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[AMPS] Filament voltage/bad advice

To: <amps@contesting.com>
Subject: [AMPS] Filament voltage/bad advice
From: W8JI@contesting.com (Tom Rauch)
Date: Wed, 20 Jun 2001 11:07:34 -0400
Hi Bob,

Harmful advice was presented in the Hint&Kinks section of July 
2001 QST.

One should NEVER *under any circumstance* reduce the filament 
voltage of a indirectly heated tube below the tube or equipment 
manufacturer's minimum recommended operating voltage. Any 
reduction below that point in a metal-oxide cathode tube can 
"poison" the cathode and permanently damage the tube.

Secondly, MOX cathode tubes can require inrush limiting, and are 
sometimes as much or more susceptible to damage than directly 
heated tubes. The indirectly heated cathode has a long thermal 
lag, causing heater areas in closer contact to the cathode to 
remain cold for a long time while areas further away from the 
cathode instantly heat. This causes hot spots in the heater, where 
resistance and heater dissipation is much higher than normal. The 
problem is not the same as in directly heated tubes, where the 
filament can mechanically distort and short to the grid, but rather 
one of reduced heater life from opening of the heater.  

Some indirectly heated tubes with larger cathodes, like the 
3CPX5000 and its little brother, the 8877, have a tendency to 
maintain high starting current for a very long time until the heater 
temperature equalizes along it's length.

One of the best guarantees of proper inrush performance is to NOT 
use "overkill" filament transformers, chokes, and wiring. Use the 
minimum size components necessary, and you will have built in 
filament inrush protection. Many amplifiers, the SB series 
Heathkits, the Ameritron series with separate transformers, and so 
on have absolutely no problem with inrush despite not having a 
filament step-start. Transformer and component resistances limit 
inrush current without external circuits. It is true that amplifiers 
using filament windings on large HV transformers are begging for 
problems unless a step-start is added.
 
Finally, reducing voltage until CW power drops, and then bringing 
voltage up until full power is just restored is no guarantee that IMD 
performance will be within spec. Normal peak emission is several 
times the average emission current, and so the tube must be 
*comfortably* above the point where full peak power is reached on 
the worse case band. Once again, indirectly heated cathodes 
should NEVER be reduced in voltage below manufacturer's 
minimum recommended voltage.  

It is unwise to randomly modify amplifiers based on folklore and 
popular opinion unless we thoroughly understand what we are 
really doing. Reducing filament voltage is mostly a spin-off of 
commercial applications where NON-linear class C PA's with 
overkill tubes (like 4CX5000A's in FM transmitters operating at 20% 
of normal rated power) that simply don't need the full emission 
capabilities of the tube. Reducing voltage can be harmful to tube 
life, as well as the quality of our emissions in linear modes. 

In amateur service, very few (if any) failures are due to voltage 
correctable emission life of tubes unless the filament or heater is 
operated ABOVE or BELOW rated voltage.          

  
73, Tom W8JI
W8JI@contesting.com 

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