On Thu, 11 Dec 1997 15:22:12 -0500 Steve Parsons
<sparsons@geronimo.legato.com> writes:
>As the owner of a dead TL-922 I figure I should add a few
>observations....
>>
>>History was:
>>
>>Amp was in retirement for 3 yrs after 3yrs of trouble free use. It
>had the
>original Eimac tubes in it.
>>
>>Powered it up 2 weeks ago, it failed instantly with a loud bang while
>in
>standby (in CW position). Lost both 15A fuses as well as the grid
>choke and
>resistor for V1.
That certainly gets your attention Steve.
The output was connected to a cantenna and a 30W 50
>ohm
>load was attached to the input.
>>
>>Checked both tubes by applying 0 - 1800v from plate to filament and
>voltage applied to filament. V2 had no problem, but the whole inside
>of V1
>would light up from gas with as little as 800V (Color was roughly pink
>to
>magenta). Subsequent visual check also showed that the fil. was
>fractured.
What are the date codes on those tubes? Are they with or without the
phenolic (or something similar) pin support ?
>It may be a coincidence that the tube went gassy over time *and* the
>filament failed, but the amp was never bumped or dropped so I suspect
>that
>the cause of the filament failure was the current surge through the
>gassy
>tube.
Possible in the case of a pin supported tube. Most likely in the
unsupported version....these can go bad by looking at them !
It's possible that disassembling the tube could reveal more
>about the
>cause of failure
Only if you are into works of fiction.
, but the gas inside lights up so nice I want to keep
>it as
>a display piece :)
>>
>>As for the zener:
>><insert>
>>That was the basis of my question last week; what goes first, the
>choke
>>or the zener? Thank you for repeating it.
>><end>
>>
>>The zener in my amp checks good, so the choke went first. Also, in
>standby, the filaments are connected to the cutoff bias supply so in a
>standby bang the zener should not be affected. In the schematic it
>looks
>like the zener only provides operating bias.
Correct Steve. It is when the choke opens and the owner repeatedly ( one
try often is all it takes) tries to get things working by brute force
that the zener goes in the TL-922. This appears to be the more common
zener failure mode in the TL-922...not to say there are not others.
My advice to anyone is that when an amp does ANYTHING that is unusual,
shut it down and go into troubleshooting mode. You could save yourself a
lot of money and down time.
>>
>>Questions:
>>
>>If I replace the grid chokes with fuses, should I leave all the
>bypass
>caps in place?
My grid fuse comment was "tongue in cheek" Steve...just looking for more
entertainment from our gold sputtering guru. The added lead length to
fuse holders would not make this a real attractive option. Going to a
lower current rated choke would be a better choice. I do not like the
idea of an hour or so down time to fix in a contest but at least
160-200ma chokes will possibly save a tube. With direct grid grounding,
you have zero protection in that amp....plus added instability.
I've nothing against direct grid grounding...in an amp designed from the
beginning for that concept.
>>
>>Any easy to deal with mail order tube suppliers out there that
>publish
>prices? Here in Canada there doesn't seem to be a 'Tubes r-us'.
RF Parts is a good source of fair pricing. 1-800-RFPARTS...even if they
are in the land of fruits and nuts.
GL Carl KM1H
>>
>>Thanks,
>>
>>Steve Parsons
>>VE3SMP
>
>
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