SV: [AMPS] SB-220 conundrum

[measures]

>
>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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