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


rbethman rbethman at comcast.net
Tue Aug 25 12:57:38 EDT 2009


John,

I don't get the 250TH to load worth a hoot!  The plate voltage is fine.  
The drive checks out good back to the pair of 807s.  I saw the evidence 
of the drive at the grid clip.

The 100THs had been set with the specified mA on the speech amp.  I 
don't like the PA mA reading so I am hesitant to leave it on while I go 
hunt around the backside to look at all three plates.

It has sat for over a year + now, as I have had shoulder surgery and 
trying to move it around even WITH a "moving man" dolly underneath is no 
simple task.  [The wife has the whole area with wall to wall 
carpeting.].  There is bare concrete underneath that.

Bob - N0DGN

John Coleman wrote:
> 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 te
>  lling me to be careful. HIHI 
> 73
> 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
>
> Question:
>
> 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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-- 
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|             AM Amateur Radio Operator    NØDGN                   |   
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| Bob Bethman                \\\|///     " The absence of a danger |
|                           \\ ~ ~ //      signal does *NOT* mean  |
| rbethman at comcast.net      (/ @ @ /)      that everything is OK " |
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