>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.
*** I know it's not necessary, Ian, but it's interesting to me and
perhaps to some other amp builders, and if you are proved right, it's
another feather in your war-bonnet. .
>
>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.
*** Wasn't there something about a big-bang? What if the bang preceeded
the bias problem? I'll take an autopsy photo over speculation any day of
the month.
>
>The tube itself did not cause the fault. The question is whether the
>tube was damaged *by* the fault.
*** ... an opinion. Maybe wromg, maybe right. An autopsy will shed some
light
>
>This was a totally DC-related fault with a DC-related explanation.
** But what about the big bang?
>So why are we now diverting/confusing ourselves by thinking about the RF
>part of the amplifier?
** Because experience tells me that a big-bang in a 3-500Z amplifier is
not a tube problem. It is an regeneration problem that results in the
hot filament being pushed sideways, presumably by EMF, except of cource
in aerobatic airplane-mobile service, of course. If the filament gets
pushed far enough, it shorts against the grid cage. [see relevant
autopsy photo in "Parasitics Revisited". September/October, 1990, *QST
Magazine*.] // Regeneration is the natural result a tube's ability to
amplify VHF combined with the tube's/valve's oft'-ignored feedback-C.
- 813s are virtually immune to this unhappy problem because they have
virtually no VHF gain, but 4CX1600Bs and 3-500Zs do.
>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'
** Based on my experiences with 3-500Zs and 8170s, I would like to do an
autopsy on the tube -- provided it's kaput.
>
>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).
>
** I don't buy it because with 0V on the grid, the HV drops to a low
potential because the 1600B is drawing c. 3A, until the mains breaker
opens uneventfully.
>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.
>
** ... which an autopsy photo will undoubtedly show. [see relevant
autopsy photo in "Parasitics Revisited". September/October, 1990, *QST
Magazine*.]
>The only way to find out is to put the tube back in the amp and try it.
** agreed
>Given the cost and difficulty of finding another spare 4CX1600B, don't
>let a few screws get in the way.
>
>
>--
>73 from Ian G3SEK ...
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|