[AMPS] L-4B

[Rich Measures]

>        ... Don't see much commentary on the L-4B, but mine just cratered. Was
>in standby when a crack and long fizzle sound came out of the RF section.
>The grid resistor opened up, and the meter needle pinned to the left
>(minimum) direction. 
>
IMO, this condition indicates that a grid to filament short exists in one 
of the tubes when the filaments are hot.  

>        I happened to be looking right at the RF section when this happened,
>and saw no arc or tube glow. Anybody know if this is a common failure mode?

It reportedly happens in about 5% of the cases, Jim. 

>Since the tubes were biased off at the time, parasitics don't seem likely,
>and I ohmed the tubes and see no shorts.

With the filaments hot, you will probably see the short.  Testing the 
tube cold with a high-pot. will tell you what's what.  A good tube will 
measure at least 6kV withstanding ability between the grid and the 
filament.  New tubes typically measure c. 9kV.  If the vacuum appears to 
be ok [10uA or less leakage at 8kV], and the filament to grid 
withstanding voltage is much less than what it should be, a bent 
filament-helix is indicated.  
>
>        I had a diode stack go out on this unit once, but never had a RF
>section pronlem in the 12 years I've had this amp.
>
RF deck trouble is reportedly less common than diode trouble.  .  

What is the measured R of the VHF suppressor resistors, Jim?

Rich---

R. L. Measures, 805-386-3734, AG6K   


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