[Amps] Parasites (parasitics)

[Ian White, G3SEK]

2 wrote:
>>Feedback C is significant, but is only one of the factors that determine
>>VHF stability. The factor that is usually more important is the length
>>and inductance from grid to ground.
>>
>>When there are no other feedback paths except through the tube, and the
>>grid ring is grounded directly to the chassis by a finger-stock, an 8877
>>amp can be unconditionally stable at all VHF frequencies - that means
>>stable with any input or output loads (or none) in any combination.
>
>**  I have autopsied 6 gold-sputtered 8877s from HF amplifiers that 
>used the Eimac 135305 grid-grounding collet.  [ref Figure 24 on my Web 
>site] The inductance of an 8877 grid + base flange appears to be c. 
>15nH.  The collet appears to have c. 7nH.

How do you arrive at those inductance figures? What do believe they 
show, and why?

>>
>>Thousands of 8877 VHF amps out there can testify to that.
>
>**  I know of no case of VHF/UHF gold-sputtering in other than HF 
>amp;ifiers

How many 887s from VHF amps have you looked at? Again, what do you 
believe that shows, and why?


If an 8877 can be made unconditionally stable in a VHF amp, with input 
and output deliberately tuned to the same frequency, and then you move 
it into an HF amp (keeping the same VHF-style grid grounding 
arrangement), then why shouldn't it still be stable at VHF?

-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek