[Amps] TL-922 Troubles

[R.Measures]

On Jan 29, 2005, at 4:33 PM, W0OXJ wrote:

> I have been running a balanced pair of 3-500ZG's for 3 years.Recently 
> the
> amp made a loud pop and began arcing while at idle.I turned it back on 
> after
> a period of time and noticed a loud hum and the filament transformaer 
> was
> getting warm.

A filament got pushed sideways and shorted against the grid cage.  Are 
you using the stock vhf suppressors?  Did you install a glitch R?
>
> One tube has fresh paint on it and the other had faded paint which 
> seemed
> strange.The tube with fresh paint does not look used at all.The tube 
> with
> faded paint had a red plate after 45 seconds of excitation under low 
> drive
> and a loaded final.I swapped out tubes with a spare Eimac 3-500 and 
> the tube
> with fresh paint proved to be causing the heavy draw on the filament
> transformer.I also notice no idle current where I used to have 150 ma.
>
> The amp seems normal,it loads but I didn't try to load it fully.The 
> relay
> seems to operate OK,the 3-500ZG plate stills glows faintly red.
>
> In the past I have noticed one tube would turn red after a period of 
> keydown
> but was not concerned as the amp has delivered 800 watts out for three 
> years
> .
>
> Now I plan on replacing the defective 3-500ZG but would like to know 
> what
> could be causing the imbalance? and is this the reason for the failed
> tube.Will a new tube be matched enough to give good service with the
> existing tube?Why no idle current?
> Both tubes check out with an ohm meter,no shorts between the three 
> elements.

The tube with the bent filament only shorts when the filament is hot.  
Operating a stock 922 with a grid-fil shorted tube will destroy T2.  
There is an article on the 922 on my Web site.
>
> I can't do any voltage checks until I replace the 3-500ZG and will 
> follow
> the instructions in the service manual.They mention a bad zener,bad 
> relay
> and a bad tube as causes for this condition.

The bad tube is the result of an intermittent oscillation c. 120MHz.  
Good luck, Bob.
>
>
> Any advice or experiences would sure help as I would like to insure a 
> few
> more years of reliable operation.
>
> Many thanks and 73,
>
> Bob
> W0OXJ
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org