DO-35 diode inside vacuum tube ?

[pjp]

Well not really!

But what is this thing that looks like a DO-35 diode inside the glass
envelope of a vacuum tube: http://www.r-type.org/pics/aaa0772b.jpg

(Like to page: http://www.r-type.org/exhib/aaa0772.htm )

 

[Miles Prower]

^^

Looks like a fuse.

 

[pjp]

The 'fuse' is not accessible. If it blows you'll have to throw out the tube.

Is there some seriously catastrophic scenario that needs such extreme protection ?
(I'm a tube noob)
(if it is a fuse, its protecting the gate)

 

[astouffer]

Looks like a fuse

How is the fuse changed when it decides to blow

 

[Richard Ellis]

Its' a small signal RF diode............the kind we all used making our first AM radios!!
__________________________________________Rick........

 

[Robert McLean]

Looking at the close up view in the last picture on the page here
http://www.r-type.org/exhib/aaa0772.htm

it appears to me to be some sort of mechanical support or stiffener. The lead going through it does not seem to actually go anywhere, it seems to be curved like a spring and is pressing against the bottom structure. Perhaps it reduced microphonics ??

If it was electrically connected somehow that I cant see then perhaps it is some sort of spark gap protection ??

 

[pjp]

It isn't a support.
Its the only connection to the grid (gate).

 

[kevinkr]

It isn't a support.
Its the only connection to the grid (gate).

It appears to me that it goes to pin 5 which is open, and the thin line through it appears to be nothing more than a distorted image of a lead behind it. To me it appears to be nothing more than a support. (I'm using a very good CRT monitor and image magnification and that is what it looks like to me.)

Tubes have grids, fets of various descriptions have gates.

 

[pjp]

It appears to me that it goes to pin 5 which is open

Now its beginning to make sense to me - perhaps the purpose is to provide mechanical support without the inductance of a lead (at VHF) ?


Is such a mechanism employed in any other tube ?

 

[refference]

................ mechanical support without the inductance of a lead (at VHF) ?

I don't think so . The purpose is to provide INSULATION , between the internal electrode where it is connected and the
external pin 5 .

kevinkr is correct !!

Regards ,

Carlos

 

[kevinkr]

I don't think so . The purpose is to provide INSULATION , between the internal electrode where it is connected and the
external pin 5 .

kevinkr is correct !!

Regards ,

Carlos

The glass bead provides a relatively high Q and very small capacitance between the adjoining leads while providing additional mechanical support. Pin 5 happens to be a convenient support point. I suspect this tube was probably intended to be relatively simple and inexpensive to make.

It is not unusual to bring out several grid leads in types intended for grounded grid operation (sometimes well into the VHF range and beyond) as this reduces the effective inductance in series with the grid, each doubling of the number of leads reduces it by half - in theory and really only if the lead lengths are equal.. (5842/417A for example has 4 grid leads)

 

[pjp]

Thank you Kevin.