SV: SV: [AMPS] SB-220 conundrum

[measures]

>
>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
>
>
>----------
>>FrĀn: 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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>>
>
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>


-  Rich..., 805.386.3734, www.vcnet.com/measures.  


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