[Amps] multi-tube amplifiers

[Tony King - W4ZT]

Harold Mandel wrote:
> Gents,
> At some point, running more than one good tube can only achieve
> limited results, measured "bang-for-buck" or "steps-in-reinventing
> the wheel."
<snip>
There are only two justifications for the multi-tube build:
1) You already have all the components and just want to use them.
2) It's something you just want and have to do.

Beyond that, the gain just doesn't justify the expense and trouble.

 > If I wanted X amount of power, the first consideration was the power
 > supply. Next, a budget for the RF department. For cheaper an
 > expenditure for everything a 3 tube 8877 would cost I could build
 > an AB2 3CX5,000A, and even a 3CX6,000A device.
<snip>
Exactly right, and therein lies the real answer. As I have said before: 
Tubes are like women. If you feel you must have two, keep them in 
separate houses and don't let them know about each other. If one isn't 
good or big enough, get a better, bigger one!

Multiple tubes can lead to problems at the higher frequencies, increased 
instability problems, balancing problems (not many folks even know how 
to build a grounded grid amp so you can meter current for the plates and 
grids of multiple tubes separately). Most of the multi-tube amps were 
built to satisfy the manufacturer's need to produce an amp that will do 
the legal limit at the time of manufacture. Most of the time, for them 
to purchase a single tube to do the job, would make the amp cost more 
than most hams would or could afford to pay. That's why you see a lot 
more pairs of 3-500Z's than you do single 8877's.

Remember, the BEST you can get by adding another tube is 3 dB.  If you 
add a third tube you've hardly made a dent in the S-meter. If you run 
four tubes you are only 1 S unit above that first one tube.

Just because you have a big tube doesn't mean you must be illegal.  A 
larger tube run at lower powers usually means a cleaner amp and a much 
extended life.  Temptation is there just as there is when the 
accelerator only goes half way down, but if you are a practical person, 
you will see the advantages.

Happy building!
73, Tony W4ZT