[AMPS] Cetron 572B tubes

[Merit W6NQ]

A few comments about the Cetron 572B and another type of filament failure....
We found that early Cetron tubes through the early 1980's were very good tubes with high reliability.  Sometime after Richardson bought Cetron, we experienced great filament failure with new production.  I think there was something wrong with the wire or assembly, as we were seeing in excess of 10% filament breakage before installation in equipment, and another 5% premature failure within about 100 hours of use.  Richardson's customer service people confirmed they were having problems.  What made it even worse, the price had more than doubled compared to the pre-Richardson takeover.  As it turned out, by 1993, the Chinese tubes had much fewer filament problems than Cetron (after our strict QC inspection & selection).   We accepted only the upper 20% of production (remainder were junked or went elsewhere).  

The FL2100B has had a history of various problems, mostly in one way or another tied to the insufficient cut off bias (-17v) when using the high tap of the plate transformer.  By changing bias circuit to voltage doubler (plus a resistor), it is possible to remove/reduce most stability problems.  The later FL2100Z has this change. Need for bias change applies to all tubes, especially new tubes with full emission.  Svetlana absolutely requires the change to -34v. Sometimes the relay closing sequence needs to be adjusted for output circuit closure before input. (I think there has been adequate discussion on the subject of fast switching TR circuits on modern transceivers and their affects on amplifier tubes.)  You should also keep an eye on the tube for signs of gas with the import tubes.

The above discussion does not touch on proper plate & input tuning, need for parasitic supressors, neutralization, etc. --just a few comments on the Cetron and use of new production tubes.

Merit     W6NQ   


At 09:46 AM 5/12/97 -0700, you wrote:
>... ...
>>Need a little help finding some Cetron 572B tubes for my Yaesu FL2100B
>>amp (yes, I'm just a little pistol, but 500-700 watts works fine for me
>>execpt when trying to work BS7H).  At least one of the finals has gone
>>south and max output is now limited to about 350 watts. 
>What failed in the tube that went south to kaputsville, Tom?
>> It used to hit
>>700+ when the finals were new after Yaesu replaced them and did some
>>other work on it.  I know Svetlana makes a 572B that is supposed to be
>>fairly heavy duty, or so I've been told, but major circuit modifications
>>are necessary to use that tube, so the Svetlana people tell me.  
>Svetlana 572B and 811A tubes apparently exhibit more VHF gain than other 
>manufacturers' tubes.  This is good news if one is trying to build a VHF 
>oscillator, but hardly good news in amplifier service.  You could lower 
>the VHF gain in your FL-2100B by using a VHF parasitic suppressor with a 
>lower VHF parallel equivalent resistance (Rp).  . An Rp suppressor 
>comparison can be found at:
>http://www.vcnet.com/measures
>see Figure 12.    
>>Anyone
>>have a source for Cetron? I understand they are more expensive than the
>>Svetlana or similar Chinese brand.  Any help appreciated.
>(Eddie) Richardson Electronics Ltd. in Chicago used to sell Cetron tubes. 
> However, the use of new Cetron tubes is no guarantee of unconditional 
>VHF stability.  Although rare, I have heard reports of a 'big-bang' and 
>the filaments suddenly breaking when new Cetron tubes were installed.  
>Even though one "expert" blames filament breakage on mechanical defect, 
>it seems a bit unlikely to me that two 572Bs  would experience 
>simultaneously broken filaments unless both 572Bs were dropped from about 
>5 feet onto a firm carpet.  
>>Tom, WW5L 
>Rich---
>
>R. L. Measures, 805-386-3734, AG6K   
>
>
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