[ARC5] Glass or Metal Tubes Preferred for Receivers

[Rich Post]

Some years ago, I bought a Philco 38-7 in beautiful original condition.
Turning it on, it suffered the mother of all positive feedback squeals.
Looking closely under the chassis, I realized that a former owner had
removed all the tube shields and cut off the matching shield parts that
were riveted under the tube sockets.  All the signal tubes had been subbed
using metal tubes.  HOWEVER, the original Philco sockets had left out the
female connector for pin 1 on each of the tube sockets.  You can guess the
results from the ungrounded metal on the replacement tubes!  The set likely
was in as nice a condition because the "expert" had put the set away in the
attic.  After noticing the problem, I wrapped a temporary wire link around
pin 1 of each of the tubes and soldered the other end of each link to a
nearby chassis ground.   The set worked fine after that.  I felt sad for
the "expert" who back in the day wanted to improve his Philco. One of these
days I will revisit the temporary repair but will have to replace the
sockets.

Rich KB8TAD



On Fri, Feb 9, 2018 at 4:38 PM, Todd, KA1KAQ <ka1kaq at gmail.com> wrote:

> On Mon, Feb 5, 2018 at 4:56 PM, Richard Knoppow <1oldlens1 at ix.netcom.com>
> wrote:
>
>>    I wonder how this happened. The shell of metal tubes normally goes to
>> pin-1 which is grounded. Some glass tubes have an internal shield to pin-1.
>> It sounds like the pin was shorted to something.
>>
>
> ----- The cold tube was cold for a reason - someone had broken off the
> keyway and stuffed it back into the socket wrong. After recovering I lined
> it back up properly and the receiver worked fine.
>
> Since then I let them heat up, then shut the set off before checking for
> cold cases.
>
> ~ Todd,  KA1KAQ/4
>
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