[Amps] Gettering 3-500 Tubes

Sun Nov 6 10:30:12 PST 2011

Great advice as ever from Carl.  Print it out and pin it on the shack door :-)

Dave G0OIL

----- Original Message -----
From: Carl <km1h at jeremy.mv.com>
Sent: 06 November 2011 02:16
To: rbethman <rbethman at comcast.net>; amps at contesting.com
Cc: R Johnson <rjohnson at tmlp.com>
Subject: Re: [Amps] Gettering 3-500 Tubes

Actually we all should be very concerned about keeping the 3-500Z gettered, 
it is one of the most prone tubes for needing it.
There is also a big difference between air leakage past the seals and 
outgassing from the anode.

Just "trying them as they are" is a great way to destroy the tubes as well 
as damage other components.

Gettering the 3-500 and many other tubes requires the anode get hot enough 
to show at least a bright red for as little as 10 minutes to many hours.The 
getter is the coating on the anode in most modern glass transmitting tubes 
with sheet metal or graphite anodes.

Unfortunately doing that at the amps operating voltage will often lead to a 
gas arc in the tube and most amps dont have sufficient protection circuitry 
to shut down fast.

The procedure Ive recommended here and elsewhere is to run the HV at a level 
that wont sustain an arc, I use 900-1000V and then apply a bias voltage that 
allows sufficient plate current to flow to color the anode.

For a 3-500 this is about 300-350W of dissipation so plenty of air flow is 
needed.

A SB-220 can be temporarily modified to do this. The filament supply is fine 
but the bias supply wont be used, disconnect it. The HV needs to be variac 
controlled and an adjustable  bias supply is needed which can be just about 
any small 0-30 or 40VDC supply since there will be no grid current being 
drawn (unless it arcs). The HV can be run a little higher if youre a bit low 
on the bias but I wouldnt go over 1200V for the initial run.
Look at the constant current curves to get an idea of how the voltages 
interact.
www.g8wrb.org/data/eimac/3-500Z.pdf

Let the first pass run for 15 minutes, there should be no gas glow. Then try 
it at 1250V and then 1500V for 15 minutes each and adjust the bias each time 
for the same dissipation. When I see just the trace of gas glow I back down 
250V and let it cook for an hour and then try a higher voltage again.. It 
takes time and patience with some tubes before they pass the test at 
2500-3000V. Dont let the SB-220 HV transformer overheat, just do one tube at 
a time...the transformer is only 500VA CCS.

Once the tubes are fully gettered and the amp is in normal use then its a 
good idea to get the tubes hot on a regular basis of once a  month or so. 
Running as an AM linear is an easy and fun way to do it.

Carl
KM1H

.

----- Original Message ----- 
From: "rbethman" <rbethman at comcast.net>
To: <amps at contesting.com>
Cc: "R Johnson" <rjohnson at tmlp.com>
Sent: Saturday, November 05, 2011 7:08 PM
Subject: Re: [Amps] Gettering 3-500 Tubes

I wouldn't be concerned with "Gettering".

*IF* there is air leakage into the tubes, they will show a cloudy gassy
look.

Go ahead and try them as they are after you've gotten the rest sorted out.

*IF* it was me, I'd replace the capacitors in the HV section.  20 years
is a long time, and Heath didn't always use the "High" end parts.

Bob - N0DGN

On 11/4/2011 12:05 AM, R Johnson wrote:
> I have just been given an old SB-220. It has been sitting in an attic for 
> 20+ years.
>
> I have removed the tubes and have been bringing the voltage up slowly
> to reform the filter caps. When I get to 120V in and the caps aren't 
> steaming
> and/or blowing smoke I'll be ready to check it with the tubes installed.
>
> I know that the need 3-500's will need to be "Gettered" to remove any gas 
> diffusion
> thru the metal to glass seals on the tube.
>
> I read a procedure on (probably) on this list on how to do it.
> Now that I need it, I can't find it.
>
> Does anyone remember this thread within the last 3 or 4 months ???
> If not,can any recommend how to cook these tubes in ???
>
> TNX es 73
> Bob, K1VU
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>

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