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?
Interesting problem though......
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: [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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