[Amps] Ameritron (et. al.) filament voltages

[Rob Stampfli]

On Thu, Jan 28, 2010 at 03:10:46PM +0800, Alek Petkovic wrote:
> Just as an aside. When the amp is run on 245V AC, here in Australia, 
> the voltage measured on the filament pins, with all 4 tubes lit up, 
> is 7.15V AC. A bit on the high side you could say. Could even call it 
> scandalous.

I've noticed the same here with an AL-80B:  On 240V, the filament
voltage runs >5.5V on a 3-500Z (nominally a 5V tube).  I spoke with
Ameritron on the phone about the issue and their tech told me that
voltages up to 5.6V are considered normal.

I understand that high filament voltages is a contentious issue among
hams and amp manufacturers.  One camp claims it drastically shortens
the tube life, whereas the other claims that in ICAS operation that
other factors will kill a tube long before the excess filament voltage
will do so.

I remain curious, though:  What is to be gained by designing amps that
run 10+% high on filament voltages?  (And, frankly, I've never heard
of a commerical amp that runs low.)

FWIW, I tacked a few current-sensing resistors into the circuit to
lower my tube's filament to a more nominal 5V, and have not noticed
any problems from having done so here, but YMMV.

Rob / KD8WK