One of the problems with the older 3-500s is if they are rocked when removed
from the sockets. This is hard on the filament seals and can let in gas.
73
Jim W7RY
--------------------------------------------------
From: "Carl" <km1h@jeremy.mv.com>
Sent: Saturday, November 05, 2011 6:16 PM
To: "rbethman" <rbethman@comcast.net>; <amps@contesting.com>
Cc: "R Johnson" <rjohnson@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@comcast.net>
> To: <amps@contesting.com>
> Cc: "R Johnson" <rjohnson@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
>>
>>
>> _______________________________________________
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>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
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