[AMPS] Length of Anode Lead To Tuning Condenser in a Linear Amplifier?

[2]

>
>From what I have been able to gather over the years, in the output circuit of 
>a GG Linear Amplifier it is good practice to keep the length of the lead as 
>short as possible from the Anode of the tube to the Tuning Capacitor of the 
>Pi-Net output circuit, 

/\  This is also true for a grid-driven amplifier.  

>and to mount the Tuning Capacitor as close as possible 
>to the base of the tube to keep ground return currents as short as possible 
>from plate to grid grounding point? If that is the case then the best 
>practice would be to mount a Vacuum Variable Capacitor vertically as close to 
>the tube as possible without bumping into the glass chimney for a glass tube 
>or bumping into the anode of an external-anode tube? 

/\  This is good engineering practice.  

> With an external-anode 
>tube like the 8877 the distance between them could be practically zero. Has 
>anyone done any research into the stray effects of hanging a large DC 
>Blocking Doorknob Capacitor in line between the Anode and the first Tuning 
>Cap? I imagine depending on the construction of the DC Blocking Capacitor it 
>could introduce some significant stray inductance and capacitance into the 
>circuit? 

/\  Most DC blocking caps. are so short that they have negligable 
inductance.  Also, the +XC of the blocking cap, tends to cancel the -XL 
of the strap.  

>This effect could be greatly reduced as well as size of the 
>Blocking 
>Cap if a Ceramic Chip Capacitor assembly could be used at this point. If the 
>distance in the Anode circuit were reduced to near-zero would this 
>significantly raise the VHF parasitic resonant frequency of the circuit?

/\  True
 
> Is it always easier to damp out a VHF parasitic if its frequency is raised? 

/\  VHF parasitic suppressors do not damp out parasitics.  Instead, they 
reduce Rp (and Q) in the VHF-resonant anode circuit so that VHF gain is 
reduced to a point where VHF regeneration is less likely.

>Would a parasitic suppressor still be needed? 

/\ Modern, high-Mu triodes such as the 8877 and 3CX800A7 have so much 
high frequency gain - some even into the lower UHF region - that good 
enginering practice indicates that a osc. suppressor be used.  For an 
example of what can happen when an 8877 is used without a suppressor, see 
Figure 24 on my Web site.  

cheers, Todd

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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