The screen current will certainly vary if the tube load resistance is
changed. Its often best to look for screen I max for best tuning,
instead of a plate current dip. But as for it being momentary, no, it is
an average screen current that moves slowly as indicated by a meter.
73
John K5PRO
On 2/19/25 8:05 AM, Lukasz wrote:
On Wed, 19 Feb 2025, 06:09 John Lyles, <jtml@losalamos.com> wrote:
(...)
Anyway, this is how i think about these things. And, as stated
below, if
you go too far off normal, the components in the matching network
are no
longer able to compensate to bring the real part of the impedance
to the
optimal point of the tube for a given plate voltage and power. That,
along with the reactance that gets transformed back to the tube,
are all
bad things, tube or transistor. But how far is bad, requires doing
the
math of calculating the circulating currents and voltages.
This explains why we (owners of this amp -it is very popular in Poland
, Russia and some other countries) can get away with tuning it for
let's say 3.7MHz and use it on entire 80m (which ends at 3.8MHz here
on the high end). All of the Pi tank elements are massively oversized.
To the point where people replace the factory 1500W output tube with
up to 5kW tubes and run it successfully . But it also explains why the
original tubes have a reputation for being "finicky"and "easy to
break" in sone circles and many replace them with two pentodes like
GU81m (which they then run at double the datasheet power). While the
Pi tank can definitely take it the tube is another story. If it's run
at 1000W I think it's going to be fine most of the time ..but let's
say we run it at 1800W (as people do). Then we go to the very end of
80m when tuned sonewhere in the beginning....
There is one more variable. This amplifier as it comes from the
factory contains the preamp and the coupling between the two is tuned.
I found this coupling is the first thing that limits the output power
(at same drive level) if I stray quite far from the original tuning
frequency .
The most popular modification of this amp is to remove that preamp
completely and drive the final tube through a small attenuator with
30W.( I drive it with 200mW)
I can definitely see how all of this together is a recipe for broken
tubes.
You mentioned higher momentary currents and voltages , depending on
where we are in the AC cycle. I wonder if this could also create high
momentary screen currents that could not show on the meter nor trigger
the protection because of how brief they are.
Very interesting . These amps were made for the military/TV service.
There they would have 10 frequencies programmed and they would sit at
100% output on one of them never straying a dozen KHz away (usually FM
was the modulation of choice but linearity was very important because
the signal they sent was a dozen telephone channels, maybe some data
too. They wanted as low IMD as possible ). It is only in our amateur
use they started to be used this way.
73, Łukasz
73
John
K5PRO
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|