[AMPS] Re: Bent filaments

Mon, 14 Jul 1997 13:12:20 +0000

> From:          km1h@juno.com (km1h @ juno.com)
> Subject:       [AMPS] Re: Bent filaments
> Date:          Mon, 14 Jul 97 13:54:44 +0000

> At least the grid choke school of thought and possibly Rich's "network"
> act as fuses and can often save the tube. 

No sale Carl.

When the tube faults the grid flys right up to full anode voltage. 
The grid just as often as not arcs to the filament in that condition, 
wiping out all the stuff in the cathode return.

Fusing the grid for HV arcs is dumb, because the grid to anode arc is 
generally in the harmless area of the very stout grid support 
or well reinforced grid cage rather than to the hotter and 
more magnetically susceptible filament structure. 

The proper place to limit fault current is in the anode, near the 
filter caps, as you suggested. The last thing in the world anyone 
would want is a grid flying up to 3000 volts.

>offers no such protection which IMO is why the AL-82 (and similar
> designs) often causes considerable damage when whatever it is does its
> mischief.

What do you call considerable damage? Ameritron uses an AL-82 
running at 4500 volts to check tubes for outgassing and arcs, and the 
ONLY components that get damaged in a fault are the zeners (on 
rare occasion) and the meter protection diode.

Perhaps you can elaborate on the components you recall seeing 
damaged? How many AL-82's and 80 series PA do you see? Be honest, I 
depend on feedback like yours to learn.

> I do know as fact, that any 220 that I have reworked with my own
> parasitic suppressors, series B+ resistor and original grid circuit has
> not lost a tube. Over the years that is a considerable number of amps. 

That sounds like a used car service center sales pitch Carl. The 
3-500Z Eimac tube had such a good service history, I don't see how you 
ever process enough amplifiers to get a good data base.

I doubt that thousands of 3-500Z's run through Radiokit's doors, and 
the failure rate of 3-500Z's is so low (other than the Chinese 
junk) it would take a very large data base to get any idea what the 
failure rate and effects of modifications really are.

Of course if CASH went in my pocket for selling mods, I might spend 
all my time saying I had a magic 100% reliable cure also, hi. ;-)

1.) The 3-500Z's number one problem since 1982 or so has been broken 
anode welds letting the anode cock and touch the grid. Over half of 
Ameritron's tube failure records indicate bad welds. When the anode 
falls off or is sitting at a large tilt, it's pretty easy to 
determine the failure's cause.

2.) Outside of two definite production periods, one about four years 
ago when Eimac changed testing specs (which they changed back) and 
another when they made the last batch of tubes and the employees all 
knew they were on the way out, gas failures and related arcs were 
rare. All of the other failures combined, bad seals, grid filament 
shorts, broken glass, etc  did not equal the weld problems.

Even though I have the utmost confidence you do the best job you can, 
I doubt a Radiokit modification could correct manufacturing defects 
in the tube welds.

Either your data base was VERY small, or you discovered a way to 
repair welds in the tube.

73, Tom W8JI 

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