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Re: [Amps] Question on installing new Amp tube

Subject: Re: [Amps] Question on installing new Amp tube
From: Lee <>
Date: Sat, 8 Aug 2015 14:11:10 -0500
List-post: <">>

I have always heard of long time "idle" 3-500Z tubes needing to be gettered because the high voltage can flash over and destroy the grid. I have never heard they have a problem emitting electrons off the filament from being idle. This is news to me. However, I would be much more concerned with gettering a gassy tube then initially worrying about electron emission. Electron emission is a mute point if you have destroyed the grid from flash over due to a gassy tube.
Lee, w0vt

On 8/8/2015 1:26 PM, Mike Tubby wrote:

I think you may be confusing two unrelated issues ...

The 'Reactivation' that PA0FRI describes is a process of making a tube work again. The 3-500Z is a directly heated cathode, i.e. the filament is the cathode - it comprises of Thoriated Tungsten. As I understand it when a tube sits, unused, for an extended period of time the surface of the filament looses its ability to emit electrons - you could say it 'oxidizes' (but their isn't any Oxygen to speak of). The re-activation process brings an otherwise 'dead' or 'partially working' tube back to life by rejuvenating it and restoring its ability to emit electrons. What you're doing is keeping the filament hot for an extended period until some of the Thorium atoms migrate back to the surface renewing the emissive layer.

The reactivation process needs only modest voltage (40-ish volts). Note that in PA0FRI's diagram the control grid is tied up to the anode. With 40V and 400mA the tube only dissipates 16W and this is really just an indication of whether your tube has sufficient emission to be able to conduct with 400mA of anode current.

What you were talking about in your previous post is "gettering" the tube which is something entirely different. The majority of electron tubes have a method of "gettering" them - or "sucking up the free oxygen atoms" or "hardening" the tube vacuum. In the case of the 3-500Z the anode has a coating of Zirconium - when this heats up (ideally to around 1000C) it mops up free Oxygen atoms - presumably by converting them to Zirconium-Oxide.

Some tubes with an indirectly heated cathode can be gettered by just turning on the heater. In the case of the 3-500Z you have to get the tube (very) hot by running it up to near full dissipation with HT and anode current.

Mike G8TIC

On 08/08/2015 17:52, Lee wrote:
This scheme makes no sense to me. It is known a 3-500Z tube has it's getter on the plate. So if you place 45 volts on the plate and draw 400 ma, you have a plate dissipation of 18 watts. 18 watts is not even close to getting the plate cherry red. In fact it won't get the plate to glow at all. Since the plate is the getter and it can't getter unless the plate is glowing, how in the world can this scheme work? There is something wrong here. This scheme is telling us the getter works without a red hot plate. This is news to me as every other gettering scheme I have seen insists on getting the plate to glow.
Lee, w0vt

On 8/8/2015 10:59 AM, Mike Tubby wrote:
If the 3-500Z has stood for a long time unused I suggest you use the 'Reactivation 3-500Z' procedure by PA0FRI, here:

When I did this with a new-old-stock tube I used 40V and had no significant current indication for about 1.5 days, then the current meter started to 'flick' a few milli-amps and back to nothing and then after a few more hours it started a constant reading of around 50-60mA which then rose until it hit my current limit.

I used a pair of 0-20DC bench power supplies in series to get 40V and set the current limit at 400mA.

After the reactivation I put the tube in a PA and it worked fine.

Mike G8TIC

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