[ARC5] 7V and 14V Tubes

[Geoff]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "J. Forster" <jfor at quikus.com>
Cc: <Arc5 at mailman.qth.net>
Sent: Monday, April 30, 2012 9:27 PM
Subject: Re: [ARC5] 7V and 14V Tubes


> On 30 Apr 2012 at 17:59, J. Forster wrote:
>
>> That logic explains the need for a separate transformer winding, but
>> not the 5 Volts.
>>
>> Having a few hundred volts between the heater and cathode of a tube
>> can lead to failures...  big time. Viz. the AC power supply for the
>> Cabadian WS19. A 6X4 failure often destroys the unobtanium power
>> transformer.
>
> I suggest that the 5 V tubes came about during the time that most tubes 
> had
> 2.5 V filaments. 2.5 X 2 = 5.0.
>
> There is probably some historical reason for it, rather than a technical
> reason.
>
> Ludwell Sibley's book may tell us.
>
> Ken W7EKB


There is some consensus that 5V was the first voltage that allowed 
sufficient emission and reliability resulting in the 1927 UX280 and 
continuing sort of by habit until the 50's when a whole bunch of 6.3V 
rectifiers arrived to support color TV, audio, industrial needs and even 
then seperate transformer windings were common.  The 280 came out at the 
same time as the 2.5V receiving tubes for the first AC operated sets. 
Another reason for 5V was likely I*R losses involved in using a 2.5V type 80 
and the combination of brittle transformer insulation as well as socket and 
wiring.

And, of course, a directly heated rectifier of any voltage required its own 
winding.