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Re: [Amps] Ten Tec 425 titan

To: "john naberezny" <we2f1@localnet.com>," AMPS" <Amps@contesting.com>
Subject: Re: [Amps] Ten Tec 425 titan
From: R.Measures <r@somis.org>
Date: Wed, 19 Nov 2003 07:12:47 -0800
List-post: <mailto:amps@contesting.com>

>Hi Folks,
>This amp has a 10 ohm wirewound 10 watt resistor in series with the plate 
>RFC, with no cap to ground between them, 

**  No cap is necessary, but 100 - 300 pF would help to attenuate 
harmonics in the VHF region.

>and I burned one up badly this 
>morning before it went pop, and took out the power supply main fuses. 

**  The chain of events:  Loose gold meltballs which  evaporated from the 
gold-plated molybdneum grid cause an anode to grounded-grid arc inside 
the 3cx800A7s.  If the glitch R is not up to the job -- i.e., it comes 
unglued during the arc.  It should either be replaced with two 
glass-coated 10 - 15 ohm resistors in series or replaced with a 
surge-protection resistor that is rated for >than the # of 
Joules/watt-seconds of energy stored in the HV filter C. [Globar SP-type]

>I replace it tonite, went back on 15, took tuner out of line, this time I 
>saw the flash, got the pop, and then the fuses blew. It cracked the 
>ceramic standoff holding the resistor, and the resistor was warm..
>   Any ideas ? Company says it in the antenna- bull !

**  Spread some of this around fruit trees to get sweeter fruit.  
--  The laugher is that when Ten-Tec read the 1988 article on low VHF-Q 
parasitic suppressors, they tried the idea but quickly abandoned it when 
the VHF suppressors got real-toasty hot at 29MHz.  However, if VHF 
suppressors don't get hot at 29MHz, they are obviously not going to do 
much to suppress/waste energy in the VHF region.  The result was that 
Ten-Tec stuck with their original high-Q suppressors which are lovely to 
behold but sometimes fail to sufficiently lower the parallel-equivalent-R 
at the anode's resonant frequency.  The fix may be as simple as tapping 
the anodes in the vertical plane (base-down) with the flat side of a 
ball-peen hammer in order to move the loose meltballs out of harm's way.  
To prevent a re-occurence, install lossier VHF parasitic suppressors.  
The trade-off is c. 2% less P-out on 10-m.  [note --  after tapping, if 
the tubes are ever inverted to base-up, re-tapping is in order]
-  There's more on gold-migration as well as non-destuctive testing for 
the problem on my Web site.  

good luck, John


> Got a problem in the 
>other direction. Any ideas? Only a 15 meter problem.Had this amp from the 
>factory 15 years now.
>          73, John WE2F
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