[AMRadio] 100TH and 250TH advice and or comment

John Coleman jc at pctechref.com
Tue Aug 25 12:39:58 EDT 2009

Do they show color on the plates or gas type ionization color when in use?  If they show gas ionization color when in use then they defiantly have air in them and could be caused by overheating due to some malfunction.  Maybe they have many more hours on them than you know about.  If the plates or just a little red but no gas ionization then the tubes or probably OK.  Make sure the grid drive stays up at the recommended level and one way to tell if a tube is losing its conduction is to check the grid drive.  If the grid drive falls off considerably when the plate supply is energized then the tube is getting weak.  A small amount of grid current fall is normal.  The neutralization must be very good to make this check else capacitive feedback will affect your findings.  I have used 250ths for some time and have never seen them loose vacuum from any normal use.  I did crack one once taking the grid connector off.  That was a little heartbreak.  Especially when Geoff W5OMR was telling me to be careful. HIHI 
John, WA5BXO

-----Original Message-----
From: amradio-bounces at mailman.qth.net [mailto:amradio-bounces at mailman.qth.net] On Behalf Of rbethman
Sent: Tuesday, August 25, 2009 11:07 AM
To: AM Radio List
Subject: [AMRadio] 100TH and 250TH advice and or comment


I have noticed that the 100THs and 250THS that are in their original
boxes, either not used at all, or maybe use for less than an hour to
test them, seem to be doing fine.

Their envelopes are crystal clear an can not see any sort of defect.

Then the ones that are in one of the two BC-610s that has the hours on
it, even though it last performed flawlessly, now appears to have gone
gassy with some milky white inside the envelope.

Is this perhaps due to over heating or maybe I should phrase this as the
convection cooling of these radios since they do NOT have fans.

Ideas?  Comments?

It has always been metered and watched with a scope for observing
waveforms, and the wattmeter is the newer Model Bird with peak reading
built-in.  (However true they really are!)

Bob - N0DGN

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