>
>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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