[Amps] Parasitic suppressor resistor

R.Measures r at somis.org
Mon Jan 2 07:05:16 EST 2006


On Jan 2, 2006, at 1:04 AM, Steve Thompson wrote:

>
>
> Bill Turner wrote:
>> ORIGINAL MESSAGE:
>>
>> At 04:22 PM 1/1/2006, R. Measures wrote:
>>
>>> **  A problem with a resistor that is c. 120mm long is that has more
>>> L than would be optimal for a parasitic suppressor.
>
>> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>>
>> I thought about that, but the L would be there anyway. The 8877 sits
>> low on the chassis and the top of the plate choke is six inches above
>> the chassis, so the wire would be that long anyway. I guess I could
>> have mounted the choke horizontal but it's too late now.  :-)
> Having the L in the resistor is different from having it in series with
> the resistor/inductor suppressor. Whether the difference matters is
> unique to your amplifier and its gain/phase characteristics. If you can
> measure them, you can design a suppressor that's spot on for the job -
> if you can measure them, you might find that you can make an amp that
> avoids oscillation without needing a suppressor.

According to a Ham who telephoned me during the grate parasite debate, 
who uses a computer program to design commercial amplifiers, the 8877 
(C-fb = 0.1pF) does not need a parasitic suppressor if the anode 
resonance can moved comfortably above 110MHz.  In practical terms, 
using 150MHz as a comfort target, this means that the total L between 
the anode and the Tune-C needs to be under 110nH.   In other words, if 
an 8877 amplifier has an anode resonance of under 150MHz and it does 
not use a VHF suppressor, it may be on the ragged edge.
>
> Steve
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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org



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