what's the purpose of pins 3 and 7 in an 8pin VR tube?

[jarthel]

it seems these are not connected to the internals.

thank you.

 

[sorenj07]

Structural support

 

[jarthel]

Structural support

if that is all its purpose, why can't they just have the pins without the wires connecting 3 and 7?

 

[Tubelab_com]

These pins have a real use. In many regulator circuits the output voltage will run away if the regulator tube is pulled out while the circuit is operational. The intent of the jumpered pins is to shut the power supply down if the tube is removed. This is pointed out in some tube manuals and data sheets. The RCA RC-30 (AES reprint) page 116 shows these pins wired is series with the line voltage supply. This explanation is offered:

"Removal of tube from socket removes voltage from load."

 

[Sheldon]

it seems these are not connected to the internals.

thank you.

They can be used as a safety link. For instance, you can put 3&7 in series with B+. If the VR tube is removed, no B+ out.

Sheldon

edit: can't think fast enough. Tubelab beat me to it.

 

[jarthel]

these pins wired is series with the line voltage supply. This explanation is offered:

do you mean "line" wire of the AC mains or B+?

 

[Sheldon]

do you mean "line" wire of the AC mains or B+?

B+. But if it made sense, you could use it for other similar purposes. I wouldn't use it for the AC line, however.

Sheldon

 

[Tubelab_com]

I wouldn't use it for the AC line, however.

I wouldn't either. I was looking at the circuit published in a tube manual written in 1970. The circuit has probably been around since the 1950's. If you have ever worked on a 1950's vintage radio you know that safety was not a concern back then. One side of the AC line was often wired DIRECTLY to the chassis. The polarized plug had not been invented yet.