DIYbanter - View Single Post - Will RF output (transmitter) be the same wattage as audio output?

Ralph Mowery

In article ,
says...

I just referred to the common audio output tubes. Wondering if a
transmitter tube (like an 813) is capable of audio output power in the
same wattage as that same tube would put out as a transmitter (asuming
the plate voltage, biasing, and signal input to the tube was about the
same.

From this thead, I did learn that these tubes need a lot higher plate
voltages than I thought (like 1500 to 2500 volts).

I also learned (after doing some research on the web), that the original
metal 6L6 tubes, did not have the same power output as the glass
version. I NEVER KNEW THERE WAS A DIFFERENCE IN THE
CHARACTERISTICS I thought they were the same tube in a different
container.

*** And this explains something that occurred 40 some years ago. When I
was running my tube power amps for my stereo, (four 6L6GC tubes
Push-Pull Parallel, in each mono block amp), one of the tubes went bad.
I went to my box of used tubes and found a 6L6 metal case, and used that
for a temporary replacement. Although that tube tested GOOD, the amp
just did not sound or perform properly. It did work, but lacked a lot of
the "punch" it had before. A few days later I bought a new 6L6GC, and
the amp worked great once again.

Now, 40 some years later, I learned why that metal 6L6 did not work
properly !!!!

I also learned in recent times that for best performance, a pair or quad
of output tubes should be MATCHED. Back then, I never knew anything
about matching. If it was the proper numbered tube, and a tube that
tested good, I'd just plug it in and GO....

The short answer is that most of the tubes will put out the same power
at audio as they will at RF. Not counting some specific tubes such as
the microwave tubes. Tubes start loosing efficency as the frequency
goes higher into the RF ranges. The common 6146 tube might put out
about 50 watts up to about 50 MHz but will work to 150 MHz but only put
out 30 watts and stay within the ratings.

There is one other thing to look at. That is the class of service.
Running class C will put out the most power but can only be used for
certain types of RF signals due ot the distortion. It can not be used
at all for normal audio amplifiers.

As with the 813 tube, most tubes rated for much over 50 watts output
will often have a plate voltage of over 1000 volts,many like to have
2000 volts or more.

For the best sound the tubes should be matched. They are operated in
what is called a push-pull circuit. Often close to class B. That is
when one tube is putting out power,the other tube isn't during a cycle.
It is at the crossover point that if they are not well matched you get
most of the distortion. Many audio amps often operate more like AB
class to help eliminate that problem. It is not much of a problem at RF
due to the way the output circuit works.