[ARC5] No Doubt Dumb Tube Question

[Neil]

From: Glen Zook
Sent: Tuesday, June 18, 2013 10:18 AM

> It is still different!  The tube has an indirectly heated cathode even if 
> the cathode
> and one side of the heater do share a common connection.  The electron 
> flow is NOT
> from the heater like in a directly heated (filament) rectifier.  The 
> electron flow IS from
> the cathode.

A minor nitpick: "indirectly heated cathode" contains a redundancy.
A rectifier is either directly heated (has only a filament) or is indirectly 
heated
(has a cathode that is heated by a filament).

You are correct that if one side or even the centertap of a filament is 
connected
to the cathode, the rectifier is still indirectly heated. The connection is 
probably
an intention by the maker to limit the possibility of a heater-cathode 
breakdown.
As someone has pointed out elsewhere, the connection means that the 
insulation
burden is now entirely on the transformer's heater winding.

If the heater happens to come into contact with the cathode at some point 
that
differs from the cathode connection, part of the filament will be 
short-circuited
and a high current will flow in the un-shorted portion(s), leading to 
eventual failure
of the filament.

73 de Neil ZL1ANM