Re: [Amps] TL 922 Problem

[R.Measures <r@somis.org>]

On Apr 23, 2004, at 1:57 PM, Michael Clutton wrote:

> 3-500Z Problem
>
> Some time ago in a chain of TS 850S driving a TL922 linear amplifier, 
> an 'incident' occurred during initial switch-on and tuning into a 
> dummy load.   There was a catastrophic failure of the transceiver, but 
> the state of the linear amplifier was at that stage unknown.  The 
> transceiver failure was eventually located, and turned out to be due 
> to the short-circuit of one of the front-end steering diodes, and when 
> this was replaced the transceiver operated normally again in all 
> respects.
>
> The TL922 was now tested in the same configuration as before, and 
> functioned normally up to about half power.  But then the plate 
> current began to increase uncontrollably.  The circuitry was examined, 
> and it appeared that the Zener bias diode had failed, but there were 
> signs that one of the grid chokes had at some time experienced a 
> flash-over, although it appeared to be otherwise sound.
>
> The Zener diode was replaced, and once again the chain was tested.  
> Again there was plate current instability at some level.  Then the 
> suspect grid choke was replaced also, but tests again indicated 
> instability at about half-power.

  Michael --  The TL-922 has a high-Q resonance in the anode circuit 
at c. 120MHz.  This resonant circuit  can be observed by coupling a 
dipmeter to either side of the DC blocking capacitor.  There is a total 
feedback capacitance of  0.3pF  (c.4000-ohms of XC at 120MHz) in 2, 
3-500Zs.   Since 3-500Zs exhibit fairly good gain at 120MHz, 
occasionally, things can get squirrelly as well as noisy.  The fix is 
not difficult.  There's an article about the TL-922 on my Web site.
> By using a tube of the same filament rating in one socket, but with no 
> plate voltage applied, a test was then made of the amplifier operating 
> with only one 3-500Z in the other socket.  The behaviour of the 
> amplifier was that one tube produced about the right output power, but 
> the other showed distinct plate-current instability at a much lower 
> power.  Now the question:
>
> Is it your experience that a 3-500Z (Eimac) can appear to be quite 
> sound in respect of filament emission, but exhibit plate-current 
> instability under some drive conditions.  In essence the question 
> amounts to what now needs to be done.  Should one or both tubes be 
> replaced by new ones, or should one look for some other fault in the 
> circuitry (which appears to be all in order)?
>
> If one or both of the tubes has had an internal flash-over, and 
> something (the grid?) is damaged, can anything be done to salvage the 
> tube(s)?

  When a 120MHz parasite occurs, there is no load to absorb the energy 
since the MF/HF tank is a low-pass filter.  This causes a burst of high 
grid-current which exerts an EMF between the grid and the source of 
grid current -- i, e., the filament/cathode.  The force produced does 
not damage the unheated molybdneum grid, but the hot (1820ºK) filament 
can sometimes be bent sideways until it shorts against the grid.  The 
fix for a  filament-grid short is to replace the tube.  However, 
usually only the inboard tube shorts, so if a tube with a similar Mu is 
used as a replacement for the Eimac, only the bad tube needs replacing. 
 RF-Parts currently sells 3-500Zs that have a Mu similar to the old 
Eimacs.
-  cheers
> Any help you can give would be appreciated.
>    de Mike
>                                                           G4VQH
>
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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