[1000mp] Amps & IMD Low power

[Tom Rauch]

> Wow. I didn't see the  msg that preceded this, but should I assume
> that the "MP" should always be run at full output, even while driving
> (in my case) an Alpha 87A? My amp only requires about 40-50 w. Maybe
> that explains some problems I've had with the units.
> 
> Incidentally, why shouldn't the MP be run at reduced power? What is
> the function  of that power control then? What am I missing?

Cathode driven triodes are much more forgiving than tetrodes for 
tuning and drive errors. The lower the ratio of driving impedance to 
the output impedance, the more negative feedback and the less 
critical the PA is to drive power. 

That is why an amp like the AL1200 will live comfortably for many 
years with 200 watts of drive, and have good IM performance, while 
the same *ratio* of excessive drive will eat up the grid in a 
3CX800A7 in a few minutes!

Same goes for mistuning. Despite having the grid wire located outside 
the electron path in tubes like the 3CX800, the grid current 
skyrockets when the plate impedance is too high for the amount of 
drive applied. That means loading and drive power are both critical 
to tube life and IM distortion products.

Tetrodes are even worse, because they have screen supplies that must 
be current limited, and bias supplies that are often poorly regulated 
for grid current changes. Not only that, they have two grids that are 
critical to operating parameters rather than one. They are the most 
unforgiving of any tube for any errors.

The general rule is the lower the potential power gain of the device, 
the more immune it is to operating errors of any type both for life 
and distortion.

Backing the power control off in some rigs, like the FT1000D, is not 
terribly harmful for IM or overshoot problems. As a matter of fact 
the FT100D is one of the few rigs **totally without** overshoot 
problems if operated properly without too much ALC (it has a drive 
and a power control).

Rigs that do it all with one control virtually always have a leading 
edge overshoot that remains constant no matter what output power you 
have set. As an extreme example the early IC775DSP I had overshot to 
as much as  350 watts until the ALC caught up, and it did that even 
if it is set at 20 watts output. It was unusable with my AL1500, 
because it would either arc the bandswitch, the tuning cap, trip the 
grid protect system, or flash my antenna tuner over on leading edges.

Since power overshoot almost always gets WORSE as output power is 
reduced, reducing power aggravates this problem. The most 
unfortunate thing is the tubes and circuits that can least take the 
overshoot are in the very same amps that require you to reduce 
drive!!

These leading-edge transients not only eat up the grids in MOx 
cathode tubes, they blow FET's in solid state amps. They can cause 
bandswitches to arc, and other voltage sensitive components to arc or 
fail.

Having an amp with low driving power compared to rig output power is 
not any type of advantage! Quite the contrary, it is often a problem 
for both signal quality and equipment life. If you do have an amp 
that drives easy, tune if for full power output at full drive 
power....even if the output is 3 or 4 kW. Then back the power off to 
legal power.

If the amp won't take full exciter drive without failing, you need a 
different amp or an attenuator pad.73, Tom W8JI
W8JI@contesting.com