[Heathkit] reactivation of a 3-500

Fri May 16 15:16:45 EDT 2014

   Eimac's Care and Feeding of Power Grid Tubes is available on line in several editions.  My memory is that they recommend running just the filament for a considerable period of time, a few days I think.  I am not sure this applies to the type of filament used in the 3-500 but a look at the books will tell you.  
   As far as running the tubes hard, I think this is left from the time Eimac used tantalum plates. Tantalum is a good getter, i.e. absorbs gas, provided its quite hot. Eimac recommended running tubes at red to yellow heat.  About 1946 Eimac began to make places from an alloy they called Pyrovac.  These plates were also coated with a zirconium compound.  The new material works as a getter at lower temperatures. It still likes to be run at red heat and will withstand higher heat but does not require it. 
    One of Eimac's recommendations is to make sure filament voltage is very close to the rated amount.  Some types of filaments will last longer when some reduction of voltage, provided the emission is sufficient but it depends on the type of filament.  
    If a tube is sufficiently gassy there is no simple way to degas it. The gas tends to "poison" the filament and drop emission substantially.  Most tubes have two kinds of getter, one works at low temperatures to maintain the vacuum while the tube is stored, the other works hot to absorb gasses during use.  The hot getter is the more effective of the two. 
     In general, transmitting tubes have the longest life when run continuously at conservative ratings. 

-----Original Message-----
>From: Garey Barrell <k4oah at mindspring.com>
>Sent: May 16, 2014 11:53 AM
>To: Chris Kepus <ckepus at comcast.net>, Heathkit Reflector <heathkit at mailman.qth.net>
>Subject: Re: [Heathkit] reactivation of a 3-500
>
>Chris -
>
>The best way if you can, is reduce the PLATE voltage somewhat.  Hopefully your amp has a current 
>limiting resistor in the HV, and protection for the meters in case of an arc. One problem is 
>that tubes that have been idle for a while often arc over when HV is applied, and this can cause 
>damage to things other than the tube.  The Grid is in this path, so don't forget the Grid meter 
>protection.
>
>Then you have to apply some positive voltage to the Grid to make the tube draw more than it's 
>'zero-bias' current and thus heat up the PLATE.
>
>Ideally, you'd have a separate test setup with a socket, air and a high voltage supply.  Most of 
>us can't do that however!
>
>I've done two sets of tubes in my L-4B with no difficulty.  I first let them run for a couple of 
>hours with just filament current.  I don't think this serves any purpose, but made me feel 
>better!  :-)  I applied CW high voltage, only about 1700 V in the L-4B and tuned it up.  I have 
>a four foot long dummy load with a large fan on one end that was able to dissipate the RF 
>generated.  I let them run for two hours at a dull red color, and that did it.
>
>If you can't do that, you can lift the filament transformer center tap and apply a positive 
>voltage of I think about 40 VDC.  I don't remember for sure, and can't find my notes. Adjust the 
>voltage until the plates glow a dull red at IDLE and wait.
>
>EIMAC discovered back in the 30's that their tubes lasted longer when 'run hard' i.e., red 
>plates, in commercial service, than they did when tubes were operated with no color.......   The 
>plates were made of tungsten then, which 'getters' very well when "HOT".....  After the war, 
>tungsten was too expensive so they came up with 'Zirconium' which when plated on tube plates 
>also performed 'gettering' when hot.
>
>73, Garey - K4OAH
>Glen Allen, VA
>
>Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
>and TR-4/C Service Supplement CDs
><www.k4oah.com>
>
>Chris Kepus wrote:
>> Garey,
>>
>> Excellent points, to be sure.
>>
>> Couple of questions. If one has a dual 3-500 amp that hasn't been fired up
>> in years, it is reported all over the web that the 3-500 doesn't do well if
>> kept in storage for extended periods. It could be a very unpleasant and
>> expensive event when the amp is once again fired up. How does one avoid
>> damage to the amp but accomplish what you described with your 3-500s? (if
>> this is already written up, please direct me to the reading room).
>>
>> Assuming the tube(s) don't short and arc over at start up, and assuming
>> there's an answer to the above question, what dummy load does one use that
>> won't cook during this hour plus long plate heating process?
>>
>> Thanks es 73,,
>> Chris
>> W7JPG
>>
>>
>>
>>
>>
>> -----Original Message-----
>> From: Heathkit [mailto:heathkit-bounces at mailman.qth.net] On Behalf Of Garey
>> Barrell
>> Sent: Friday, May 16, 2014 7:52 AM
>> To: Ulrich Graf; heathkit at mailman.qth.net >> Heathkit Reflector
>> Subject: Re: [Heathkit] reactivation of a 3-500
>>
>> Uli -
>>
>> The 'heating' must be more than just the filament.  You need plate
>> dissipation.  The plates need to be heated to red hot for some period of
>> time.  IF these are EIMAC tubes there is a coating on the plates that is
>> 'activated' by red heat.  I have had tubes recover after only an hour at red
>> heat.  Be sure and keep air going, especially on the glass/metal seams of
>> the pins and plate cap..
>>
>> 73, Garey - K4OAH
>> Glen Allen, VA
>>
>> Drake 2-B, 2-C/2-NT, 4-A, 4-B, C-Line
>> and TR-4/C Service Supplement CDs
>> <www.k4oah.com>
>>
>> Ulrich Graf wrote:
>>> Hi to the list,
>>>
>>> got an SB-1000 linear amplifier recently second hand. While activating
>>> PTT I noticed that the bias current is continuously increasing. Bias
>>> network seems to be ok. So the tube must have drawn some gas. How can
>>> I activate the getter the easiest way to reuse this tube again? Many
>>> hours of heating do not work.
>>>
>>> vy 73, Uli, DK4SX
>>>
>>
>
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