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Re: [Amps] 4CX1600B - Bangs

To: <gdaught6@stanford.edu>, " AMPS" <amps@contesting.com>
Subject: Re: [Amps] 4CX1600B - Bangs
From: R.Measures <r@somis.org>
Date: Sun, 16 Nov 2003 01:56:40 -0800
List-post: <mailto:amps@contesting.com>

>KL7RA wrote:
>
>> Interesting debate on the 4CX1600B tube. This is a real workhorse tube
>> and it's too bad it has been dropped by the amp makers due supply
>> problems. 
>
>I agree.
>
>> I never heard from anyone that had the tube "bang" and found it still
>> good or bad unless I missed it from the postings. 
>
>OK.  I've had a "bang" event in my HB '1600b amp, one time.  Afterwards, 
>the tube 
>was fine.  The only damaged component was the shunt resistor for the 
>screen grid 
>current meter, which exploded.  The meter movement was fine (back-to-front 
>diodes 
>across it, of course).  The amp has run reliably and uneventfully since 
>(about 4 
>years.) [Crossed fingers, salt-over-the-shoulder.]
>
>Also, I had a "bang" in my 4cx800a MLA2500B conversion.  The exploded 
>resistor(s) 
>there were the "cathode degeneration" resistors from cathode to ground.  
>Whether 
>they all went at once, or one at a time, I don't know.  I replaced them 
>with a single 
>Caddock high power resistor which was up to the task, and the amp has been OK 
>ever since.  [Crossed toes, salt-over-the-other-shoulder.]  The tube is fine.
>
>Summary:  two Russian tetrodes; 

***   Does/did Svetlana gold-plate the grid in the 4cx800 and 4cx1600?

>two bangs;  exploding resistors in both 
>cases (in 
>different parts of the circuits); tubes fine in both cases.  Root causes 
>of bang events 
>unknown.  I can't ascribe them to parasites, ...

***  Nor did I until an old timer (W7MOI) suggested that I measure the 
resistance of the parasite suppressor resistor in an amplifier that went 
bang.  When I told him that the suppressor resistor was still in mint 
condition, he replied that resistance counts for more than appearance .   
I measured the R of the VHF suppressor resistors at c. 4x the marked R.  
Whatever did such damage had to be fast and furious since the internal 
heat did not have time to reach the outside of the resistor.   IOW, the 
resistor was probably not damaged by an ordinary overload.  Also, since 
the resistor was paralled by 4-turns of #16 Cu buswire, the energy that 
damaged the resistor must have been well into the VHF region. 
-   After Eimac autopsied the tubes from this amplifier, they concluded 
that the tubes were damaged by an oscillation condition.  The questions 
that needs to asked are:  1.  Can a tube's feedback-C be safely ignored 
by those of us who have not been blessed with a free, lifetime supply of 
tubes?, and 2.   Are all of the resonant circuits In an amplifier shown 
On the schematic diagram?

cheers, George
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