[Boatanchors] SB-220 with parasitics

[Bill Cotter]

Keith,

I have repaired many 3-500Z amplifiers that suffered reported 
flashovers. None of which I could honestly say (with certainty) was 
due to parasitics. Not to say that wasn't the cause, but not 
repeatable. In every case I do a hipot test of the tubes and 
usually there is evidence of grid to filament or grid to plate short.

The test I do is 3-step: 1) filament to plate 6kV, 2) filament to 
grid 3kV, and 3) grid to plate 6kV. Anything over .100mA I get 
suspicious. Nearly every 3-500Z that has flashed over trips out 
(2mA limit) well below 1kV.

The most common cause of failures I have seen is plate to grid 
shorts where the plate is visibly misaligned with the grid support. 
Evidence of a drop, or perhaps stored for a long time on its side. 
The second most common is grid to filament short, which could have 
been a parasitic event or possibly overdriven. It is amazing how 
many ops will drive their amps past 240mA to get the last watt out 
of a pair....

I have a dozen or so failed 3-500Z's in a box that I have tried all 
the rejuvenation tactics, to no avail. They are reserved for future 
testing of any technique for revival. The bent plate may be only 
good for target practice.

73 Bill N4LG








At 11:23 AM 12/24/2021, Keith Densmore wrote:
>Safe Holidays to Everyone,
>I have agreed to look at a Heath SB-220 amp for a friend. The amp, 
>tubes
>look like new but one of the 3-500's has had a parasitic bang, at 
>least
>once and maybe twice. (On the same tube.)
>I cannot find much info on the web but the question is, can such a 
>tube be
>reformed? Run the filament for some hours? Power it up with lower 
>plate
>and/or filament voltage? Any hope or is it dead?
>If anyone has experience in this area, your advice is welcome.
>
>Thanks,
>Keith ve3gem
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