[Amps] Question for the tetrode experts

[Carl]

> Quoting Vic K2VCO <k2vco.vic at gmail.com>:
>> If I had it to do over again, I think I would use a triode!
>
> Nothing wrong with a triode, but you can have more control with a tetrode.
>
> Working with a 4cx12,000 last night, I noticed that with zero screen 
> voltage the transmitter was able to make around 10% rated power output.
> Add a little voltage with the screen variac, and it smoothly rises to 125%
> with no more than 50ma screen current at any point.
> The 4CX20,000s are even better at making power with low screen current.
> The 1000s are much more temperamental, but the small pentodes ... the 
> 5cx1500s
> will make 2.2kw dc carrier in FM service 24/7 for 18 months or more.
> Swap out the 4cx1000 for a 5cx1500, and in ham service, it could be  your 
> last tube.
>
> Don W4DNR

My big tube experience stops at a 4CX5000A!

I dont know how many hours my 3CX1000A7 had on it in TV translator service 
but when I built a 2M GG amp it had no trouble making 1500W +.  Now, almost 
30 years later with the same driver brick and transverter, and tuning 
settings, it does the same.
That is what I call a GOOD tube plus I have another that tests the same just 
in case.

For tetrodes I converted a NCL-2000 engr prototype to 6M in 1964 and 
replaced the early 8122's with no screen supports a few years later with 
1967 date codes. The power held at 1200W until about 10 years ago when it 
slowly decreased and they were replaced about 5 years ago at 700W. Since it 
was only retuned when a new antenna was used and screen current was always 
at 10-12ma it didnt get the abuse of a typical regularly retuned HF amp.
The only mods were to add Ians Tetrode Boards in the mid 90's and convert 
the plate relay to 12VDC and drop the DC to the 12VAC TX/RX relay to 3.3V 
where it continues to do its job.
It is on its third set of electrolytics in those 50 years since built, 
original transformers, meters and blower.

Carl
KM1H