[Johnson] What is a tube shield insert?

[David C. Hallam]

On 1/22/2012 9:45 PM, Lee Craner wrote:
> Hello list:
>
> As I complete my Pacemaker restoration, I have noted in the manual's parts list, "22.1493 Inserts, tube shield, XS4, XS5."
>
> What is a tube shield insert?  The tube shields themselves are listed separatly (part number 22.1218-1 for the 9 pin variety).  V4 and V5 are the ECC-81/12AT7 balance modulator tubes.
>
> On a related note, the 9 pin shields for the 12BH7 and 6CL6 seem to be too short (they fit, but with no room for ventalation.  Is this normal, or do I have the wrong shields?
>
> 73,
> Lee WB6SSW
> ______________________________________________________________
> Johnson mailing list
> Home: http://mailman.qth.net/mailman/listinfo/johnson
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Johnson at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
Lee,
If you search the R-390 group archives, you will find long threads 
discussing tube shields, yes or no and if yes, what kind to use.

My take from these discussions was, tube shields probably should not be 
used unless there was a specific reason for having them, i.e., tube 
affected by external magnetic fields.  In that case you really need a mu 
metal shield, high nickel.  The bright shiny shields are not effective 
in providing tube cooling, in fact tube run hotter with them than 
without.  The only shields effective in reducing bulb temperature are 
the ones referred to as "IERC".  They are black and have an insert that 
fits tightly to the bulb and shield.

My Pacemaker did not come with any inserts.  Tubes of the 12BHY type 
take what is called an extra tall.  They can be a little hard to find.

David
KW4DH