There are those who will defend until death that since
Eimac does not specify the need for inrush current limiting
that it is not required. Most tubes are in commercial service
and are not turned on and off as they are in amateur service.
My personal opinion is that it is just good common sense and
good engineering practice to include inrush protection in amp
design. There are several ways to do this. If the correct filament
transformer or filament winding is selected, this in itself is self-
limiting because saturation occurs in the core of the transformer
before excessive current can be delivered to the load. (heater)
A cheap and easy method with a multi-tapped filament transformer
is to set the filament voltage slightly high and insert a low-ohm
power resistor in series with the primary of the transformer. It can
be left in the circuit all the time. The same thing can be done with
a small variac and series resistor if the transformer is not tapped.
This allows for precise adjustment of heater voltage at the tube
pins and limits inrush at the same time. The problem becomes a little
more complex if the heater winding is on the same core as the HV
winding, but is doable. The cost is just a few bucks, and is good
insurance to protect those costly tubes. It certainly does not hurt anything.
99.9% of the light bulb failures I have witnessed in the past 60 years
have occurred when the switch was turned on. This shock has got
to be no good for a cold heater assuming a robust filament transformer
is installed.
(((73)))
Phil, K5PC
Subject: [AMPS] Filament Inrush Current Limiting
>
> Specifically, is it necessary to limit inrush current at turn-on in a 8877
> tube?
>
> If some filaments do not require current limiting, what is the characteristic
> of a filament which requires limiting the current (I understand that a cold
> filament has less resistance than a hot one) ?
>
> I have not seen a definitive answer although I have not been subscribed here
> for all that long.
>
> Thank you to those who contribute here to make information available.
>
> Dave, WT8R
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