[ARC5] 7V and 14V Tubes

[Richard Knoppow]

----- Original Message ----- 
From: "Geoff" <geoffrey at jeremy.mv.com>
To: <kgordon2006 at frontier.com>; "J. Forster" 
<jfor at quikus.com>
Cc: <Arc5 at mailman.qth.net>
Sent: Tuesday, May 01, 2012 10:10 AM
Subject: Re: [ARC5] 7V and 14V Tubes


>
> ----- 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.
>

     The voltages may have been chosen because early sets 
were run from batteries.  The filaments may have been run 
from dry batteries but more likely from wet cells of some 
sort. Also most early sets had some means of adjusting 
filament current such as a variable resistor in series with 
them.
     I suppose some sort of authentic answers may exist 
buried deep in old literature but one would need a lot of 
time to find them.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com