SV: SV: SV: [AMPS] SB-220 conundrum

[sm5ki]

Thanks Rich,and this SB 220 I repaired had been at a commercial company full
of transmitter specialist and everybody looked for that parasitic that was
no parasitic at all , hi! Just by replacing this lonely diode and its, of
course, reservoire condenser solved the problem. No chain is stronger
than..... etc etc. 

Prosit de Hans

----------
>Från: measures <measures@vcnet.com>
>Till: "sm5ki" <sm5ki@algonet.se>, "mike baker" <bakerhouse@uswest.net>,
"pete smith" <n4zr@contesting.com>, "amps reflector" <amps@contesting.com>
>Ämne: Re: SV: SV: [AMPS] SB-220 conundrum
>Datum: fre 21 jan 2000 14.37
>

>
>>
>>Interesting info Rich, but Pete has a SB220 that probably once worked OK
>>with those parasitic resonances you mentioned. But, something has happened
>>and maybe those resonances moved to another frequency because of a failure
>>in one of the amps components. And could generate a new nasty parasitic
>>situation. 
>>
>>Remember my message some months ago ( nobody reacted and no
>>comments...funny)?
>>I said then, that I once had a problem with a shorted diode in the relay
>>power supply of my SB220 and the grids were subjected to AC that was
>>rectified by the 3-500Z grids and showed up as grid current on the meter.
>>Everybody said the amp was oscillating at some parasitic frequency but it
>>was plain rectified AC! So, maybe Petes problem is something similar simple?
>>
>When the 110v PS diode shorts, the filter C looks pretty much like a dead 
>short.  I would like to see a high-pot test of the grid/filament 
>withstanding potentials.  
>>
>>Interesting problem though......
>>
>Indeed, Hans
>>
>>
>>----------
>>>FrÂn: measures <measures@vcnet.com>
>>>Till: "sm5ki" <sm5ki@algonet.se>, "Mike Baker" <bakerhouse@uswest.net>,
>>"Pete Smith" <n4zr@contesting.com>, "AMPS reflector" <amps@contesting.com>
>>>Ÿmne: Re: SV: [AMPS] SB-220 conundrum
>>>Datum: ons 19 jan 2000 15.09
>>>
>>
>>>
>>>>
>>>>Hello Pete:
>>>>
>>>>I usually start my parasitics hunt with the old Millen grid dip meter and
>>>>the amp disconnected from the mains. You will be surprised how many odd
>>>>resonces there may be.
>>>>
>>>>If you find a resonant frequency in the grid or cathode circuitry try to
>>>>rotate your C1. If there is a change in the dip you may have a tgtp
>>>>oscillator at that frequency.
>>>>
>>>>Also I try to parasitic test my amplifiers with higher then normal resting
>>>>current. The small signal gain will be higher and any tendency for
>>>>parasitics will be increased.
>>>>
>>>>I never tried this on a SB220 but could there be some new resonant
>>>>frequencies in the 3-500Z grid circuitry?  
>>>
>>>There are, in the 88MHz range.  
>>>
>>>>The paralell circuit of the
>>>>negative feedback condensers and grid chokes maybe are resonant somewhere?
>>>
>>>Direct-grounding the grid makes little change in the resonance.  
>>>
>>>>And the plate choke maybe has the same resonant frequency?  Maybe you tried
>>>>earlier to run the amp on one of the  bands not avaible the SB 220 and one
>>>>of the grid chokes was hurt by a parasitic and changed its self resonant
>>>>frequency? 
>>>
>>>The grid RFC is essentially bypassed at the parasitic freq. of c. 110MHz 
>>>by the (3) 200pF grid capacitors.  
>>>
>>>>If I remember right, the new bands where not avaible in 1968-69
>>>>when the SB 220 came out and the plate choke nay very well be resonant on
>>>>the newer bands?
>>>
>>>The HV-RFC is resonant above 35MHz. 
>>>>
>>>>...
>>>
>>>cheers, Hans.......
>>>
>>>-  Rich..., 805.386.3734, www.vcnet.com/measures.  
>>>
>>>
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>>>FAQ on WWW:               http://www.contesting.com/ampsfaq.html
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>>>
>>
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>>
>
>
>-  Rich..., 805.386.3734, www.vcnet.com/measures.  
>
>
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>

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