[Amps] 4CX1600B

[Ian White, G3SEK]

R. Measures wrote:
>
>
>>Yo 4CX1600B owners
>>
>>After many years of faithful service, (rode hard, put away wet), my QRO
>>3k amp "banged" during the WW. Easy repair, the bridge rectifier for the
>>-160 VDC control grid was shorted. Took longer to replace all the screws
>>holding the QRO HFDX-3K amp together then find and repair the problem.
>>
>>If this was a 4-1000 in my home brews I would replace the parts and never
>>give a second thought to continue using the tube, but as it's a 4CX1600B I
>>replaced the tube with a new spare.
>>
>>Anyone have a history on this tube? Once they arc are they ready for the
>>(as far as I can throw) pitch in the woods?
>
>***  I would not conclude that the tube arced until an autopsy is done.
>If you heard a loud bang, it is unlikely that it was caused by an an
>internal arc since an arc inside the envelope makes only a little sound
>outside the envelope.
>- - -   Does the circuit in which the 4cx1600 is used have a grid
>terminating R?  If so, is there an inductor in series with said R?  If
>so, how much L?  tnx.
>>
>>I could put it back in the amp and see what happens but next summer after
>>the contest season.
>
>**  Rich -- If it proves to be a dud, I would like to disembowel it and
>publish the autopsy photos of the guts on my Web site.

That would be interesting, but not necessary.

Let's be logical about this. KL7RA has already told us what happened to 
this tube: the negative grid bias failed, so it pulled lots of current.

The tube itself did not cause the fault. The question is whether the 
tube was damaged *by* the fault.

This was a totally DC-related fault with a DC-related explanation. So 
why are we now diverting/confusing ourselves by thinking about the RF 
part of the amplifier? When the bias supply was repaired and the tube 
was replaced, the amp produced RF again as normal. Since nothing 
RF-related is broken, it don't need fixin'

Also there is no logical reason to assume an arc here. That large pulse 
of current was quite probably normal electron current from the cathode, 
simply because the grid bias had failed. An arc is a different way of 
pulling lots of current, involving positive ions as well as electrons. 
Arcs in tubes sometimes do happen, but this time we have no reason to 
suppose that an arc took place.

The bang would have come from the external circuit, where that same 
large current was also flowing (for example from a fuse blowing).

So the only *relevant* question is whether that pulse of current has 
damaged the tube... possibly by pulling too many electrons from the 
oxide cathode surface.

The only way to find out is to put the tube back in the amp and try it. 
Given the cost and difficulty of finding another spare 4CX1600B, don't 
let a few screws get in the way.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek