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Securing anode tube cap?

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LAN_403

dmg

Thu Nov 24 2011, 09:16PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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Posts: 627

Normally I see the ceramic pressfit adaters used to sit on the tube as the cap, but since I don't have one of those nor want to buy one, I decided to machine it from some scrap aluminum..

However this is where I had a bit of a concern, which hopefully someone can shed some light on.

I wanted to use a 1/8" set screw on the side of the aluminum cap to fasten it to the tube's cap, but fear the tube will crack once its in use from heat (expantion) and likewise a tight pressfit.
Is it actually possible for the tube to expand that much for it to become a threat to crack at the cap and against the set screw?

This is for an 811a tube, I thought of putting a 1/8" set screw on the side of the cap to hold it to the tube, the top philips screw is just for electrical connection.

Vtcap003

Thank you.

radiotech

Thu Nov 24 2011, 10:00PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546

If it rests on the glass at the top it will be a sink that will cool that part of the tube
and when power is switched off the tube glass will be subject to stresses because
of uneven cooling. The correct caps are available from MFJ electronics.

dmg

Thu Nov 24 2011, 10:06PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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While its not apparent in the angle of the photo, the cap itself sits only on the tube's cap, there is a 1/16" gap between itself and the glass (it forms a lip which prevents the cap from sitting any lower)... unless what you are refering to is the tube's cap making transfer from the glass to the aluminum?

radiotech

Thu Nov 24 2011, 10:19PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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It not bearing on the glass, then it may rock the cap of the tube. Also 811
caps were insulated in most equipment to prevent shock. Many high power
caused the cement holding the cap on to become brittle. This made cap removal
as a possible cause of damaging the tubes solder connection.

Is your 811 going to be mounted base down?

edit- just noticed you are in Canada. Why not contact a local ham radio club. Someone will
have a cap for the tube.

dmg

Thu Nov 24 2011, 10:23PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
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No, ultimately the tube will be mounted horrizontally. (I will refer to the datasheet on its propper orientation, note I will do this with the RCA tube, I don't trust the chinese ones to not deform or melt in horrizontal.). Once everything is fastened down, I really don't intend to touch the tube again unless it fails.

...

Fri Nov 25 2011, 07:36AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You should be fine with a set screw, the top caps are just glued on so the worst that can happen is the glue cracks and you have to reglue it.

I would not worry too much about uneaven heating, I have seen top caps that are designed to act as a heatsink to help cool the anode, so having a slightly larger than normal mount should be fine.

Steve Conner

Fri Nov 25 2011, 07:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I've got a (used) 4-400C tube here that came with a heatsink cap, secured with a set screw exactly like GBD's.

However, the plate terminal on this tube is a thick metal rod that goes straight through the glass and the plate itself dangles from the bottom end. In operation the plate will send a huge amount of heat up the rod, so the heatsink is vital to stop the glass-to-metal seal getting overheated.

The 811 has its plate cap connected by a thin wire, so heat conduction won't be a problem, the cap and seal should run much cooler. You may still want to use the heatsink cap for looks, just don't do that set screw up too tight as the cap is thin and hollow.

dmg

Fri Nov 25 2011, 02:57PM

Registered Member #2628 Joined: Fri Jan 15 2010, 12:23AM
Location:
Posts: 627

I realized the cap looked pretty thin (looks like some the anode wire is just soldered to the top, and the cap is cemented ontop of the tube).

I stuck the set screw on and tightend it just enough so it sits steady. looks like the only points of contact from the tube's cap to my cap is the base of the tube cap (the cap has a wider base) and the set screw. so it should still give me a good electrical contact, but not fully constrict the cap at the top, so I guess it can expand a bit if it really has too.

Thanks for your help, I'll see how it will fair up in usage.

TCA1

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