[Amps] Parasites (parasitics)

2 2 at vc.net
Thu Mar 27 19:09:17 EST 2003



>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.

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


-  R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K, 
www.vcnet.com/measures.  
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