HW-16 Final Tube Meltdown

[Kenneth G. Gordon]

On 5 Sep 2008 at 17:51, Robert Wood wrote:

> All,
> 
> 
> 
> I am encountering a very unusual problem with my HW-16.  All was
> working well, and I operated it for about a week during the evenings. 
> Last night when I turned it on and monitoring 40 meters, the final
> turned red hot.  I moved the switch to the current position, and
> noticed it was drawing current.

How much? Pegged?

>  I removed the 40 meter xtal and key
> and turned it on while switching bands and encountered the same
> issue.  I shut it off and retired for the evening.
> 
> After getting home from work this afternoon, I decided to test the
> 6GE5 in my Syncore tube tester.  I was hoping the tube may have a
> short, but the tube tested ok.  I put the it back into the rig with
> same results. Before I tear into the rig, does anyone have any
> suggestions?

It sounds to me as though your grid bias is missing. Measure the 
voltage at the grid with the key unplugged. Do this quickly so you don't 
ruin the tube. The voltage at the grid with no key plugged in, or with 
the key UP, should be around -150 VDC.

If it is zero or positive, then you have something between the -160 
VDC line and the Grid that is open, or some other problem. 

If the voltage is zero, perhaps L-15, a 0.5 mH choke, which is 
connected right at the grid, pin 3 of the 6GE5 is open.

IF there is a high positive voltage on the grid, then the 100 pfd 
coupling cap coming from the driver plate and which is also connected 
directly to pin 3 of the 6GE5 may be shorted.

Measure the voltage at the grid of the 6GE5 and report back.

Do this very quickly so you don't ruin the tube. Connect your meter, 
set it at a safe setting, then turn the rig on. 6GE5s are very rugged 
tubes, but no tube of that sort lasts very long with a red plate.

Ken Gordon W7EKB 

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