get NOS, American made, 811s for five bucks each. Today they cost you fifteen bucks each and are made in China.
Over the years I have homebrewed linears, used linears made from kits, and also owned commercial amps. This equipment used 813s, 811s, 4-400s, 572Bs, 4CX300s, and 4CX1000s. Out of all of these choices the amps using the 811s were the most reliable, most inexpensive, and easiest to operate. Here is why.
811s are perfectly happy running at 1500 volts on the plate. The lower plate voltage reduces power supply cost and allows less capable components to be used in the pi-network of the plate circuit.
Filament requirements are easily met. 6.3vac at 10 amps is more than sufficient to power a quad set of 811s. You don’t need to wind a special filament transformer or buy a special 5.0 volt high current transformer.
Each tube requires a 4-pin socket which is a fairly inexpensive and standard item, mounts with two screws, and does not require any special sized chassis hole.
Cooling 811s is easily done using simple fans. No need for high volume blowers. Some applications (like the Tiny Tim) can be implemented without need for any forced air cooling at all.
A set of four 811s will easily run a 1KW input without need for a special 220vac line connection.
811 linears are easy to homebrew because low cost parts are still readily available.
Of the commercial 811 linears the Collins 30L-1 is the best by far. Quiet and strong it is a marvel of good engineering and reliability. It is also very fairly priced and can be purchased, used, in good condition for as little as $600.
In contrast all the 4-xxxx series of tube linears fall far short of the 811s benefits by requiring higher plate voltages, higher voltage components, special cooling considerations, expensive hardware and sockets, and special biasing and driving circuits. In short all the things that make the 811 so desirable are missing in the 4-xxxx series of tube linears.
Probably the most significant thing is the missing lower cost. While four 811s will set you back about sixty bucks, you could easily spend ten times that on a 4-xxxx tube equivalency.
I still have four linears. One is a dual 811 Tiny Tim. Another is a Yeasu FL-2000B running a pair of 572Bs. A third is an underpowered pair of 4-400s in a short rack mount cabinet. A fourth is a desktop pair of 813s. All four of these are all-band, band switching linears. I am seriously considering turning the Tiny Tim into a 75 meter only amp, converting the 4-400 amp to a quad of 811s, and setting up the 813 on 20 meters permanently.
That still leaves me with one all-band amp and allows me a higher degree of operating convenience. At least on 75 and 20 meters.