[Amps] Parasites (parasitics)

[Ian White, G3SEK]

2 wrote:
>
>However, the tradeoff with ceramic tubes is that they are rated for 
>"Amplifier and Oscillator Service" to higher frequencies, so there 
>could still be surprises lurking about,.  For example, the 3-500Z is 
>rated at 110MHz and it has 0.15pF of feedback C.  Since the 8877 has 
>only 0.1pF of feedback C, it would seem to be a more stable device. The 
>sticky-wicket is that the 8877 is rated at several times the amp/osc 
>frequency of the 3-500Z, so the 8877 actually has fewer reactive ohms 
>of feedback at its F-max than does the 3-500Z.   However, although this 
>may be bad news for amplifier builders, for oscillator builders, it's 
>good news.
>
Yawn... here we go again...

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.

Thousands of 8877 VHF amps out there can testify to that.

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