HW-16 Final Tube Meltdown

[Kenneth G. Gordon]

On 6 Sep 2008 at 10:43, Robert Wood wrote:

> The bad news is the HW16 is now QRP.  Even after going through the
> neutralization steps, the best she is able to do is about 2 watts. 
> Prior to the meltdown she was able to put out about 45 watts.

That problem MAY be because the RF choke inductance is 
now too low, and the circuit impedance has changed too 
much.

I suggest that you replace that 0.5 mH choke with any 
choke you have which is of higher value and try the rig 
again before you install a new final amp tube. 

IMHO, 0.5 mH is barely enough inductance to do the job 
any way.

I would try a 1 mH, or even a 2.5 mH RF choke. There 
should be plenty of room.

There is also the possibility that your plate voltage is too 
low: that something else was damaged when the final amp 
drew so much current. The plate RF choke, L-16, 
immediately comes to mind.

With the transmitter running, but without being keyed, 
measure (CAREFULLY!) the voltage at the plate, AND at 
the "bottom" (power supply side) of the plate RF choke. It 
should be at least 600 VDC.

You should also measure the screen voltage. If that is very 
low (should be at least 200 VDC), then check the "Power 
Level" control to see if it might be open, or that some other 
component in that circuit might have failed. Screen voltage, 
according to the schematic, should be 260 VDC at 
maximum.

Remember, if you have no plate voltage but DO have 
screen voltage, you can burn out the screen if the tube 
draws excessive current. 

If you do need a new 6GE5, I have bought them quite 
inexpensively off Ebay. They really are a tough tube and 
can stand a lot of abuse before failing. I think both of my 
HW-16s have the original finals in them. I have never had 
to replace one, but then again, I have never run one with a 
red-hot plate either. :-)

Ken Gordon W7EKB

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