[Amps] Parasitic suppressor resistor

[Steve Thompson]

R. Measures wrote:
> 
> On Jan 2, 2006, at 7:53 AM, Steve Thompson wrote:
> 
>>
>>
>> R. Measures wrote:
>>
>>> 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.
>>
>>
>> Did he (or she)
> 
> 
> it was a dude.  Wimmen is too dam stuped to know much about RF.
> 
>> indicate whether the grid grounding and/or cathode
>> impedance made any difference?
> 
> 
> I don't think there was an entry field for grid grounding L.    Cathode 
> Z is fixed at c. 54-ohms, however, the Z of the cathode-ground path at 
> the parasite frequency would make a difference in how much feedback 
> arrived from the anode to the cathode through C-fb.
I can't help but think that a low inductance Xc=5-10 ohms from cathode 
to ground would make a big difference to the calculation. Cutting the 
loop gain by 7-10dB is a good recipe for better stability.

Steve