DOES ANYONE KNOW, IS THERE A DIFFERENCE BETWEEN A 12SK7 THAT HAS A BLACK METAL CASE AND A 12SK7GT/G THAT IS A GLASS TUBE? I HAVE A RCA 61-8 THAT THE 12SK7 METAL TUBE WAS BAD IN AND WAS ABLE TO PICK UP A COUPLE 12SK7GT/G AT AN ANTIQUE STORE FOR $1. DOES ANYONE KNOW IF THIS GLASS TUBE WILL WORK IN PLACE OF THE METAL TUBE? THANKS.

:DOES ANYONE KNOW, IS THERE A DIFFERENCE BETWEEN A 12SK7 THAT HAS A BLACK METAL CASE AND A 12SK7GT/G THAT IS A GLASS TUBE? I HAVE A RCA 61-8 THAT THE 12SK7 METAL TUBE WAS BAD IN AND WAS ABLE TO PICK UP A COUPLE 12SK7GT/G AT AN ANTIQUE STORE FOR $1. DOES ANYONE KNOW IF THIS GLASS TUBE WILL WORK IN PLACE OF THE METAL TUBE? THANKS.
:
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Yes, there is a difference. The metal tube has its envelope grounded to the radio chassis via pin 1. This provides an electrostatic shield for the tube, so no external metal shield is needed. The glass tube has no built-in shield. However, it will work fine if you equip it with an external shield that is electrically grounded to the chassis. The radio may even work acceptably without the tube shield, but if you get squealing/motorboating, that's a sign that a shield is necessary. Try it without a shield; you can't hurt anything. At least you will be able to quickly tell if the tubes that you bought were any good or not.

Other than the shielding method (and the use of pin 1) there is no difference in performance between the two tubes.

The following pentodes: #SK7/#SJ7, #K7, #J7, 1LN5, 1N5, 1LC5, 78, and many more that I am unaware of, have built in shielding. The plate that you see (or screen mesh) is actually tied to the suppressor grid. The actual anode is underneath this. Often the metal shielding structure completely surrounds the other elements, even on top and on bottom (where the mica spacers are located).

These tubes may work just fine without shields, even where shields might otherwise be required (close tube spacing).

Tubes like the 6U7 and 6D6 (56, 57, etc.) frequently do require shielding, and have no internal shielding provided what-so-ever.

#SA7, #A7, #A8, #SQ7,#Q7, do not have any internal shielding, and so there may be a marked difference in performance when switching from a metal to a glass tube, when a shield is not used.

:The following pentodes: #SK7/#SJ7, #K7, #J7, 1LN5, 1N5, 1LC5, 78, and many more that I am unaware of, have built in shielding. The plate that you see (or screen mesh) is actually tied to the suppressor grid. The actual anode is underneath this. Often the metal shielding structure completely surrounds the other elements, even on top and on bottom (where the mica spacers are located).
:
:These tubes may work just fine without shields, even where shields might otherwise be required (close tube spacing).
:
:Tubes like the 6U7 and 6D6 (56, 57, etc.) frequently do require shielding, and have no internal shielding provided what-so-ever.
:
:#SA7, #A7, #A8, #SQ7,#Q7, do not have any internal shielding, and so there may be a marked difference in performance when switching from a metal to a glass tube, when a shield is not used.

Another problem that sometimes arises is when a G or GT is replaced with a metal, and the #1 pin has been used for a tie point by the manufacturer. This connects the metal of the tube to whatever is tied to pin 1. In your case, of course, that will not be a problem, just something to keep in mind.
Lewis
:

::The following pentodes: #SK7/#SJ7, #K7, #J7, 1LN5, 1N5, 1LC5, 78, and many more that I am unaware of, have built in shielding. The plate that you see (or screen mesh) is actually tied to the suppressor grid. The actual anode is underneath this. Often the metal shielding structure completely surrounds the other elements, even on top and on bottom (where the mica spacers are located).
::
::These tubes may work just fine without shields, even where shields might otherwise be required (close tube spacing).
::
::Tubes like the 6U7 and 6D6 (56, 57, etc.) frequently do require shielding, and have no internal shielding provided what-so-ever.
::
::#SA7, #A7, #A8, #SQ7,#Q7, do not have any internal shielding, and so there may be a marked difference in performance when switching from a metal to a glass tube, when a shield is not used.
:
:
:Another problem that sometimes arises is when a G or GT is replaced with a metal, and the #1 pin has been used for a tie point by the manufacturer. This connects the metal of the tube to whatever is tied to pin 1. In your case, of course, that will not be a problem, just something to keep in mind.
:Lewis
::
:
Thanks for the info guys. The 12SK7 GT/G glass tube works perfectly. I got the radio to work and it is crisp and crystal clear.

::The following pentodes: #SK7/#SJ7, #K7, #J7, 1LN5, 1N5, 1LC5, 78, and many more that I am unaware of, have built in shielding. The plate that you see (or screen mesh) is actually tied to the suppressor grid. The actual anode is underneath this. Often the metal shielding structure completely surrounds the other elements, even on top and on bottom (where the mica spacers are located).
::
::These tubes may work just fine without shields, even where shields might otherwise be required (close tube spacing).
::
::Tubes like the 6U7 and 6D6 (56, 57, etc.) frequently do require shielding, and have no internal shielding provided what-so-ever.
::
::#SA7, #A7, #A8, #SQ7,#Q7, do not have any internal shielding, and so there may be a marked difference in performance when switching from a metal to a glass tube, when a shield is not used.
:
:
:Another problem that sometimes arises is when a G or GT is replaced with a metal, and the #1 pin has been used for a tie point by the manufacturer. This connects the metal of the tube to whatever is tied to pin 1. In your case, of course, that will not be a problem, just something to keep in mind.
:Lewis
::
:
Thanks for the info guys. The 12SK7 GT/G glass tube works perfectly. I got the radio to work and it is crisp and crystal clear.